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Glossary

Glossary

**A**
ABS PIPE: Acrylonitrile-butadiene-styrene plastic pipe used for water, drains, waste and venting.
ABSOLUTE PRESSURE: Gauge pressure plus the pressure of the atmosphere, normally 14,696 at sea Level at 70°F.
ABSOLUTE ZERO TEMPERATURE: Temperature measured from absolute zero, a point of temperature, theoretically equal to minus 273.18° C or minus 459.72° F; the hypothetical point at which a substance would have no molecular motion and no heat.
ABSORBENT (attractant): The salt solution used to attract water in an absorption chiller.
ABSORBER: That part of absorption chiller.
ABSORPTION: The process by which one substance is absorbed by another.
ABSORPTION AIR-CONDITIONING CHILLER: A system using a salt substance, water, and heat to provide cooling for air conditioning system.
“A” COIL: An evaporator coil that can be used for upflow, downflow, and horizontal-flow applications. It can actually consists of two coils shaped like a letter “A”.
ACCA MANUAL J: The Air Conditioning Contractors of America Manual that facilitates the calculations of heat gains and heat losses in residential structures.
ACCUMULATOR: A storage tank located in the suction line. It allows small amounts of liquid refrigerant to boil away before entering the compressor. Sometimes used to store excess refrigerant in heat pump systems during the winter cycle.
ACETYLENE: A gas often used with air oxygen for wielding, brazing, or soldering applications.
ACH: An acronym for (air changes per hour).
ACH50: An acronym for air changes per hour of air leaking while the blower door is connected and pulling 50 Pascal of vacuum on the house.
ACID-CONTAMINATED SYSTEM: A refrigeration system that contains acid due to contamination.
ACR TUBING: Air-conditioning and refrigeration tubing that is very clean, dry nitrogen. The tubing is sealing at the ends to contain the nitrogen.
ACTIVATED ALUMINA: A chemical desiccant used in refrigerant driers.
ACTIVATED CHARCOAL: A substance manufactured from coal or coconut shells into pellets. It is often used to absorb solvents, other organic materials, and odors.
ACTIVE RECOVERY: Recovering refrigerant with the use of recovery machine that has its own built-in compressor.
ACTIVE SENSOR: Sensors that send information back to the controller in terms of milliamps (mA) or volts.
ACTIVE SOLAR SYSTEM: A system that uses electrical and/or mechanical devices to help collect, store, and distribute the sun’s energy.
ACTUATOR: Device used to operate a valve using electric, pneumatic or hydraulic means. Often used for remote control or sequencing of valve operations.
ADSORPTION: The process by which a thin film of liquid or gas adheres to the surface of a solid substance.
AIR-ACETYLENE: A mixture of air and acetylene gas that when ignited is used for soldering, brazing, and other applications.
AIR, COMPRESSED: Air at any pressure greater than atmospheric pressure.
AIR-CONDITIONER: Equipment that conditions air by cleaning, cooling, heating, humidifying, or comfort cooling equipment.
AIR CONDITIONING: A process that maintains comfort conditions in a defined area.
AIR CONDITIONING, HEATING AND REFRIGERATION INSTITUTE (AHRI): A non-profit association that regulates equipment manufacturers and rates the capacity of equipment.
AIR-COOLED CONDENSER: One of the four main components of an air-cooled refrigeration system. It receives hot gas from the compressor and rejects heat to a place where it makes no difference.
AIR, FREE: Air which is not contained and which is subject only to atmospheric conditions.
AIR FRICTION CHART: A chart used to determine proper round duct sizes.
AIR GAP: The clearance between the rotating rotor and the stationery winding on an open motor. Known as a vapor gap in a hermetically sealed compressor motor.
AIR HANDLER: The device that moves the air across the heat exchanger in a forced air system – normally considered to be the fan and its housing.
AIR HEAT EXCHANGER: A device used to exchange heat between air and another medium at different temperature levels, such as air-to-air, air-to-water, or air-to-refrigerant.
AIR LOOP: The heat pump’s heating and cooling ducted air system, which exchanges heat with the refrigerant loop.
AIR PRESSURE CONTROL (switch: Used to detect air pressure drop across the coil in a heat pump outdoor unit due to ice buildup.
AIR SENSOR: Devices that are placed in an air-stream for the purpose of controlling airflow.
AIR STANDARD: Dry air at 70°F and 14.696 psi, at which it has a mass density of 0.075 lb/ft³ and a specific volume of 13.33ft³/lb, ASHRAE.
AIR-TO-CLOSE ACTION: See NORMALLY OPEN.
AIR-TO-OPEN ACTION: See NORMALLY CLOSED.
AIR VALVE: Valve that is used to control air flow. Flow is generally small.
AIR VENT: A fitting used to vent air manually or automatically from a system.
ALCOHOL: Antifreeze solution used in the water loop of geothermal heat pumps.
ALGAE: A form of green or black, slimy plant life that grows in water systems.
ALGORITHM: A computer code or set instructions to make specific calculations.
ALKYLBENZINE: A popular synthetic lubricant that works best with HCFC-based refrigerant blends. This lubricant can be used with CFC and HCFC refrigerants.
ALLEN HEAD: A recessed hex head in a fastener.
ALLOY STEEL: A steel consisting primarily of iron with some percentage of one or more other elements such as chromium, nickel, manganese, or vanadium deliberately added to enhance its properties.
ALL-WEATHER SYSTEM: A system providing year-round conditioning of the air.
ALTERNATING REFRIGERANTS: Newer refrigerants that are replacing the traditional CFC and HCFC refrigerants that have been used for many years. Many of these refrigerants have very low ozone depletion and global-warming indices. Some are completely chlorine-free.
ALTITUDE ADJUSTMENT: An adjustment to a refrigerator thermostat to account for a lower-than-normal atmospheric pressure such as may be found at a high altitude.
AMBIENT TEMPERATURE: The prevailing temperature of the environment immediately surrounding an object – generally considered to be -20° F to +100° F.
AMERICAN STANDARD PIPE THREAD: Standard thread used on pipe to prevent leaks.
AMMETER: A meter used to measure current flow in an electrical circuit.
AMPERAGE: Amount (quantity) of electron or current flow (the number of electrons passing a point in a given time) in an electrical circuit.
AMPERE: Unit of current flow.
ANALOG ELECTRONIC DEVICES: Devices that generate continuous or modulating signals within.
ANALOG SIGNAL: A continuous signal within an infinite number of steps.
ANALOG VOM: A volt-ohm-millimeter constructed so that the meter indicator is a needle over a printed surface.
ANEMOMETER: An instrument used to measure the velocity of air.
ANEROID BAROMETER: An instrument used to measure atmosphere pressure. This barometer uses a closed bellows linked to a needle and can be easily carried from place to place.
ANGLE VALVE: A valve with one opening at a 90° angle from the other opening, pneumatically actuated valve with a Y body design. The linear pneumatic actuator lifts a piston off the Y body seat to control flow.
ANNUAL FUEL UTILIZATION EFFICIENCY (AFUE): The US Federal Trade Commission requires furnace manufacturers to provide this rating so consumers may compare furnace performance before purchasing. This value is derived in a laboratory and used for heating appliance in which it accounts for cycling losses chimney losses and furnace jacket losses.
ANNUAL FUEL UTILIZATION INDEX (AFUI): The AFUI compares the amount of (seasonal) energy used in BTUs to the (seasonal) output of BTUs and expresses the ratio as a percentage. AFUI is calculated by the manufacturer of fuel burning combustion equipment 
ANODE: A terminal or connection point on a semiconductor.
ANSI: Abbreviation for the American National Standards Institute.
ANSI CLASS: A strength designation for valves which show the maximum pressures at various temperatures at which a valve can be expected to work safely.
ANTICIPATOR: Resistance beaters in thermostats that lessens the effects of system lag and overshoot.
ANTIFREEZE SOLUTION: A solution that will mix with water that has a freezing point much lower than water. Different solutions are mixed with water to lower the freezing point to the desired level.
APPROACH: The temperature difference between the water leaving the cooling tower and the wet-bulb temperature of the tower’s entering air.
APPROACH TEMPERATURE: The difference in temperature between the refrigerant and the leaving water in a chilled-water system.
AROMATIC OIL: A refrigeration mineral oil in which the alkylbenzine lubricant group originates.
ASA: Abbreviation for the American Standards Association (now known as American National Standards Institute (ANSI)).
ASHRAE: Abbreviation for the American Society of Heating, Refrigerating and Air-Conditioning Engineers.
ASME: Abbreviation for the American Society of Mechanical Engineers.
ASPECT RATIO: The ratio of the length of width of component.
ASTM: An acronym for American Society for Testing and Materials.
ATMOSPHERIC PRESSURE: The weight of the atmosphere’s gases pressing down on the earth. Equal to 14.696 psi at sea level and 70°F.
ATOM: The smallest particle of an element.
ATOMIZATION: The process of using pressure to change liquid to small particles of vapor.
ATOMIZING HUMIDIFIER: A type of humidifier that discharges tiny water droplets into the air which evaporate very rapidly into the duct air stream or directly into the conditioned space.
ATTRACTANT: A type of salt solution, usually lithium bromide, used as an absorbent in absorption refrigeration.
AUSTENITIC STAINLESS STEEL: The common stainless steel, where the primary microstructure is austenite and the composition primarily iron but also includes both chromium and nickel. The steels are designated as 300 series such as 304, 316, CF8M, etc.
AUTOMATIC CHANGEOVER THERMOSTAT: A thermostat that changes from cool to heat automatically by room temperature.
AUTOMATIC COMBINATION GAS VALVE: A gas valve for gas furnaces that incorporates a manual control, gas supply for the pilot, adjustment and safety features for the pilot, pressure regulator, and the controls for and the main gas valve.
AUTOMATIC CONTROL: Controls that react to a change in conditions to cause the condition to stabilize.
AUTOMATIC DEFROST: Using automatic means to remove ice form a refrigeration coil a timed intervals.
AUTOMATIC EXPANSION VALVE (AXV): A refrigerant control valve that maintains a constant pressure in an evaporator. Sometimes abbreviated AEV.
AUTOMATIC PUMPDOWN SYSTEM: A control scheme in refrigeration consisting of a thermostat and liquid-line solenoid valve that clears refrigerant from the compressor’s crankcase, evaporator, and suction line just before the compressor off cycle.
AUXILIARY DRAIN PAN: A separate drain pan that is placed under an air-conditioner evaporator to catch condensate in the event that the primary drain pan runs over.
AUXILIARY LINE: See whipline.
AZEOTROPIC BLEND: Two or more refrigerants mixed together that will have only one boiling and/or condensing point for each system pressure. Negligible fractionation or temperature glide will occur.
**B**
BACKDRAFTING: A continuous spillage of combustion by-products usually caused from a pressure difference that causes a reversal of the vent or chimney’s normal flow upward.
BACK ELECTROMOTIVE FORCE (BEMF): This is the voltage-generating effect of an electric motor’s rotor turning within the motor.
BACK PRESSURE: The pressure on the low pressure side of a refrigeration system (also known as suction pressure).
BACK SEAT: The position of refrigeration service valve body and seated, shutting off the service port. A shoulder on the stem of a gate or globe valve which seals against a mating surface inside the bonnet to prevent leakage of media through the bonnet stuffing box when the valve is fully opened.
BAFFLE: A plate used to keep fluids from moving back and forth at will in a container.
BALANCE POINT: The temperature in a structure where the heat pump will run full-time and remain the temperature in the structure. Auxiliary heat must be used to accompany the heat pump when the temperature is below the balance point.
BALANCED-PORT TXV: A valve that will meter refrigerant at the same rate when the condenser head pressure is low.
BALL CHECK VALVE: A valve with a ball-shaped internal assembly that only allows fluid flow in one direction.
BALL: The closure element of a ball valve.
BALL VALVE: A valve that uses a round ball in the flow chamber in the valve. The round ball is bored to the approximate inside diameter of the pipe and also offers very little resistance or pressure drop to the flow of water or fluid flow. The handle only has to be turned 90° to fully open or close the valve.
BAROMETER: A device used to measure atmospheric pressure that is commonly calibrated in inches or millimeters of mercury. There are two types: mercury column and aneroid. 
BAROMETRIC DAMPER: A place for drawing indoor air (dilution air) into a chimney after the combustion process for open combination type oil fired furnaces and boilers.
BASE: A terminal on a semiconductor.
BASE LOADS: Base loads are consistent, everyday consumers of energy in residential structures which includes freezers, refrigerators, lights, water heaters, clothes dryers, copiers, cell phone chargers, computers, and printers.
BASEBOARD HEATING: Convection heaters providing whole-house, spot or individual room healing. The heat is normally provided by electrical resistance or hot water.
BATCH PROCESS: A process in which the flow of both the control agent and the controlled medium is intermittent or gradually changes over a wide range. A process of relatively small magnitude or time expanse as compared to a continuous process.
BATT: The term used to describe precut panels of insulation that fit in between wall studs or ceiling joists.
BATTERY: A device that produces electricity (DC power) from the interaction of metals and acid.
BEARING: A device that surrounds a rotating shaft and provides a low-friction contact surface to reduce wear from the rotating shaft.
BEARING WASHOUT: A cleaning of the compressor’s bearing surfaces, which causes lack of lubrication. It is usually caused by liquid refrigerant mixing with the compressor’s crankcase oil due to liquid floodback or migration.
BELLOWS: An accordion-like device that expands and contracts when internal pressure changes.
BELLOWS SEAL: A method of sealing a rotating shaft or valve stem that allows rotary movement of the shaft or stem without leaking.
BELLY-BAND-MOUNT MOTOR: An electric motor mounted with a strap around the motor secured with brackets on the strap.
BENDING SPRING: A coil spring that can be fitted inside or outside a piece of tubing to prevent its walls from collapsing when being formed.
BEVEL GEAR OPERATOR: Device facilitating operation of a gate or globe valve by means of a set of bevel gears having the axis of the pinion gear at right angles to that of the larger ring gear. The reduction ratio of this gear set determines the multiplication of torque achieved.
BIMETAL: Two dissimilar metals fastened together to create a distortion of the assembly with temperature changes.
BIMETAL STRIP: Two dissimilar metal strips fastened back to back.
BINARY: Consisting of 1s and 0s. The 1s and 0s represent numbers, words, or signals that can be stored in the computer’s memory for future use. Calculators and computers are digital systems.
BIOFILM: Consists of a slimy bacterial secretion or film that forms a protective canopy to protect the bacteria beneath it from chemical biocides in cooling tower and boiler water systems.
BLEEDING: Allowing pressure to move from one pressure level to another very slowly.
BLEED VALVE: A valve with a small port usually used to bleed pressure from a vessel to the atmosphere.
BLOCKED SUCTION: A method of cylinder unloading. The suction line passage to a cylinder in reciprocating compressor is blocked, thus causing that cylinder to stop pumping.
BLOWDOWN: A system in a cooling tower whereby some of the circulating water to dilute the sediment in the sump.
BLOW-OFF SYSTEM: A piping system used for blowing scale, sediment, etc, from boilers, tanks or receivers. Also called “blow-down.”
BLOW-OFF VALVE: A valve designed specifically for blow-off service and used in blow-off lines. Also called “blowdown valve”.
BODY: The principle pressure containing part of a valve in which the closure element and seats are located.
BOILER: A container in which a liquid may be heated using any heat source. When the liquid is heated to the point that vapor forms and is used as the circulating medium, it is called a steam boiler.
BOILING POINT: The temperature level of a liquid at which it begins to change to a vapor. The boiling temperature is controlled by vapor pressure above the liquid.
BOILING TEMPERATURE: The boiling temperature of the liquid can be controlled by controlling the pressure. The standard boiling pressure for water is an atmospheric pressure of 29.92 in. Hg (mercury) where water boils at 212°F.
BOLTED BONNET: A bonnet which is connected to a valve body with bolts or studs and nuts.
BOLTED CONSTRUCTION: Describes a valve construction in which the pressure shell elements (such as body and closures of a trunnion ball valve) are bolted together and can be taken apart and repaired in the field.
BONDED ROOF: A roof that is insured against damage resulting from leaks. 
BONNET: The top part of a valve, attached to the body, which contains the packing gland, guides the stem, and adapts to extensions or operators.
BOOSTER COMPRESSOR: The compressor on a parallel compressor system that is dedicated to the coldest evaporators.
BOOSTER PUMP: An additional pump that is used to build the pressure above what the primary pump can accomplish.
BOOT: The connection between the branch line duct and the floor or ceiling register. It transitions the branch duct to the register size. It may be from rectangular.
BORE: The inside diameter of a cylinder, e.g., inside diameter of a seat ring, diameter of hole through ball in a ball valve.
BOURDON TUBE: C-shaped tube manufactured of thin metal and closed on one end. When pressure is increased inside, it tends to straighten. It is used in a gauge to indicate pressure.
BRAZING: High-temperature (above 800°F) melting of a filler metal for joining of two metals.
BREAKAWAY TORQUE: A value depicting the turning force required to initiate rotation of the closure element of a rotary valve.
BREAKER: A heat-activated electrical device used to open an electrical circuit to protect it from excessive current flow.
BRITISH THERMAL UNIT (BTU): The amount (quantity) of heat required to raise the temperature to 1 lb. of water 1°F.
BTU: Abbreviation for British thermal unit.
BUBBLE POINT: The refrigerant temperature at which bubbles begin to appear in a saturated liquid. Bubble point values are used to calculate subcooling on systems that operate with blended refrigerants.
BUBBLE TIGHT: A commonly used term to describe the ability of a control valve or regulator to shut off completely against any pressure on any fluid.
BUILDING PERFORMANCE INSTITUTE (BPI): A nationally recognized energy certification. For home energy auditors and contractors. A non-profit organization that accredits auditors, contractors, and other building professional that specialize in evaluating building systems and potential energy savings in homes.
BULB, SENSOR: The part of a sealed automatic control used to sense temperature.
BURNER: A device used to prepare and burn fuel.
BURR: Excess material squeezed into the end of tubing or pipe after a cut has been made. This burr must be removed.
BUSHING: A fitting used to reduce the size of an opening.
BUTANE GAS: A liquefied petroleum gas burned for heat.
BUTTERFLY VALVE: A valve constructed with a disk in the valve chamber that can be adjusted across the water or fluid stream. Imagine a coin that has a handle that can be turned crossways of the water or fluid only travels 90°, similar to the ball valve.
BUTT-WELD END: Lips are formed on the ends of the valve to exactly butt against connecting pipe in the system. These lip ends of both pipe and valve are machined, forming an inside groove to accommodate a backup ring for welding.
B-VENT: A double-walled venting system for gas stoves or other gas appliances using approved piping through the roof or into a chimney.
BYPASS: An auxiliary loop in a pipeline, usually for diverting flow around a valve, or other piece of equipment.
BYPASS VALVE: A valve used to divert the flow around or past the part of the system through which it normally passes.
**C**
CAD CELL: A device containing cadmium sulfide used to prove the flame in an oil-burning furnace or boiler.
CALIBRATION: The adjustment of instruments or gauges to the correct setting for known conditions.
CALORIMETER: An instrument of laboratory-grade quality used to measure heat absorbed into a substance.
CAPACITANCE: The term used to describe the electrical storage ability of a capacitor.
CAPACITIVE CIRCUIT: When the current in a circuit leads the voltage by 90°.
CAPACITOR: An electrical storage device used to start motors (start capacitor) and to improve the efficiency of motors (run capacitor).
CAPACITOR-START MOTOR: A single-phase motor with a start and run winding that has a capacitor in series with the start winding, which remains in the circuit until the motor gets up to about 75% of the motor’s rated speed.
CAPACITOR-START-CAPACIT-RUN MOTOR: A single-phase motor that has start capacitor in series with the start winding that is disconnected after start-up and run capacitor that is also in parallel with the start windings that stays in the circuit while running. This run capacitor is built for full-time duty and is used to increase the running efficiency of the motor.
CAPACITY: The rating system of equipment used to heat or cool substances. The mass flow rate through a valve under specified conditions.
CAPILLARY ATTRACTION: The attraction of a liquid material between two pieces of material such as two pieces of copper or copper and brass. For instance, in a joint made up of copper tubing and a brass fitting, the solder filler material has a greater attraction to the copper and brass than to itself and is drawn into the space between them.
CAPILLARY TUBE: A fixed-bore metering device. This is a small-diameter tube that can vary in length from a few inches to several feet. The amount of refrigerant flow needed is predetermined and the length and diameter of the capillary tube is sized accordingly.
CARBON DIOXIDE: A by-product of natural gas combustion that is not harmful.
CARBON MONOXIDE: A poisonous, colorless, odorless, tasteless gas generated by incomplete combustion.
CARBON STEEL: – Iron containing carbon in the form of carbides, about 0.1 to 0.3 percent carbon with no other alloying elements other than the sulfur, phosphorus, and other elements present in almost all steels.
CARRIER FREQUENCY: The speed at which the variable frequency drive’s power device switches the waveform’s positive and negative halves on and off is referred to as the switch frequency or carrier frequency.
CASTING: A product or the act of producing a product made by pouring molten metal into a mold and allowing it to solidify, thus taking the shape of the mold.
CAST IRON: The common term for cast gray iron or iron containing flake carbon in the range of 2% to 4.3%. Cast iron is brittle, exhibiting very little ductility before fracturing.
CATALYTIC COMBUSTOR STOVE: A stove that contains a cell-like structure consisting of a substrate, washcoat, and catalyst that produces a chemical reaction causing pollutants to be burned at much lower temperatures.
CATHODE: A terminal or connection point on a semiconductor.
CAVITATION: A vapor formed due to a drop in pressure in a pumping system. Vapor at a pump inlet may be caused at a cooling tower if the pressure is low and water is turned to vapor. Cavitation is a concern for liquid services where cavities or bubbles form and then collapse. It is the two-stage process of vaporization and condensation of a liquid.
CAVITY FILLER: Teflon seat that fills the area around the ball of a valve eliminating cavities where product can potentially accumulate.
CELLULOSE: A substance formed in wood plants from glucose or sugar.
CELSIUS SCALE: A temperature scale with 100-degree graduations between water freezing (0°C) and water boiling (100°C).
CENTIGRADE SCALE: See Celsius scale.
CENTRIFUGAL COMPRESSOR: A compressor used for large refrigeration systems that uses centrifugal force to accomplish compression. It is not positive displacement, but it is similar to a blower.
CENTRIFUGAL PUMP: A pump that uses centrifugal force to move a fluid. An impeller is rotated rapidly within the pump, causing the fluid to fly away from the center, which forces the fluid through a piping system.
CENTRIFUGAL SWITCH: A switch that uses a centrifugal action to disconnect the start windings from the circuit.
CFM: Cubic feet per minute.
CFM50°: The air flow measured in cubic feet per minute (CFM) through a blower door while maintaining a constant vacuum of 50 Pascals inside a home is referred to as CFM50°.
CHANGE OF STATE: The conditions that occurs when a substance from one physical state to another, such as ice to water and water to steam.
CHARGE OF REFRIGERANT: The quantity of refrigerant in a system.
CHARGING CURVE: A graphical method of assisting a service technician with charging an air conditioning or heat pump system.
CHARGING CYLINDER: A device that allows the technician to accurately charge a refrigeration system with refrigerant.
CHECK VALVE: A device that permits fluid flow in one direction only. A single-directional valve which automatically prevents back flow when the fluid in the line reverses.
CHILL FACTOR: A factor or number that is combination of temperature humidity and wind velocity that is used to compare a relative condition to a known condition.
CHILLED WATER SYSTEM: An air-conditioning system that circulates refrigerated water picks up heat from the area, thus cooling the area.
CHILLER PURGE UNIT: A system that removes air or non-condensables from a low pressure chiller.
CHIMNEY: A vertical shaft used to convey flue gases above the rooftop.
CHIMNEY DRAFT: Another name for stack draft.
CHIMNEY EFFECT: A term used to describe air or gas when it expands and rises when heated.
CHLOROFLUOROCARBONS (CFC): Those refrigerants thought to contribute to the depletion of the ozone layer.
CIRCUIT: An election or fluid flow path that makes a complete loop.
CIRCUIT BREAKER: A device that opens an electric circuit when an overload occurs.
CLAMP-ON AMMETER: An instrument that can be clamped around one conductor in an electrical circuit and measure the current.
CLASS: A rating using in valve piping systems. (i.e Class 150# or 300#) Note: The pressure class rating charts do not directly relate to the actual working pressure of the valve itself.
CLEARANCE VOLUME: The volume at the top of the stroke in a reciprocating compressor cylinder between the top of the piston and the valve plate.
CLOSED CIRCUIT: A complete path for electrons to flow on.
CLOSED-CIRCUIT COOLING TOWER: May have a wet/dry mode, an adiabatic mode, and a dry mode.
CLOSED LOOP: Piping circuit that is complete and not open to the atmosphere.
CLOSED-LOOP CONTROL CONFIGURATION: A closed loop control configuration can maintain the controlled variable, like the air in a duct, at its set point because the sensor is now located in the air being heated or cooled (controlled medium).
CLOSED-LOOP HEAT PUMP: Heat pump system that reuses the same heat transfer fluid, which is buried in plastic pipes within the earth or within a lake or pond for the heat source.
COAXIAL HEAT EXCHANGER: A tube-within-a-tube liquid heat exchanger. Typically it is used for water source heat pumps and small water cooled air conditioners.
CODE: The local, state, or national rules that govern safe installation and service of systems and equipment for the purpose of safety of the public and trade personnel.
COEFFICIENT OF PERFORMANCE (COP): The ratio of usable output energy divided by input energy.
CO2 INDICATOR: An instrument used to detect the quantity of carbon dioxide in flue gas for efficiency purposes.
COLD: The word used to describe heat at lower levels of intensity.
COLD ANTICIPATOR: A fixed resistor in a thermostat that is wired in parallel with the cooling contacts. This starts the cooling system before the thermostat calls for cooling, which allows the system to get up capacity before the cooling is actually needed.
COLD JUNCTION: The opposite junction to the hot junction in a thermocouple.
COLD TRAP: A device to help moisture during the evacuation of a refrigeration system.
COLD WALL: The term used in comfort heating to describe a cold outside wall and its effect on human comfort.
COLLECTOR: A terminal on a semiconductor.
COMBUSTION: A reaction called rapid oxidation or burning produced with the right combination of a fuel, oxygen and heat.
COMBUSTION AIR: Primary and/or secondary air. Primary air enters the furnace before the combustion process takes place. Secondary air is air introduced into furnace after combustions and supports combustion. 
COMBUSTION ANALYZER: An instrument used to measure oxygen concentrations with flue gases. This analyzer can test smoke and test for carbon monoxide and other gases.
COMBUSTION APPLIANCE: Any heating appliance that incorporates the combustion process which may include; gas and oil furnace and boilers, water heaters, stoves, fireplace and space heaters.
COMBUSTION APPLIANCE ZONE (CAZ): An area or zone within a residence or building where combustion appliances reside and operate.
COMBUSTION BLOWER MOTOR: A motor mounted either before or after the heat exchanger of a furnace or boiler which turns a fan to either push or pull combustion gases through the heat exchanger.
COMFORT: People are said to be comfortable when they are not aware of the ambient air surrounding them. They do not feel cool or warm or sweaty.
COMFORT CHART: A chart used to compare the relative comfort of one temperature and humidity condition to another condition.
COMPOUND GAUGE: A gauge used to measure the pressure above and below the atmosphere’s standard pressure. It is a Bourdon tube sensing device and can be found on all gauge manifolds used for air-conditioning and refrigeration service work.
COMPRESSION: A term used to describe a vapor when pressure is applied and the molecules are compacted closer together.
COMPRESSION RATIO: A term used with compressors to describe the ratio between the high-and-low-pressure sides of the compression cycle. It is calculated by dividing the absolute discharge pressure by the absolute suction pressure.
COMPRESSOR: A vapor pump that pumps vapor (refrigerant or air) from one pressure level to a higher pressure level.
COMPRESSOR CRANKCASE: The internal part of the compressor that houses the crankshaft and lubricating oil.
COMPRESSOR DISPLACEMENT: The internal volume of compressor’s cylinders, used to calculate the pumping capacity of the compressor.
COMPRESSOR HEAD: The component that sits on top of the compressor cylinder and holds the components together.
COMPRESSOR OIL COOLER: One or more piping systems used for cooling the crankcase oil.
COMPRESSOR SHAFT SEAL: The seal that prevents refrigerant inside the compressor from leaking around the rotating shaft.
CONCENTRATOR: The part of an absorption chiller where the dilute salt solution is boiled to release the water and concentrate the solution.
CONDENSATE: The moisture that forms on an evaporator coil.
CONDENSATE PUMP: A small pump used to pump condensate to a higher level.
CONDENSATION: Liquid formed when a vapor condenses.
CONDENSE: Changing a vapor to a liquid.
CONDENSER: The component in a refrigeration system that transfers heat from the system by condensing refrigerant.
CONDENSER FLOODING: An automatic method of maintaining the correct head pressure in mild weather by using refrigerant from an auxiliary receiver.
CONDENSER SPLITTING: Split the condenser into two separate and identical condenser circuits to reduce the amount of extra refrigerant charge needed for changing outdoor ambient conditions.
CONDENSING BOILER: High-efficiency boiler designed to operate with low flue-gas temperatures. These boilers allow for the condensing of flue gas.
CONDENSING OIL FURNACE: An oil furnace with a coil that causes moisture to condense from the flue gases, producing latent heat. This increases the efficiency of the furnace. 
CONDENSING PRESSURE: The pressure that corresponds to the condensing temperature in a refrigeration system.
CONDENSING TEMPERATURE: The temperature at which a vapor changes to a liquid.
CONDENSING UNIT: A complete unit that includes that includes the compressor and condensing coil.
CONDITIONED SPACE: Areas of a structure or residence that are heated or cooled.
CONDUCTION: Heat transfer from one molecule to another with a substance or from one substance to another.
CONDUCTIVITY: The ability of a substance to conduct electricity or heat.
CONDUCTOR: A path for electrical energy to flow on.
CONNECTING ROD: A rod that connects the piston to the crankshaft.
CONTRACTOR: A larger version of the relay. It can be repaired or rebuilt and has movable and stationary contacts.
CONTAMINANT: Any substance in a refrigeration system that is foreign to the system, particularly if it causes damage.
CONTINUOUS COIL VOLTAGE: This is the maximum back electromotive force (BEMF) that the relay’s coil can tolerate “continuously” without overheating and circuit.
CONTINUOUS PROCESS: A process in which the flow of the controlled media is continuous and fluctuates over a narrow range. The opposite of “batch process.”
CONTINUOUS PROCESS CONTROL: That process in which materials flow more or less continuously through a plant while being treated.
CONTROL: A device for stopping, starting, or modulating the flow of electricity or fluid to maintain a preset condition.
CONTROL AGENT: The control agent is fluid that transfers energy or mass to the controlled medium. It is the control agent (such as hot or cold water) that will flow through a final control device (water valves).
CONTROL LOOP: A sensor, a controller, and a controlled device. A control made up of a number of devices acting as individual transfer systems which are joined to form a network. 
CONTROL, MECHANICAL: A control actuated by linkages, gears, cams or other mechanical elements. 
CONTROL, PNEUMATIC: A control actuated by air or other gas pressure.
CONTROL POINT: The actual value of the controlled variable at a particular point in time.
CONTROL SYSTEM: A network of controls to maintain desired conditions in a system or space.
CONTROL VALVE: A valve which regulates the flow or pressure of a medium which affects a controlled process. Control valves are operated by remote signals from independent devices using any of a number of control mechanism such as pneumatic, electric, and electro-hydraulic.
CONTROLLED DEVICE: May be any control that stops, starts, or modulates fuel, fluid flow, or air to provide expected conditions in the conditioned space.
CONTROLLED ENVIRONMENT: The conditioned space the HVACR system is trying to maintain.
CONTROLLED MEDIUM: The controlled medium is what absorbs or releases the energy or mass transferred to or from the HVACR process. Controlled mediums can be air in duct or water in a pipe.
CONTROLLED OUTPUT DEVICE: Any device of an array of dampers, fans, variable-frequency drives (VFDs), cooling coil valves, heating coil valve, relays, and motors – all of which control the controlled environment.
CONTROLLER: A device that provides the output to the controlled device.
CONVECTION: Heat transfer from one place to another using a fluid.
CONVERSION FACTOR: A number used to convert from one equivalent value to another.
COOLER: A walk-in or reach-in refrigerated box.
COOLING TOWER: The final device in many water cooled systems, which rejects heat from, the system into the atmosphere by evaporation of water.
COPPER PLATING: Small amounts of copper are removed by electrolysis and deposited on the ferrous metal parts in a compressor.
CORROSION: A chemical action that eats into or wears away material from a substance.
COTTER PIN: Used to secure a pin. The cotter pin is inserted through a hole in the pin, and the ends spreads to retain it.
COUNTER EMF: Voltage generated or induced above the applied voltage in a single phase motor.
COUNTERFLOW: Two fluids flowing in opposite directions.
COUPLING: A device for joining two fluid-flow lines. Also the device connecting a motor driveshaft to the driven shaft in a direct drive system.
CPVC (CHLORINATED POLYVINYL CHLORIDE): Plastic pipe similar to PVC except that it can used with temperatures up to 180°F at 100 psig.
CRACKAGE: Small spaces in a structure that allow air to infiltrate the structure.
CRADLE-MOUNT MOTOR: A motor with a mounting cradle that fits the motor end housing on each end and is held down with a bracket.
CRANKCASE HEAT: Heat provided to the compressor crankcase.
CRANKCASE PRESSURE-REGULATING VALVE (CPR VALVE): A valve installed in the suction line, usually close to the compressor. It is used to keep a low-temperature compressor from overloading on a hot pulldown by limiting the pressure to the compressor.
CRANKSHAFT: In a reciprocating compressor, the crankshaft changes the round-and-round motion of the pistons using off-center devices called throws.
CRANKSHAFT SEAL: Same as the compressor shaft seal.
CRANKSHAFT THROW: The off center portion of a crankshaft that changes rotating motion to reciprocating motion.
CREOSOTE: A mixture of unburned organic material found in the smoke from a wood burning fire.
CRISPER: A refrigerated compartment that maintains a high humidity and low temperature.
CRITICAL POINT: A point on the pressure/enthalpy diagram where the critical temperature and critical pressure exist. No liquid refrigerant can be produced above a refrigerant’s critical point.
CRITICAL TEMPERATURE: The critical temperature of a gas is the highest temperature the gas can have and still be condensable by the application of pressure.
CROSS CHARGE: A control with a sealed bulb that contains two different fluids that work together for a common specific condition.
CROSS LIQUID CHARGE BULB: A type of charge in the sensing bulb of the TXV that has different characteristics from the system refrigerant. This is designed to help prevent liquid refrigerant from flooding to the compressor at start-up.
CROSS VAPOR CHARGE BULB: Similar to the vapor charge bulb but contains a fluid different from the system refrigerant. This is special-type charge and produces a different temperature/pressure relationship under different conditions.
CRYOGENIC VALVE: A term used to describe valves designed to operate below –40°C.
CRYSTALLIZATION: When a salt solution becomes too concentrated and part of the solution turns to salt.
CUPRONICKEL: A material used in heat exchangers. This material is made with an alloy of copper for a higher corrosion resistance for acid cleaning.
CURB: Frame that is used to mount and support HVAC equipment.
CURRENT, ELECTRICAL: Electrons flowing along a conductor.
CURRENT RELAY: An electrical device activated by a change in current flow.
CURRENT SENSING RELAY: An inductive relay coil usually located around a wire used to sense current flowing through the wire. Its action usually opens or closes a set of contacts.
CUT-IN AND CUT-OUT: The two points at which a control opens and closes its contacts based on the condition it is supposed to maintain.
Cv: Flow coefficient expressed as the number of gallons of 60° water that would flow through an opening, such as a valve port, in 1 minute under a differential pressure of 1 psi pressure drop under stated conditions.
CYCLE: A complete sequence of events (from start to finish) in a system.
CYLINDER: A circular container with straight sides used to contain fluids or to contain the compression process (the piston movement) in a compressor.
CYLINDER COMPRESSOR: The part of the compressor that contains the piston and its travel.
CYLINDER HEAD COMPRESSOR: The top to the cylinder on the high pressure side of the compressor.
CYLINDER REFRIGERANT: The container that holds refrigerant.
CYLINDER UNLOADING: A method of providing capacity control by cylinder in a reciprocating compressor to stop pumping.
**D**
DAMPER: A component in an air-distribution system that restricts airflow for the purpose of air balance.
DATABASE: A collection of data or information stored in the memory of the controller at a later point in time.
DC CONVERTER: A type of rectifier that changes alternating current (AC) to direct current (DC).
DC MOTOR: A motor that operates on direct current (DC).
DCV: See Demand Control Ventilation.
DEAD ZONE: A range of measured values or variable in which an instrument cannot detect or initiate corrections.
DECLINATION ANGLE: The angle of the tilt of the earth on its axis.
DECORATIVE APPLIANCE: A term used by ANSI to indicate a gas appliance that is not designed for larger space-heating applications.
DEEP VACUUM: An attained vacuum that is below 250 microns.
DEFROST: The melting of ice.
DEFROST CONDENSATE: The condensate or water from a defrost application of a refrigeration system.
DEFROST CYCLE: The portion of the refrigeration cycle that melt the ice off the evaporator.
DEFROST TERMINATION SWITCH: A temperature activated switch that stops the defrost cycle and returns the equipment to the refrigeration cycle.
DEFROST TIMER: A timer used to start and stop the defrost cycle.
DEGASSING: Removing air and other non-condensable gasses from a system using a vacuum pump.
DEGREASER: A cleaning solution used to remove grease from parts and coils.
DEHUMIDIFY: To remove moisture from air.
DEHYDRATION: Removing moisture from a sealed system or a product.
DELTA TRANSFORMER CONNECTION: A transformer connection that results in a voltage output of 115V and 230V.
DELTA-T: The temperature difference at two different points, such as the inlet and outlet temperature difference across a water chiller.
DEMAND CONTROL VENTILATION, DCV: Ventilation strategy that calculates required ventilations based on actual, not, expected occupancy levels.
DEMAND METERING: In his system, the power company changes the customer based on the highest usage for a prescribed period of time during the billing period. The prescribed time for demand metering may be any 15 or 30min period within the billing period.
DENSITY: The weight per unit of volume of a substance.
DEPARTMENT OF TRANSPORTATION (DOT): The governing body of the US government that makes the rules for transporting items, such as volatile liquids.
DERIVATIVE (DIFFERENTIAL) CONTROLLER: A control mode where the controller will change its output signal according to the rate of change of the error or offset.
DESICCANT: Substance in a refrigeration system drier that collect moisture.
DESICCANT DRIER: A device that dehumidifies compressed air for use in controls or processing.
DESIGN PRESSURE: The pressure at which system is designed to operate under normal conditions.
DE-SUPERHEATING: Removing heat from the superheated hot refrigerant gas down to the condensing temperature.
DETECTOR: A device used to search and find.
DETENT OR SNAP ACTION: The quick opening and doing of an electrical switch.
DEVIATION: The difference at any given time between the control point and the set point.
DEW: Moisture droplets that form on a cool surface.
DEW POINT TEMPERATURE: The exact temperature at which moisture begins to form. The central temperature of a gas or liquid at which condensation or evaporation occurs.
DIAC: A semiconductor often used as a voltage sensitive switching device.
DIAGNOSTIC THERMOSTAT: A thermostat that can receive information from various sensors and determine when a system is having a problem. It may give a fault signal in the form of a code indicating the problem.
DIAPHRAGM: A thin flexible material (metal rubber, or plastic) that separates two pressure differences.
DIAPHRAGM VALVE: A valve that has a thin sheet of metal between the valve and the fluid flow. This thin sheet is called the diaphragm and contains the pressure in the system from the action of the valve. A bi-directional valve, operated by applying force to a diaphragm. They are often used in slurries as well as hygienic applications as they are less susceptible to clog.
DIE: A tool used to make an external thread such as on the end of a piece of pipe.
DIFFERENTIAL: The difference in the cut-in and cut-out points of a control, pressure, time, temperature, or level.
DIFFERENTIAL PRESSURE: The maximum difference in pressure measured between the valve inlet and outlet, against which the valve is required to operate.
DIFFUSE RADIATION: Radiation from the sun that reaches the earth after it is reflected from other substances, such as moisture or other particles in space.
DIFFUSER: The terminal or end device in an air-distribution system that directs air in a specific direction using louvres.
DIGITAL ELECTRONIC DEVICES: Devices that generate strings of data or groups of logic consisting of 1s and 0s.
DIGITAL ELECTRONIC SIGNAL: An electrical signal, usually 0 to 10 V DC or 0 to 20 milliamps DC that is used to control system conditions.
DIGITAL VOM: A volt-ohm-milliammeter that displays the reading in digits or numbers.
DILUTION AIR: Excess air introduced after combustion and usually enters at the end of the heat exchanger. It is brought in by the draft hood of the natural draft furnace.
DIODE: A solid-state device composed of both P-type and N-type material. When the diode is reversed, current will not flow.
DIP (DUAL INLINE PAIR): A very small low-amperage, single-pole, double-throw switch used in electronic circuits to set up the program in the circuit.
DIRECT ACTING (SOLENOID VALVE): A direct-acting actuator(coil) is one in which the actuator stem extends to push-down-to-close plug and seat orientation.
DIRECT ACTING (POSITIONER): A direct-acting positioner or a direct-acting controller outputs an increase in signal in response to an increase in set point.
DIRECT CURRENT: Electricity in which all electron flow goes continuously in one direction.
DIRECT DIGITAL CONTROL (DDC): Very low voltage control signal, usually 0 to 10 V DC or 0 to 20 milliamps DC.
DIRECT DRIVE COMPRESSOR: A compressor that is connected directly to the end of the motor shaft. No pulleys are involved.
DIRECT DRIVE MOTOR: A motor that is connected directly to the load, such as an oil burner motor or a furnace fan motor.
DIRECT EXPANSION: The term used to describe an evaporator with an expansion device other than a low-side float type.
DIRECT-FIRED ABSORPTION SYSTEM: An absorption refrigeration system that uses gas or oil as the heat source.
DIRECT GEOEXCHANGE SYSTEM: A geothermal system where a primary, phase-changing refrigerant is circulated in a copper tubing directly in contact with the earth.
DIRECT MOUNT VALVES: Valves that offer ISO 5211 mounting pads and recessed packing nuts. Can accommodate actuators without mounting brackets making them direct mount.
DIRECT RADIATION: The energy from the sun that reaches the earth directly.
DIRECT-SPARK IGNITION (DSI): A system that provides direct ignition to the main burner.
DIRECT VENT: A venting system that provides direct ignition to the main burner.
DISCHARGE PRESSURE: The pressure on the high-pressure side of a compressor.
DISCHARGE VALVE: The valve at the top of a compressor cylinder that shuts on the downstroke to prevent high-pressure gas from reentering the compressor cylinder, allowing low pressure gas to enter.
DISCUS COMPRESSOR: A reciprocating compressor distinguished by its disc-type valve system.
DISCUS VALVE: A reciprocating compressor valve design with a low clearance volume and larger bore.
DISTRIBUTOR: A component installed at the outlet of the expansion valve that distributes the refrigerant to each evaporator circuit.
DIVERTER TEE: Special tee fitting used on one pipe hydronic systems to facilitate water flow through remote terminal units.
DOMESTIC HOT WATER LOOP: Domestic hot water heated by the refrigerant loop of the heat pump.
DOPING: Adding an impurity to semiconductor to produce a desired charge.
DOUBLE BLOCK AND BLEED: The capability of a valve under pressure to obtain a seal across both the upstream and downstream seat rings and to have its body cavity bled down/vented to atmospheric pressure.
DOUBLE FLARE: A connection used on copper, aluminum, or steel tubing that folds tubing wall to a double thickness.
DOWEL PIN: A pin, which may or may not be tapered, used to align and fasten two parts.
DOWNFLOW FURNACE: This furnace sometimes is called a counterflow furnace. The air intake is at the top, and the discharge air is the bottom.
DRAFT: A force that allows air travel into the combustion chamber and to force the combustion gases out through a vent or chimney.
DRAFT DIVERTER: A place for drawing indoor air (dilution air) into a vent or chimney after the combustion process for open combustion type gas furnaces and boilers. Part of its function is to prevent down drafts from affecting burner performance.
DRAFT GAUGE: A gauge used to measure very small pressures (above and below atmospheric) and compare them with the atmosphere’s pressure. Used to determine the flow of flue gas in a chimney or vent.
DRAFT HOOD: A place for drawing indoor air (dilution air) into a vent or chimney after the combustion process for open combustion.
DRAIN PLUG: A fitting at the bottom of a valve, the removal of which permits draining and flushing the body cavity.
DRIER: A device used in refrigerant line to remove moisture.
DRILLED WELL: A well with a well casing, electric pump and electric line that is drilled into the ground into a water source.
DRIP PAN: A pan shaped to collect moisture condensing on an evaporator coil in an air-conditioning or refrigeration system.
DRIP TIME: Once the defrost cycle is terminated; the fans must remain of for a period of time to prevent moisture from being blown off the coil and on to the product. This fan-off time after defrost is referred to as “drip time”.
DRIP PROOF MOTOR: A motor that can stand water dripping on it, such as condenser fan motor in an outdoor condensing unit.
DRIVE CLIP: One of the fasteners that holds square or rectangular steel ducts together.
DROP OUT VOLTAGE: The minimum back electromotive force (BEMF) that the potential relay coil must experience or its armature will drop out and the contacts between terminals 1 and 2 will close.
DRY BULB TEMPERATURE: The temperature measured using a plan thermometer.
DRY WELL: A well-used for the discharged water in an open-loop geothermal heat pump.
DUAL PRESSURE CONTROL: Two controls that are mounted in the same housing, typically low and high pressure controls together.
DUCT: A sealed channel used to convey air from the system to and from the point of utilization.
DUCT BLOWER: An instrument used to pressurize and test ductwork systems and air handler cabinets for air leakage.
DUST MITES: Microscopic spider-like insects. Dust mites and their remains are thought to be a primary irritant to some people.
DUTY CYCLE: The percentage of time a device is allowed to operate over a given period of time. Expressed in percent it equals “time on” divided by “time off”.
**E**
EARTH-COUPLED HEAT PUMP: Another name for a closed loop heat pump.
ECCENTRIC: An off-center device that rotates in a circle around a shaft.
ECCENTRIC BALL VALVE: Similar to a ball valve in construction except that it usually contains a segmented ball or half ball. Used typically in Waste Water applications.
ECM: An electronically commutated motor. This DC motor uses electronics to commutate the rotor instead of brushes. It is typically built or under 1 hp.
ECONOMIZER: Mechanical device used on HVAC equipment to reduce energy consumption by allowing outside air to be used for cooling purpose if air conditions desirable.
EDDY CURRENT TEST: A test with an instrument to find potential in evaporator or condenser tubes.
EEPROM: An acronym for electrical erasable programmable read-only memory.
EFFECTIVE TEMPERATURE: Different combinations of temperature and humidity that provide the same comfort level.
ELASTOMER: A natural or synthetic elastic material, often used for o-ring seals. Typical materials are Viton®, buna-n, EPDM.
ELECTRIC ACTUATOR: Electric valve actuators which when mounted on valves in response to a signal, automatically move to a desired position using an outside power source.
ELECTRIC FORCED AIR FURNACE: An electrical resistance type of heating furnace used with a duct system to provide heat to more than one room.
ELECTRIC HEAT: The process of converting electrical energy, using resistance into heat.
ELECTRIC HYDRONIC BOILER: A boiler using electrical resistance heat, which often has a closed loop piping system to distribute heated water for space heating.
ELECTRICAL POWER: Electrical power is measured in watts. One watt is equal to one ampere flowing with potential of one volt. Watt = Volts x Amperes (P=E x I).
ELECTRICAL SHOCK: When an electrical current travels through a human body.
ELECTRODES: Electrodes carry high voltage to the tips, where an arc is created for the purpose of ignition for oil or gas furnaces.
ELECTROMAGNET: A coil of wire wrapped around a soft iron core that creates a magnet.
ELECTROMECHANICAL CONTROLS: Electromechanical controls convert some form of mechanical energy to operate an electrical function, such as a pressure-operated switch.
ELECTROMOTIVE FORCE: A term often used for voltage indicating the difference of potential in two charges.
ELECTRON: The smallest portion of an atom that carries a negative charge and orbits around the nucleus of an atom.
ELECTRONIC AIR FILTER: A filter that charges dust particles using a high-voltage direct current and then collects these particles on a plate of opposite charge.
ELECTRONIC CHARGING SCALE: An electronically operated scale used to accurately charge refrigeration systems by weight.
ELECTRONIC CIRCUIT BOARD: A phenolic type of plastic board that electronic components are mounted on. Typically, the circuits are routed on the back side of the board and with prongs of wire that are soldered to the circuits on the back. These can be mass-produced  and coated with material that keeps the circuits separated if moisture and dust accumulate.
ELECTRONIC CONTROLS: Controls that use solid state semiconductors for electrical and electronic functions.
ELECTRONIC EXPANSION VALVE (EXV): A metering valve that uses a thermistor as a temperature-sensing element that varies the voltage to a heat motor-operated valve.
ELECTRONIC LEAK DETECTOR: An instrument used to detect gases in very small portions by using electronic sensors and circuits.
ELECTRONIC OR PROGRAMMABLE THERMOSTAT: A space thermostat that is electronic in nature with semiconductors that provide different timing programs for cycling the equipment.
ELECTRONIC RELAY: A solid-state relay with semiconductors used to stop start, or modulate power in a circuit.
ELECTRONICS: The use of electron flow in conductors, semiconductors, and other devices.
ELECTROPORATION: The high frequency, pulsing action of a signal generator damages the membrane of the planktonic bacteria in cooling tower and boiler water systems by creating small “pores” in their outer membrane.
ELECTROSTATIC PRECIPITATOR: Another term for an electronic air cleaner.
EMERGENCY SHUT DOWN VALVE (ESD): A valve that uses energy which is stored in the actuator to close rapidly in an emergency.
EMITTANCE: The ability of a material to emit radiant energy from its surface.
EMITTER: A terminal on a semiconductor.
ENCAPSULATION: Bacteria are attracted to the powder in cooling tower and boiling water systems and become entrapped in the powder particle.
END BELL: The end structure of an electric motor that normally contains the bearings and lubrication system.
END CONNECTION: The type of connection supplied on the ends of a valve which allows it to be connected to various piping -(i.e- butt weld end, flanged end, threaded or socket weld)
END-MOUNT MOTOR: An electric motor mounted with tabs or studs fastened to the moor housing end.
END PLAY: The amount of lateral travel in a motor or pump shaft.
ENERGY: The capacity for doing work.
ENERGY CONSERVATION MEASURE (ECM): A method or process that will conserve or decrease energy waste.
ENERGY EFFICIENCY RATIO (EER): A equipment efficiency rating that is determined by dividing the output in Btu/h by the input n watts. This does not take into account the start-up and shutdown for each cycle.
ENERGY INDEX: A way to compare one home to another and to characterize their energy efficiency.
ENERGY MANAGEMENT: The use of computerized or other methods to manage the power to a facility. This may include cycling of nonessential equipment, such as water fountain pumps or lighting, when it may not be needed. The air-conditioning and heating system is also operated at optimum times when needed instead of around the clock.
ENERGY RECOVERY VENTILATOR (ERV): In winter, this ventilation equipment recovers heat and moisture from exhaust air. In summer, heat and moisture are removed from incoming air and transferred to exhaust air.
ENTHALPY: The amount of heat a substance contains form a predetermined base or point.
ENTHALPY CONTROL: Device used on economizers to determine the heat content, Btu/lb, of an air stream.
ENTROPHY: Term to describe the change in heat content of a pound of refrigerant per degree Rankine. Expressed in units of Btu/lb/°R.
ENVIRONMENT: Our surroundings, including the atmosphere.
ENVIRONMENTAL PROTECTION AGENCY (EPA): A branch of the federal government dealing with the control of ozone-depleting refrigerants and other chemicals and the overall welfare of the environment.
EPA: Abbreviation for the Environmental Protection Agency.
ERROR: The signed or mathematical difference between the set point and the control point of a control process that tells whether the control point is above or below the control set point.
ERROR CODES: Codes that may read out on an electronic panel that tell the technician what the problem may be.
ERV: See Energy Recovery Ventilator.
ESTER: A popular synthetic lubricant that performs best with HFCs and HFC-based blends.
ETHANE GAS: The fossil fuel, natural gas, used for heat.
ETHYLENE GLYCOL: An antifreeze commonly used in chilled water air conditioning systems to prevent the freezing of the water coils.
EUTECTIC SOLUTION: A mixture of two substances in such a ratio that it will provide for the lowest possible melting temperature for that solution.
EVACUATION: The removal of any gases not characteristic to a system or vessel.
EVAPORATIVE CONDENSER (COOLING TOWER): A combination water cooling tower and condenser the refrigerant from the compressor is routed to the cooling tower where the tower evaporates water to cool the refrigerant. In the evaporative condenser the refrigerant is routed to the tower and the water circulates only in the tower. In a cooling tower, the water is routed to the condenser at the compressor location.
EVAPORATIVE COOLING: Devices that provides this type of cooling use fiber mounted in a frame with water slowly running down the fiber. Fresh air is drawn in and through the water soaked fiber and cooled by evaporation to a point close to the wet-bulb temperature of the air.
EVAPORATIVE HUMIDIFIER: A humidifier that provides moisture on a media surface through which air is forced. The air picks up the moisture from the media as a vapor.
EVAPORATOR: The component in a refrigeration system that absorbs heat into the system and evaporates the liquid refrigerant.
EVAPORATOR FAN: A fan or blower used to improve the efficiency of an evaporator by air movement over the coil.
EVAPORATOR PRESSURE-REGULATING VALVE (EPR VALVE): A mechanical control installed in the suction line at the evaporator outlet that keeps the evaporator pressure from dropping below a certain point.
EVAPORATOR TYPES: Flooded – an evaporator where the liquid refrigerant level is maintained to the top of the heat exchange coil. Dry type- an evaporator coil that achieves the heat exchange process with a minimum of refrigerant charge.
EVEN PARALLEL SYSTEM: Parallel compressors of equal sizes mounted on a steel rack and controlled by a microprocessor.
EXCESS AIR: Excess air consists of combustion air and dilution air.
EXFILTRATION: The outflow of air from a structure. Air flow out of a structure usually from a pressure difference, usually accompanied by an equal inflow of air.
EXHAUST VALVE: The movable component in a refrigeration compressor that allows hot gas to flow to the condenser and prevents it from refilling the cylinder on the down stroke.
EXPANSION JOINT: A flexible portion of a piping system or building structure that allows for expansion of the materials due to temperature changes.
EXPANSION (METERING) DEVICE: The component between the high-pressure liquid line and the evaporator that feeds the liquid refrigerant into the evaporator.
EXPLOSION-PROOF MOTOR: A totally sealed motor and its connections that can be operated in an explosive atmosphere, such as in a natural gas plant.
EXTERNAL DRIVE: An external type of compressor motor drive, as opposed to a hermetic compressor.
EXTERNAL EQUALIZER: The connection from the evaporator outlet to the bottom of the diaphragm on a thermostatic expansion valve.
EXTERNAL HEAT FROST: A defrost system for a refrigeration system where the heat comes from some external source. It might be an electric strip heater in an air coil, or water in the case of an ice maker. External means others than hot gas defrost.
EXTERNAL MOTOR PROTECTION: Motor overload protection that is mounted on the outside of the motor.
**F**
FACE TO FACE DIMENSIONS: Is the distance or space between the end to end or flanges of a valve or fitting. These dimensions are governed by ANSI/ISA specifications.
FACING: The finish of the gasket contact surface of a flange.
FAHRENHEIT SCALE: The temperature scale that places the boiling point of water at 212°F and the freezing point at 32°F.
FAIL-SAFE: The timer on the time clock will eventually bring the system out of defrost if early defrost termination is inoperative.
FAN: A device that produces a pressure difference in air to move it.
FAN ASSISTED DRAFT: A forced draft system that either pushes or pulls combustion gasses through the heat exchanger using a combustion blower motor and fan. These systems can either forced or induced draft systems.
FAN CYCLING: The use of a pressure control to turn a condenser fan on and off to maintain a correct pressure within the system.
FAN RELAY COIL: A magnetic coil that controls the starting and stopping of a fan.
FARAD: The unit of capacity of a capacitor. Capacitors in our industry are rated in microfarads.
FEEDBACK: Part of a closed loop system which brings back information about the condition under control for comparison to the target value.
FEEDBACK CONTROLLER: A mechanism which measures the value of the controlled variable, accepts the value of command and as the result of comparison, manipulates a controlled system in order to maintain an established relationship between the controlled variable and the command.
FEEDBACK CONTROL SYSTEM: A control system which maintains a prescribed relationship of one system variable to another by comparing functions of these variables and using the difference as a means of control.
FEEDBACK LOOP: The circular data route in a control loop that usually travels from the control medium’s sensor to the controller, then to the controlled process to the sensor again as a change in the control.
FEEDBACK SIGNAL: The signal responsible to the value of the control variable. This signal is returned to the input of the system and compared with the reference signal to obtain an actuated signal which returns the controlled variable to the desired value.
FEMALE THREAD: The internal thread in a fitting.
FILL OR WETTED-SURFACE METHOD: Water in a cooling tower is spread out over a wetted surface while air is passed over it to enhance evaporation.
FILM FACTOR: The relationship between the medium giving up heat and the heat exchange surface (evaporator). This relates to the velocity of the medium passing over the evaporator. When the velocity is too slow, the film between the air and the evaporator becomes greater and becomes an insulator which slows the heat exchange.
FILTER: A fine mesh or porous material that removes particles from passing fluids.
FILTER DRIER: A type of refrigerant filter that includes a desiccant material that has an attraction for moisture. The filter drier will remove particles and moisture from refrigerant and oil.
FIN COMB: A hand tool used to straighten the fins on an air-cooled condenser.
FINNED-TUBE EVAPORATOR: A copper or aluminum tube that has fins, usually made of aluminum, pressed onto the copper lines to extend the surface area of the tubes.
FIRE-SAFE: A valve design that is capable of passing a fire test with specified limits on leakage to the atmosphere after being closed or opened subsequent to fire exposure.
FITTING: Any component, other than valves, used with pipe as part of the pressure system and normally referring to items covered by a national standard.
FITTING, COMPRESSION: A fitting which seals and grips by manual adjustable deformation.
FIXED BORE DEVICE: An expansion device with a fixed diameter that does not adjust to varying load conditions.
FIXED RESISTOR: A non-adjusted resistor. The resistance cannot be changed. 
FLAME IMPINGEMENT: When flame touches components where it is not supposed to. For example, the flame should not touch the heat exchanger in an oil or gas furnace. The flame should stay in middle of the heat exchanger.
FLAME-PROVING DEVICE: A device in a gas or oil furnace to ensure there is a flame so that excess fuel will not be released, which may cause overheating in an oil furnace and explosion in a gas appliance.
FLAME RECTIFICATION: In a gas furnace, the flame changes the normal current to direct current. The electronic components in the system will only energize and open the gas valve with a direct current.
FLANGE: A rim on the end of a pipe, valve or fitting for bolting to another pipe element.
FLANGE ENDS: Refers to a valve or fitting having flanges for joining to other piping elements. Flanged ends can be plain faced, raised face, large male and female, small male and female, large tongue and groove, small tongue and groove and ring joint.
FLAPPER VALVE: See Reed Valve.
FLARE: The angle that may be fashioned at the end of a piece of tubing to match a fitting and create a leak-connection.
FLARE NUT: A threaded connector used in a flare assembly for tubing.
FLASH GAS: A term used to describe the pressure drop in an expansion device when some of the liquid passing through the valve is changed quickly to a gas and cools the remaining liquid to the corresponding temperature.
FLOAT VALVE OR SWITCH: An assembly used to maintain or monitor a liquid level.
FLOATING BALL: A ball valve design in which the ball is not rigidly held on its rotational axis and so is free to float between the seat rings.
FLOATING HEAD PRESSURE: Letting the head pressure (condensing pressure) fluctuate with the ambient temperature from season to season for lower compression ratios and better efficiencies.
FLOODED EVAPORATOR: A refrigeration system operated with the liquid refrigerant level very close to the outlet of the evaporator coil for improved heat exchange.
FLOODING: The term applied to a refrigeration system when the liquid refrigerant reaches the compressor.
FLOW COEFFICIENT (CV): expressed as the number of gallons of 60° water that would flow through a valve port, in 1 minute under a differential pressure drop of 1 under stated conditions.
FLOW CHARACTERISTIC: The relationship between valve capacity and valve travel. It is usually expressed graphically in the form of a curve. Control valves have two types of characteristics: inherent and installed.
FLOW METER: A device which indicates either flow rate, total flow or a combination of both for a given liquid.
FLOW RATE: The volume, mass, or weight of a fluid passing through any conductor per unit of time.
FLOW-THROUGH RECEIVER: Flow through receiver can store the cool subcooled liquid refrigerant for some time before releasing it to the liquid line and liquid header. This can cause a gain in temperature with the liquid and sub-cooling can be lost.
FLUE: The duct that carries the product of combustion out of a structure for a fossil or a solid fuel system.
FLUE DRAFT: Another name for chimney draft and stack draft.
FLUE GAS ANALYSIS INSTRUMENTS: Tools used to analyze the operation of fossil fuel-burning equipment such as oil and gas furnaces by analyzing the flue gases.
FLUID: The state of matter of liquids and gases.
FLUID EXPANSION DEVICE: Using a bulb or sensor, tube and diaphragm filled with fluid the device will produce movement at the diaphragm when the fluid is heated or cooled. A bellows may be added to produce more movement. These devices may contain vapor and liquid.
FLUSH: The process of using a fluid to push contaminants from a system.
FLUX: A substance applied to soldered and brazed connections to prevent oxidation during the heating process.
FOAMING: A term used to describe oil when it has liquid refrigerant boiling out of it.
FOOT-POUND: The amount of work accomplished by lifting 1 lb. of weight 1 ft.; a unit of energy.
FORCE: Energy Exerted.
FORCED CONVECTION: The movement of fluid by mechanical means.
FORCED DRAFT: Forced draft systems push or blow combustion gases through the heat exchanger and cause a positive pressure in the heat exchanger. Forced draft systems have the combustion blower motor on the inlet of the heat exchanger.
FORCED DRAFT COOLING TOWER: A water cooling tower that has a fan on the side of the tower that pushes air through the tower, as opposed to an induced draft tower, which has the fan on the side and draws air through the tower.
FORCED DRAFT EVAPORATOR: An evaporator over which air is forced to spread the cooling more efficiently. This tem usually refers to domestic refrigerator or freezer.
FOSSIL FUELS: Natural gas, oil, and coal formed millions of years ago from dead plants and animals.
FOUR-WAY VALVE: The valve in a heat pump system that changes the direction of the refrigerant flow between the heating and cooling cycles.
FRACTIONATION: When a zeotropic refrigerant blend phase changes, the different vapor pressures. This causes different vaporization and condensation rates and temperatures as they phase change.
FREE-COOLING: The process of cooling a structure using only outside air without operating the mechanical air conditioning system.
FREEZER BURN: The term applied to frozen food when it becomes dry and hard from dehydrations due to poor packaging.
FREEZESTAT: A temperature-sensing device typically intended to protect water coils from freezing.
FREEZE UP: Excess ice or frost accumulation on an evaporation to the point that airflow may be affected.
FREEZING: The change of state of water from a liquid to a solid.
FREON: The previous trade name for refrigerants manufactured by E.I. duPont de Nemours & Co. Inc.
FREQUENCY: The cycles per second (cps) of the electrical current supplied by the power company. This is normally 60cps in the United States.
FRICTION LOSS: The loss of pressure in a fluid flow system (air or water) due to the friction of the fluid rubbing on the sides. It is typically measured in feet of equivalent loss.
FRONT SEATED: A position on a service valve that will not allow refrigerant flow in one direction.
FROST BACK: A condition of frost on the suction line and even the compressor body.
FROSTBITE: When skin freezes.
FROZEN: The term used to describe water in the solid state also used to describe rotating shaft that will not turn.
FUEL OIL: The fossil fuel used for heating a petroleum distillate.
FULL BORE (FULL OPENING): Describes a valve in which the bore (port) is nominally equal to the bore of the connecting pipe.
FULL-LOAD AMPERAGE (FLA): Mathematical calculation used to get Underwriters Laboratories (UL) approval for a motor that was used by compressor manufacturer before 1972. FLA value was replaced with a rated-load amperage (RLA) value after 1972. It should not be used to determine the proper refrigerant charge or to determine if a compressor motor is running properly.
FURNACE: Equipment used to convert heating energy, such as fuel oil, gas or electricity to usable heat. It usually contains a heat exchanger a blower and the controls to operate the system.
FUSE: A safety device used in electrical circuits for the protection of the circuit conductor and components.
FUSIBLE LINK: An electrical safety device normally located in a furnace that burns and opens the circuit during an overheat situation.
FUSIBLE PLUG: A device (made of low-melting temperature metal) used in pressure vessels that is sensitive to high temperatures and relieves the vessel contents in an overwhelming situation.
FUSION IRON: A device used to heat plastic pipe to 500°F for a permanent, water-tight connection to another plastic pipe.
**G**
GAUGE: An instrument used to indicate pressure.
GAUGE MANIFOLD: A tool that may have more than one gauge with a valve arrangement to control fluid flow.
GAUGE PORT: The service port used to attach a gauge for service procedures.
GAIN: The term used to describe the sensitivity of a control for example how much change in output signal per degree of temperature change.
GAS: The vapor state of matter.
GAS-PRESSURE SWITCH: Used to detect gas pressure before gas burners are allowed to ignite.
GAS VALVE: A valve used to stop, start, or modulate the flow of natural gas.
GASKET: A thin piece of flexible material used between two metal plates to prevent leakage. A component whose purpose is to seal a joint between two larger components, softer than the surfaces of the joint being sealed and usually squeezed by means of bolting to affect the seal.
GATE: A terminal or semiconductor. The closure element of a gate valve (sometimes called wedge or disc).
GATE VALVE: Valve that has a sliding wedge shaped piece (gate) inside it that is concerned to handle. The handle can be turned to force the sliding gate into a fitted seat to stop or control water flow. Gate valves have very little pressure drop or resistance to water or fluid flow when wide open because the gate moves all of the way out of the water system flow. A straight through pattern valve in which closure element is a wedge situated between two fixed seating surfaces, with means to move it in or out of the flow stream in a direction perpendicular to the pipeline axis. Used as a block valve, or on-off valve.
GEOTHERMAL HEAT PUMP: A heat pump that uses the earth or water in the earth for its heat sources and sinks.
GERMANIUM: A substance from which many semiconductors are made.
GLAND: The part of the valve which retains or compresses the stem packing in a stuffing box. Also called gland bushing.
GLAND PACKING: A soft conformable material fitted to a valve stuffing box to create a seal between the process fluid and the atmosphere.
GLOBAL WARMING: An earth-warming process caused by the atmosphere’s absorption of the heat energy radiated from the earth’s surface.
GLOBAL WARMING POTENTIAL (GWP): An index that measures the direct effect of chemicals emitted into the atmosphere.
GLOBE VALVE: Valve that uses a disk and a seat to stop or vary fluid flow. The handle raises or lowers the disk over the valve seat, which is built into the valve body. When the valve is fully open, the water must make two 90° turns to pass through the valve creating pressure drop. A valve whose closure element is a flat disc or conical plug sealing on a seat which is usually parallel to the flow axis. Can be used for throttling services.
GLOW COIL: A device that automatically reignites a plot light if it goes out.
GLYCOL: Antifreeze solution used in the water loop of geothermal heat pumps.
GRADUATED CYLINDER: A cylinder with a visible column of liquid refrigerant used to measure the refrigerant charged into a system. Refrigerant temperatures can be dialed on graduated cylinder.
GRAIN: Unit of weight. One pound = 7000 grains
GRAM: Metric measurement term used to express weight.
GREASE FITTING: A device which permits injection of grease into a bearing surface.
GREEN: Companies looking to go “Green” incorporate sustainable design into their building and facilities.
GREENHOUSE EFFECT: An earth-warming process caused by the atmosphere’s absorption of the heat energy radiated from the earth’s surface. Often referred to as global warming.
GRILLE: A louvered, often decorate, component in the air system at the inlet or the outlet of the airflow.
GROMMET: A rubber, plastic or metal protector usually used where wire or pipe goes through a metal panel.
GROUND, ELECTRICAL: A circuit or path for electron flow to earth ground.
GROUND FAULT CIRCUIT INTERRUPTER (GFCI): A circuit breaker that can detect very small leaks to ground, which, under certain circumstances, could cause an electrical shock. This small leak, which may not be detected by a conventional circuit breaker, will cause the GFCI circuit breaker to open the circuit.
GROUND LOOP: These loops of plastic pipe are buried in the ground in a closed-loop-geo-thermal heat pump system and contain a heat transfer fluid.
GROUND WIRE: A wire from the frame of an electrical device to be wired to the earth ground.
GROUT: A cement like material injected between the well casing and the drilled hole for a well. Grout adds rigidity and prevents contamination from entering the well.
GUIDE VANES: Vanes used to produce capacity control in a centrifugal compressor. Also called prerotation guide vanes.
**H**
HALIDE REFRIGERANTS: Refrigerant chemicals that contain halogen: R-12, R-22, R-500 and R-502 are among them.
HALIDE TORCH: A torch type leak detector used to detect the halogen refrigerants.
HALOGENS: Chemicals substances found in many refrigerants containing chlorine, bromine, iodine and fluorine.
HANDWHEEL: A manual override device used to stroke a valve or limit its travel. The handwheel is sometimes referred to as a hand jack. It may be top-mounted, side-mounted, in-yoke mounted or shaft-mounted and declutchable.
HARD TRUCK: A two-wheeled piece of equipment that can be used for moving heavy objects.
HANGER: A device used to support tubing, pipe, duct or other components of a system.
HEAD: Another term for pressure, usually referring to gas or liquid.
HEAD PRESSURE CONTROL: A control that regulates the head pressure in a refrigeration or air-conditioning system.
HEADER: A pipe or containment to which other pipe lines are connected.
HEAT: Energy that causes molecules to be in motion and raise the temperature of a substance.
HEAT ANTICIPATOR: A device that anticipates the need for cutting off the heating system prematurely so the system does not overshoot the set point temperature.
HEAT COIL: A device made of tubing or pipe designed to transfer heat to a cooler substance by using fluids.
HEAT EXCHANGER: A device that transfers heat from one substance to another.
HEAT FUSION: A process that will permanently join sections of plastic pipe together.
HEAT GAIN: Rate at which heat enters a structure in the cooling season.
HEATING SEASONAL PERFORMANCE FACTOR (HSPF): The ratio of the seasonal heat output in the BTUs to the seasonal watt-hours used. The higher the HSPF, the more efficient the unit is in moving BTUs of heat energy per BTU of energy consumed. This efficiency factor is applied to heat pumps in the heating mode and takes into consideration the energy used during the entire heating season.
HEAT ISLAND: Urban air or surface temperatures that are higher than nearby rural areas.
HEAT LOSS: Rate at which heat leaks from a structure in the heating season.
HEAT OF COMPRESSION: That part of the energy from the pressurization of gas or a liquid converted to heat.
HEAT OF FUSION: The heat released when a substance is changing from a liquid to a solid.
HEAT OF RESPIRATION: When oxygen and carbon hydrates are taken in by a substance or when carbon dioxide and water are given off. Associated with fresh fruits and vegetables during their aging process while stored.
HEAT PUMP: A refrigeration system used to supply heat or cooling using valves to reverse the refrigerant gas flow.
HEAT RECLAIM: Using heat from a condenser for purposes such as space and domestic water heating.
HEAT RECOVERY: The process of transferring heat from one area to another as needed to help reduce energy consumption by the HVAC equipment.
HEAT SINK: a low-temperature surface to which heat can transfer.
HEAT TAPE: electric resistance wires embedded into a flexible housing usually wrapped around a pipe to keep it from freezing.
HEAT TRANSFER: the transfer of heat from a warmer to a colder substance.
HELIX COIL: a bimetal formed into a helix-shaped coil that provides longer travel when heated.
HEPA FILTER: an abbreviation for high-efficiency particulate arrestor.
HERMETIC COMPRESSOR: A motor and compressor that are totally sealed by being welded in a container.
HERMETIC SYSTEM: An enclosed refrigeration system where the motor and compressor are sealed within the same system  with the refrigerant.
HERS INDEX: HERS is an acronym for Home Energy Rating Systems. It is a standardized rating procedure administered by the Residential Energy Services Network (RESNET). 
HERTZ: Cycles per second.
Hg: Abbreviation for the element mercury.
HIDDEN HEAT: See Latent heat.
HIGH-PRESSURE CONTROL: A control that stops a boiler heating device or a compressor when the pressure becomes too high.
HIGH SIDE: A term used to indicate the high pressure or condensing side of the refrigeration system.
HIGH-TEMPERATURE REFRIGERATION: A refrigeration temperature range starting with evaporator temperatures no lower than 35°F, a range usually used in air conditioning (cooling). 
HIGH-VACUUM PUMP: A pump that can produce a vacuum in the low micron range.
HOAR FROST: The outer layers of frost that form on the hard surface of ice covering the evaporator coil often resemble snow or flaked ice and will hold a lot of entrained air.
HOLLOW WALL ANCHOR: Can be used in plaster, wallboard, gypsum board, and similar materials. Once the anchor has been set, the screw may be removed as often as necessary.
HORSEPOWER (hp): A unit equal to 33000 ft-lb of work per minute.
HOT GAS: The refrigerant vapor as it leaves the compressor. This is often used to defrost evaporators.
HOT GAS BYPASS: Piping that allows hot refrigerant gas into the cooler low-pressure side of a refrigeration system usually for system  capacity control.
HOT GAS DEFROST: A system where the hot refrigerant gases are passed through the evaporator to defrost it.
HOT GAS LINE: The tubing between the compressor and condenser.
HOT JUNCTION: That part of a thermocouple or thermopile where heat is applied.
HOT PULLDOWN: The process of lowering the refrigerated space to the design temperature after it has been allowed to warm up considerably over this temperature.
HOT SURFACE IGNITION: A silicon carbide or similar substance is placed in the gas stream of a gas furnace and allowed to get very hot. When the gas impinges on this surface, immediate ignition should occur.
HOT WATER HEAT: A heating system using hot water to distribute the heat. 
HOT WIRE: The wire in an electrical circuit that has a voltage potential between it and another electrical source or between it and ground.
HRV: See Heat Recovery Ventilator.
HUMIDIFIER: A device used to add moisture to the air.
HUMIDISTAT: A control operated by a change in humidity.
HUMIDITY: Moisture in the air.
HUNTING: The open and close throttling of a valve that is searching for its set point.
HYDRAULICS: Producing mechanical motion by using liquids under pressure.
HYDROCARBONS(HCS): Organic compounds containing hydrogen and carbon found in many heating fuels.
HYDROCHLOROFLUOROCARBONS(HCFCS): Refrigerants containing hydrogen, chlorine, fluorine, and carbon, thought to contribute to the depletion of the ozone layer, although not to the extent of chlorofluorocarbons.
HYDROFLUOROCARBON(HFC): A chlorine-free refrigerant containing  hydrogen, fluorine, and carbon with zero ozone depletion potential.
HYDROFLUOROETHER(HFE): A nontoxic, secondary fluid used in refrigeration systems. HFEs have a zero ozone depletion potential.
HYDROMETER: An instrument used to measure the specific gravity of a liquid.
HYDRONIC: Usually refers to a hot water heating system.
HYDROSTATIC PRESSURE: Extreme pressure buildup when a cylinder is 100% full of liquid and increases in temperature. 
HYGROMETER: An instrument used to measure the amount of moisture in the air.
**I**
IAQ: See Indoor Air Quality.
IBBM: Iron body, bronze mounted – common term for valves with cast iron body and bonnet and bronze trim (seating surfaces, stem, bushings).
IC: Acronym for direct insulation contact. Applies to recessed light (can light) fixtures ratings.
ICAT: An acronym for (insulation contact and air tightness). Applies to recessed light (can light) fixtures ratings.
ID: The measurement of the inside diameter of a pipe.
IDLER: A pulley on which a belt rides. It does not transfer power but is used to provide tension or reduce vibration.
IGNITION OR FLAME VELOCITY: The rate at which the flame travels through the air and fuel mixture. Different gases have a different rate. The faster the rate of travel, the more explosive the gas.
IGNITION TRANSFORMER: Provides a high-voltage current, usually to produce a spark to ignite a furnace fuel, either gas or oil.
IMPEDANCE: A form of resistance in an alternating current circuit.
IMPELLER: The rotating part of a pump that causes the centrifugal force to develop fluid flow and pressure difference.
IMPINGEMENT: The condition in a gas or oil furnace when the flame strikes the sides of the combustion chamber, resulting in poor combustion efficiency.
INCLINED WATER MANOMETER: Indicates air pressures in very low-pressure systems.
INDOOR AIR QUALITY (IAQ): This term generally refers to the study or research of air quality within buildings and the procedures used to improve air quality. 
INDUCED DRAFT: Induced draft systems have a combustion blower motor located on the outlet of the heat exchanger. They pull or suck combustion gases through the heat exchanger usually causing a slight negative pressure in the heat exchanger itself.
INDUCED MAGNETISM: Magnetism produced, usually in a metal, from another magnetic field. 
INDUCTANCE: An induced voltage producing a resistance in an alternating current circuit. 
INDUCTION MOTOR: An alternating current motor where the rotor turns from induced magnetism from the field windings.
INDUCTIVE CIRCUIT: When the current in a circuit lags the voltage by 90°.
INDUCTIVE REACTANCE: A resistance to the flow of an alternating current produced by an electromagnetic induction.
INEFFICIENT EQUIPMENT: Equipment that is not operating at its design level of capacity because of some fault in the equipment, such as a cylinder not pumping in a multicylinder compressor.
INERT GAS: A gas that will not support most chemical reactions, particularly oxidation. 
INFILTRATION: Air that leaks into a structure through cracks, windows, doors, or other openings due to less pressure inside the structure than outside the structure. Usually accompanied by an equal outflow of air.
INFRARED HUMIDIFIER: A humidifier that has infrared lamps with reflectors to reflect the infrared energy onto the water. The water evaporates rapidly into the duct airstream and is carried throughout the conditioned space.
INFRARED RAYS: The rays that transfer heat by radiation.
IN. Hg VACUUM: The atmosphere will support a column of mercury 29.92 in. high. To pull a complete vacuum in a refrigeration system, the pressure inside the system must be reduced to 29.92 in. Hg vacuum.
IN. WC: Acronym for inches of water column. 
INHERENT MOTOR PROTECTION: This is provided by internal protection such as a snap-disc or a thermistor.
INJECTION: Term used to describe any hydronic system that uses a circulator pump to push water into a secondary heating loop or circuit.
IN-PHASE: When two or more alternating current circuits have the same polarity at all times. 
INPUT/OUTPUT BOARD: A solid-state electronic board that receives a signal from a microprocessor and initiates a response to cycle compressors or initiate defrost.
INSULATION, ELECTRIC: A substance that is a poor conductor of electricity.
INSULATION, THERMAL: A substance that is a poor conductor of the flow of heat.
INSULATOR: A material with several electrons in the outer orbit of the atom making them poor conductors of electricity or good insulators. Examples are glass, rubber, and plastic. 
INTEGRAL CONTROLLER: A control mode where the controller will change the output signal according to the length of time the error or offset exists. It will calculate the amount of error that exists over a specific time interval.
INTEGRAL FLANGE: A valve body whose flange connection is an integral or cast part of the body.
INTEGRAL SEAT: The flow control orifice and seat that is an integral part of the valve body or cage. The seat is machined directly out of the valve body and is normally not replaceable without replacing the body itself.
INTERLOCKING COMPONENTS: Mechanical and electrical interlocks that are used to prevent a piece of equipment from starting before it is safe to start. For example, the chilled-water pump and the condenser water pump must both be started before the compressor in a water-cooled chilled-water system. 
INTERMEDIATE ZONES: Another name for unintentionally conditioned spaces.
INTERMITTENT IGNITION: Ignition system for a gas furnace that operates only when needed or when the furnace is operating.
INTERNAL HEAT DEFROST: Heat provided for defrost from inside the system, for example, hot gas defrost.
INTERNAL MOTOR OVERLOAD: An overload that is mounted inside the motor housing, such as a snap-disc or thermistor.
INTERRUPTED IGNITION: Ignition method that uses an electric spark only at the beginning of the burner’s run cycle to ignite the atomized fuel.
INVERTER: The switching or transistor section of the variable frequency drive (VFD) that produces an AC voltage at just the right frequency for motor speed control. This section converts the DC voltage back to AC voltage.
ION GENERATOR: A device that charges particles. These particles are then passed through positive- and negative-charged plates where the particles are repelled by the positive-charged plates and attracted to the negative-charged plates.
ISO: International Organization for Standardization (ISO) is an international standard setting body composed of representatives from various national standards organizations.
ISO 5211: The standard that specifies the requirements for the attachment of part-turn actuators, with or without gearboxes, to industrial valves.
ISOBARS: Lines on a chart or graph that represent a constant pressure.
ISOLATION RELAYS: Components used to prevent stray unwanted electrical feedback that can cause erratic operation.
ISOTHERMS: Lines on a chart or graph that represent a constant temperature.
**J**
JOULE (J): Metric measurement term used to express the quantity of heat.
JUNCTION BOX: A metal or plastic box within which electrical connections are made.
**K**
KELVIN: A temperature scale where absolute 0 equals 0 or where molecular motion stops at 0. It has the same graduations per degree of change as the Celsius scale.
KILOPASCAL: A metric unit of measurement for pressure used in the air-conditioning, heating, and refrigeration field. There are 6.89 kilopascals in 1 psi.
KILOWATT: A unit of electrical power equal to 1000 watts.
KILOWATT-HOUR: 1 kilowatt (1000 watts) of energy used for 1 hour.
KING VALVE: A service valve at the liquid receiver’s outlet in a refrigeration system.
**L**
LAG SHIELD ANCHORS: Used with lag screws to secure screws in masonry materials.
LAMEL: An acronym for (lighting, appliance, and miscellaneous, electrical loads). Another way to express a base load of a residence.
LAMINAR FLOW: The flow of a fluid where each particle of the fluid follows a smooth path with all particles moving in straight lines parallel to the pipe walls.
LATENT HEAT (HIDDEN HEAT: Heat energy absorbed or rejected when a substance is changing state and there is no change in temperature.
LATENT HEAT OF CONDENSATION: The latent heat given off when refrigerant condenses.
LATENT HEAT OF VAPORIZATION: The latent heat absorbed when refrigerant evaporates.
LEADERSHIP IN ENERGY AND ENVIRONMENTAL DESIGN (LEED): A voluntary national rating system for developing high-performance, sustainable buildings. It is referred to as the LEED Green Building Rating System.
LEADS: Extended surfaces inside a heat exchanger used to enhance the heat transfer qualities of the heat exchanger.
LEAK: Fluid (gas and/or liquid) escaping at different times and at different rates from a system.
LEAK DETECTOR: Any device used to detect leaks in a pressurized system. 
LEED: An acronym for Leadership in Energy and Environmental Design.
LEED (NC): An acronym for Leadership in Energy and Environmental Design (New Construction).
LEVER: A hand operating device for quarter-turn valves.
LEVER TRUCK: A long-handled, two-wheeled device that can be used to lift and assist in moving heavy objects.
LIGHT-EMITTING DIODE (LED): A diode that emits light and often is used as a self-diagnostic tool within a microprocessor. The lights can spell out error codes or flash numbers.
LIGNIN: A fibrous material found in trees and that strengthens the cell wall.
LIMIT CONTROL: A control used to make a change in a system, usually to stop it when predetermined limits of pressure or temperature are reached.
LIMIT SWITCH: A switch that is designed to stop a piece of equipment before it does damage to itself or the surroundings, for example, a high limit on a furnace or an amperage limit on a motor.Set of electrical contacts that are activated mechanically at a preselected position.
LINE SET: A term used for tubing sets furnished by the manufacturer.
LINE TAP VALVE: A device that may be used for access to a refrigerant line.
LINE-VOLTAGE THERMOSTAT: A thermostat that switches line voltage. For example, it is used for electric baseboard heat.
LINE WIRING DIAGRAM: Sometimes called a ladder diagram, this type of diagram shows the power-consuming devices between the lines. Usually, the right side of the diagram consists of a common line.
LINEAR FLOW CHARACTERISTIC:  A characteristic where flow capacity or (Cv) increases linearly with valve travel. Flow is directly proportional to valve travel. This is the preferred valve characteristic for a control valve that is being used with a distributive control system (DCS) or programmable logic controller (PLC).
LINEAR VALVE: Another name for a globe valve. It refers to the linear or straight-line movement of the plug and stem.
LIQUID: A substance where molecules push outward and downward and seek a uniform level.
LIQUID CHARGE BULB: A type of charge in the sensing bulb of the thermostatic expansion valve. This charge is characteristic of the refrigerant in the system and contains enough liquid so that it will not totally boil away.
LIQUID COLLECTOR DESIGN: A solar collector design in which either water or an antifreeze solution is passed through the collector and heated by the sun.
LIQUID-FILLED REMOTE BULB: A remote-bulb thermostat that is completely liquid filled, such as the mercury bulb on some gas furnace pilot safety devices. 
LIQUID FLOODBACK: Liquid refrigerant returning to the compressor’s crankcase during the running cycle.
LIQUID HAMMER: The momentum force of liquid causing a noise or a disturbance when hitting against an object.
LIQUID LINE: A term applied in the industry to refer to the tubing or piping from the condenser to the expansion device.
LIQUID NITROGEN: Nitrogen in liquid form.
LIQUID RECEIVER: A container in the refrigeration system where liquid refrigerant is stored.
LIQUID REFRIGERANT CHARGING: The process of allowing liquid refrigerant to enter the refrigeration system through the liquid line to the condenser and evaporator.
LIQUID REFRIGERANT DISTRIBUTOR: This device is used between the expansion valve and the evaporator on multiple circuit evaporators to evenly distribute the refrigerant to all circuits.
LIQUIFIED PETROLEUM: Liquified propane, butane, or a combination of these gases. The gas is kept as a liquid under pressure until ready to use. 
LIQUID SLUGGING: A large amount of liquid refrigerant in the compressor cylinder, usually causing immediate damage.
LITHIUM-BROMIDE: A type of salt solution used in an absorption chiller. 
LOAD MATCHING: Trying to always match the capacity of the refrigeration or air-conditioning system with that of the heat load put on the evaporators.
LOAD SHED: Part of an energy management system where various systems in a structure may be cycled off to conserve energy.
LOCKED-ROTOR AMPERAGE (LRA): The current an electric motor draws when power is applied to the motor, but its rotor is not yet turning. A rule-of-thumb often used by service technicians is that the LRA is normally five times the rated-load amperage (RLA).
LOCKING DEVICE: Any valve attachment whose purpose is to prevent the operation of the valve by unauthorized persons.
LOW-AMBIENT CONTROL: Various types of controls that are used to control head pressure in air-cooled air-conditioning and refrigeration systems that must operate year-round or in cold weather.
LOW-BOY FURNACE: This furnace is approximately 4 ft high, and the air intake and discharge are both at the top.
LOW-LOSS FITTING: A fitting that is fastened to the end of a gauge manifold that allows the technician to connect and disconnect gauge lines with a minimum of refrigerant loss.
LOW-PRESSURE CONTROL: A pressure switch that can provide low charge protection by shutting down the system on low pressure. It can also be used to control space temperature.
LOW SIDE: A term used to refer to that part of the refrigeration system that operates at the lowest pressure, between the expansion device and the compressor.
LOW-TEMPERATURE REFRIGERATION: A refrigeration temperature range starting with evaporator temperatures no higher than 0°F for storing frozen food. 
LOW-VOLTAGE THERMOSTAT: The typical thermostat used for residential and commercial air-conditioning and heating equipment to control space temperature. The supplied voltage is 24 V.
LP FUEL: Liquefied petroleum, propane, or butane. A substance used as a gas for fuel. It is transported and stored in the liquid state.
**M**
MAGNETIC FIELD: A field or space where magnetic lines of force exist.
MAGNETIC OVERLOAD PROTECTION: This protection reads the actual current draw of the motor and is able to shut it off based on actual current, versus the heat-operated thermal overloads, which are sensitive to the ambient heat of a hot cabinet.
MAGNETISM: A force causing a magnetic field to attract ferrous metals, or where like poles of a magnet repel and unlike poles attract each other. 
MAKEUP AIR: Air, usually from outdoors, provided to make up for the air used in combustion.
MAKEUP WATER: Water that is added back into any circulating water system due to loss of water. Makeup water in a cooling tower may be quite a large volume.
MALE THREAD: A thread on the outside of a pipe, fitting, or cylinder; an external thread.
MANDREL (CLINCHING): Part of the pin rivet assembly that is pulled to the breaking point to swell the rivet into the drilled hole.
MANIFOLD: A device where multiple outlets or inlets can be controlled with valves or other devices. Our industry typically uses a gas  manifold with orifices for gas-burning appliances and gauge manifolds used by technicians. A conductor that provides for multiple connection ports.
MANOMETER: An instrument used to check low vapor pressures. The pressures may be checked against a column of mercury or water.
MANUAL RESET: A safety control that must be reset by a person, as opposed to automatically reset, to call attention to the problem. An electrical breaker is a manual reset device.
MAPP GAS: A composite gas similar to propane that may be used with air.
MARINE WATER BOX: A water box on a chiller or condenser with a removable cover.
MASS: Matter held together to the extent that it is considered one body.
MASS SPECTRUM ANALYSIS: An absorption machine factory leak test performed using helium.
MATTER: A substance that takes up space and has weight.
MECHANICAL CONTROLS: A control that has no connection to power, such as a water-regulating valve or a pressure relief valve.
MEDIUM-TEMPERATURE REFRIGERATION: Refrigeration where evaporator temperatures are 32°F or below, normally used for preserving fresh food.
MEGOHMMETER: An instrument that can detect very high resistances, in millions of ohms. A megohm is equal to 1,000,000 ohms.
MELTING POINT: The temperature at which a substance will change from a solid to a liquid. 
MEMORY: Electronic storage space located internally in a controller.
MERCURY BULB: A glass bulb containing a small amount of mercury and electrical contacts used to make and break the electrical circuit in a low-voltage thermostat.
METERING DEVICE: A valve or small fixed-size tubing or orifice that meters liquid refrigerant into the evaporator.
METHANE: Natural gas composed of 90% to 95% methane, a combustible hydrocarbon.
METRIC SYSTEM: System International (SI); system of measurement used by most countries in the world.
MICRO: A prefix meaning 1/1000000.
MICRO-FOAM SOLUTION: A solution usually sprayed or brushed in the vicinity of a system leak which will form small cocoons of foam when in contact with a refrigerant or other type of gas leak.
MICROFARAD: Capacitor capacity equal to 1/1000000 of a farad.
MICROMETER: A precision measuring instrument.
MICRON: A unit of length equal to 1/1000 of a millimeter, 1/1,000,000 of a meter.
MICRON GAUGE: A gauge used when it is necessary to measure pressure close to a perfect vacuum.
MICROPROCESSOR: A small, preprogrammed, solid-state microcomputer that acts as a main controller.
MIDSEATED (CRACKED): A position on a service valve that allows refrigerant flow in all directions.
MIGRATION OF OIL OR REFRIGERANT: When the refrigerant moves to some place in the system where it is not supposed to be, such as when oil migrates to an evaporator or when refrigerant migrates to a compressor crankcase.
MILLI: A prefix meaning 1/1000.
MINERAL OIL: A traditional refrigeration lubricant used in CFC and HCFC systems.
MINIMUM EFFICIENCY REPORTING VALUE (MERV): Air filter rating system ranging from 1 to 20, with upper levels providing the most filtering.
MODULATING CONTROL VALVE: A valve which can be positioned anywhere between fully open and fully closed to proportion the rate of flow in response to a modulating controller.
MODULATING FLOW: Controlling the flow between maximum or no flow. For example, the accelerator on a car provides modulating flow.
MODULATOR: A device that adjusts by small increments or changes.
MOISTURE INDICATOR: A device for determining moisture.
MOLD: A fungus found where there is moisture that develops and releases spores. Can be harmful to humans.
MOLD (manufacturing): A hollow cavity, frequently in packed sand, for giving a desired shape to a material in a molten or plastic shape.
MOLECULAR MOTION: The movement of molecules within a substance.
MOLECULE: The smallest particle that a substance can be broken into and still retain its chemical identity.
MONOCHLORODIFLUOROMETHANE: The refrigerant R-22.
MONTREAL PROTOCOL: An agreement signed in 1987 by the United States and other countries to control the release of ozone-depleting substances.
MOTOR SERVICE FACTOR: A factor above an electric motor’s normal operating design parameters, indicated on the nameplate, under which it can operate. 
MOTOR STARTER: Electromagnetic contactors that contain motor protection and are used for switching electric motors on and off.
MOTOR TEMPERATURE-SENSING THERMOSTAT: A thermostat that monitors the motor temperature and shuts it off for the motor’s protection.
MUFFLER, COMPRESSOR: Sound absorber at the compressor.
MULLION: Stationary frame between two doors. 
MULLION HEATER: Heating element mounted in the mullion of a refrigerator to keep moisture from forming on it.
MULTIMETER: An instrument that will measure voltage, resistance, and milliamperes.
MULTIPLE CIRCUIT COIL: An evaporator or condenser coil that has more than one circuit because of the coil length. When the coil is too long, there will be an unacceptable pressure drop and loss of efficiency.
MULTIPLE EVACUATION: A procedure for evacuating a system. A vacuum is pulled, a small amount of refrigerant allowed into the system, and the procedure duplicated. This is often done three times.
**N**
NAMUR: Standardization of drilling patterns on pneumatic actuators allowing interface with solenoid valves.
NAPHTHENIC OIL: A refrigeration mineral oil refined from California and Texas crude oil.
NATIONAL ELECTRICAL CODE® (NEC®): A publication that sets the standards for all electrical installations, including motor overload protection.
NATIONAL FIRE PROTECTION ASSOCIATION (NFPA): An association organized to prevent fires through establishing standards, providing research, and providing public education. 
NATIONAL PIPE TAPER (NPT): The standard designation for a standard tapered pipe thread.
NATURAL CONVECTION: The natural movement of a gas or fluid caused by differences in temperature.
NATURAL-DRAFT TOWER: A water cooling tower that does not have a fan to force air over the water. It relies on the natural breeze or airflow.
NATURAL GAS: A fossil fuel formed over millions of years from dead vegetation and animals that were deposited or washed deep into the earth.
NATURAL REFRIGERANTS: Natural refrigerants occur naturally in nature.
NEAR-AZEOTROPIC BLEND: Two or more refrigerants mixed together that will have a small range of boiling and/or condensing points for each system pressure. Small fractionation and temperature glides will occur but are often negligible.
NEEDLEPOINT VALVE: A device having a needle and a very small orifice for controlling the flow of a fluid. A multi-turn device with a needle-shaped closing element. Similar in design to globe valves, but often much smaller. Good in applications with low flow rates.
NEGATIVE ELECTRICAL CHARGE: An atom or component that has an excess of electrons.
NEOPRENE: Synthetic flexible material used for gaskets and seals.
NET OIL PRESSURE: Difference in the crankcase pressure and the compressor oil pump outlet pressure.
NET REFRIGERATION EFFECT (NRE): The quantity of heat in BTU/lb. that the refrigerant absorbs from the refrigerated space to produce useful cooling.
NET STACK TEMPERATURE: The temperature difference between the ambient temperature and the flue-gas temperature, typically for oil- and gas-burning equipment. 
NEUTRALIZER: A substance used to counteract acids.
NEUTRON: Neutrons and protons are located at the center of the nucleus of an atom. Neutrons have no charge.
NEWTON/METER2: Metric unit of measurement for pressure. Also called a pascal.
NICHROME: A metal made of nickel chromium that when formed into a wire is used as a resistance heating element in electric heaters and furnaces.
NITROGEN: An inert gas often used to “sweep” a refrigeration system to help ensure that all refrigerant and contaminants have been removed.
NITROGEN DIOXIDE: A combustion pollutant that causes irritation of the respiratory tract and can cause shortness of breath.
NOMINAL: A rounded-off stated size. The nominal size is the closest rounded-off size. 
NONCATALYTIC STOVE: Wood-burning stoves without catalytic combustors and that were manufactured after July 1, 1990, are allowed to emit no more than 7.5 g/hr. of particulates.
NONCONDENSABLE GAS: A gas that does not change into a liquid under normal operating conditions.
NONFERROUS. Metals containing no iron.
NONPOLYCYCLIC ORGANIC MATTER: Air pollutants from wood-burning stoves considered to be health hazards.
NON-POWER-CONSUMING DEVICES: Devices that do not consume power, such as switches, that pass power to power-consuming devices.
NORMALLY CLOSED: Applying to a normally closed control valve assembly: One which closes when the actuator supply air pressure is reduced to atmosphere. In multi-port valves the normal position must be specified.
NORMALLY OPEN: Applying to a normally open control valve assembly: One which opens when the actuator supply air pressure is reduced to atmosphere. In multi-port valves the normal position must be specified.
NORMALLY CLOSED SOLENOID VALVE: A valve in which the inlet orifice is closed when the solenoid coil is de-energized and opened only when the solenoid coil is energized.
NORMALLY OPEN SOLENOID VALVE: A valve in which the inlet orifice is open when the solenoid coil is de-energized and closed only when the solenoid coil is energized.
NORTH POLE, MAGNETIC: One end of a magnet or the magnetic north pole of the earth. 
NOZZLE: A drilled opening that measures liquid flow, such as an oil burner nozzle.
NPS NOMINAL PIPE SIZE: dimensionless number used to indicate sizes of pressure pipe and valves – used interchangeably with valve size in inches.
NPT NATIONAL PIPE THREAD: standard tapered thread for pressure pipe and components. Requirements defined in ASME B1.20.1.
N-TYPE MATERIAL: Semiconductor material with an excess of electrons that move from negative to positive when a voltage is applied.
NUT DRIVER: These tools have a socket head used primarily to turn hex head screws on air conditioning, heating, and refrigeration cabinets.
**O**
OD: The measurement of the outside diameter of a pipe.
OFF CYCLE: A period when a system is not operating.
OFF-CYCLE DEFROST: Used for medium-temperature refrigeration where the evaporator coil operates below freezing but the air in the cooler is above freezing. The coil is defrosted by the air inside the cooler while the compressor is off cycle.
OFFSET: The absolute (not signed + or –) difference between the set point and the control point of a control process.
OFFSET: The position of ductwork that must be rerouted around an obstacle.
OHM: A unit of measurement of electrical resistance to current flow. 
OHMMETER: A meter that measures electrical resistance.
OHM’S LAW: A law involving electrical relationships discovered by Georg Ohm: E = I x R.
OIL LEVEL REGULATOR: A needle valve and float system located on each compressor of a parallel compressor system. It senses the oil level in the compressor’s crankcase and adds oil if necessary. It receives its oil from the oil reservoir.
OIL-PRESSURE SAFETY CONTROL (SWITCH): A control used to ensure that a compressor has adequate oil lubricating pressure.
OIL, REFRIGERATION: Oil used in refrigeration systems.
OIL RESERVOIR: A storage cylinder for oil usually used on parallel compressor systems. It is located between the oil separator and the oil level regulators. It receives its oil from the oil separator.
OIL SEPARATOR: Apparatus that removes oil from a gaseous refrigerant.
ONETIME RELIEF VALVE: A pressure relief valve that has a diaphragm that blows out due to excess pressure. It is set at a higher pressure than the spring-loaded relief valve in case it fails.
ON-OFF CONTROL: A control system in which the final control element has only two positions from which to select. Also known as two-position control.
OPEN COMBUSTION DEVICE: An atmospheric draft heating appliance.
OPEN COMPRESSOR: A compressor with an external drive.
OPEN-LOOP CONTROL CONFIGURATION: In an open-loop control configuration, the sensor is located upstream of the location where the control agent is causing a change in the controlled medium.
OPEN-LOOP HEAT PUMP: Heat pump system that uses the water in the earth as the heat transfer medium and then expels the water back to the earth in some manner. 
OPEN WINDING: The condition that exists when there is a break and no continuity in an electric motor winding.
OPERATING PRESSURE: The actual pressure under operating conditions.
ORGANIC: Materials formed from living organisms.
O-RING: An elastomeric or synthetic seal ring of circular cross section.
ORIFICE: A small opening through which fluid flows.
OS&Y: Outside Screw & Yoke – A valve design in which the stem threads are above the packing gland or outside the valve body and there is a yoke to support the top or outer end of the stem.
OSHA: Abbreviation for the Occupational Safety and Health Administration.
OUTWARD CLINCH TACKER: A stapler or tacker that will anchor staples outward and can be used with soft materials.
OVERLOAD PROTECTION: A system or device that will shut down a system if an overcurrent condition exists.
OVER-THE-FIRE DRAFT: Draft taken over the fire and will be a lower reading lower than chimney draft.
OXIDATION: The combining of a material with oxygen to form a different substance. This results in the deterioration of the original substance. Rust is oxidation.
OZONE: A form of oxygen (O3). A layer of ozone is in the stratosphere that protects the earth from some of the sun’s ultraviolet radiation.
OZONE DEPLETION: The breaking up of the ozone molecule by the chlorine atom in the stratosphere. Stratosphere ozone protects us from ultraviolet radiation emitted by the sun. 
OZONE DEPLETION POTENTIAL (ODP): A scale used to measure how much a substance will deplete stratospheric ozone ODP ranges from 0 to 1.
**P**
PACKAGE EQUIPMENT (UNIT): A refrigerating system where all major components are located in one cabinet.
PACKING: A soft material that can be shaped and compressed to provide a seal. It is commonly applied around valve stems. A sealing system that normally consists of a deformable material such as PTFE, graphite, etc. It is usually in the form of solid or split rings contained in a packing box that are compressed so as to provide an effective pressure seal.
PACKING BOX: The chamber located in the bonnet that surrounds the stem and contains the packing and other stem-sealing components.
PARAFFINIC OIL: A refrigeration mineral oil containing some paraffin wax, which is refined from eastern U.S. crude oil.
PARALLEL CIRCUIT: An electrical or fluid circuit where the current or fluid takes more than one path at a junction.
PARALLEL COMPRESSOR: Many compressors piped in parallel and mounted on a steel rack. The compressors are usually cycled by a microprocessor.
PARALLEL FLOW: A flow path in which many paths exist for the fluid to flow.
PART-WINDING START: A large motor that is actually two motors in one housing. It starts on one and then the other is energized. This is to reduce inrush current at start-up. For example, a 100-hp motor may have two 50-hp motors built into the same winding. It will start using one motor followed by the start of the other one. They will both run under the load. 
PASCAL: A metric unit of measuring pressure. One Pascal is equivalent to 0.004 inches of water column.
PASSIVE RECOVERY: Recovering refrigerant with the use of the refrigeration system’s compressor or internal vapor pressure.
PASSIVE SENSOR: Sensors that send information back to the controller in terms of resistance (ohms).
PASSIVE SOLAR DESIGN: The use of non-moving parts of a building to provide heat or cooling, or to eliminate certain parts of a building that cause inefficient heating or cooling.
PE (POLYETHYLENE): Plastic pipe used for water, gas, and irrigation systems. 
PELLET STOVE: A stove that burns small compressed pellets made from sawdust, cardboard, or sunflower or cherry seeds.
PERCENT REFRIGERANT QUALITY: Percent vapor. 
PRECIPITATION: A process where dissolved minerals in a solution reach a certain concentration and become solid.
PERMANENT MAGNET: An object that has its own permanent magnetic field.
PERMANENT SPLIT-CAPACITOR MOTOR (PSC): A split-phase motor with a run capacitor only. It has a very low starting torque.
PEX TUBING: PEX stands for polyethylene cross (X)-linked tubing, a common name for polyethylene tubing.
PHASE: One distinct part of a cycle.
PHASE-CHANGE LOOP: The loop of piping, usually in a geothermal heat pump system, where there is a change of phase of the heat transfer fluid from liquid to vapor or vapor to liquid.
PHASE FAILURE PROTECTION: Used on three-phase equipment to interrupt the power source when one phase becomes de-energized. The motors cannot be allowed to run on the two remaining phases or damage will occur.
PHASE REVERSAL: Phase reversal can occur if someone switches any two wires on a three-phase system. Any system with a three-phase motor will reverse, and this cannot be allowed on some equipment.
PITCH POCKET: Flanged piece of roof flashing that is positioned around irregularly shaped roof penetrations.
PICK-UP VOLTAGE: The back electromotive force (BEMF) voltage that must be generated across the potential relay’s coil to cause the relay armature to move and open the relay contacts between terminals 1 and 2.
PICTORIAL WIRING DIAGRAM: This type of diagram shows the location of each component as it appears to the person installing or servicing the equipment.
PIERCING VALVE: A device that is used to pierce a pipe or tube to obtain a pressure reading without interrupting the flow of fluid. Also called a line tap valve.
PILOT DUTY RELAY: A small relay that is used in control circuits for switching purposes. It is small and cannot take a lot of current flow, such as to start a motor.
PILOT LIGHT: The flame that ignites the main burner on a gas furnace.
PILOT-OPERATED: A device in which energy transmitted through the primary element is either supplemented or amplified by energy from another source.
PILOT POSITIONER: Used in a pneumatic air system to control the large volume of air that must be used to operate large pneumatic devices. The pneumatic thermostat sends a very small air signal to the pilot positioner in a 1/4” line and the pilot positioner controls the airflow in a 3/8” line for the large volume of air to the controlled device.
PILOT VALVE: Device for controlling the flow of an auxiliary fluid used to amplify the power of a controller measuring system in effecting control, i.e. a small valve used to operate a large valve.
PINCH VALVE: A flexible hose, pinched between two or more moving elements to stop flow. It is commonly used in slurry applications since it does not easily clog.
PINION SHAFT: The external input shaft of certain gear operators which drive the internal reduction gearing.
PIN RIVET ASSEMBLY: Also known as a blind rivet, this is a fastener assembly of hollow rivets assembled on a pin, often called a mandrel. The rivets are used to join two pieces of sheet metal and are inserted and set from only one side of the metal.
PISTON: The part that moves up and down in a cylinder.
PISTON DISPLACEMENT: The volume within the cylinder that is displaced with the movement of the piston from top to bottom. 
PITOT TUBE: Part of an instrument for measuring air velocities.
PLANNED DEFROST: Shutting the compressor off with a timer so that the space temperature can provide the defrost.
PLENUM: A sealed chamber at the inlet or outlet of an air handler. The duct attaches to the plenum.
PLUG VALVE: A quarter turn valve which uses a rotating plug as the closing element. When the valve is open, the media flows through a hole in the plug, which can be cylindrical or truncated.
PNEUMATIC ACTUATOR:  A device which converts pneumatic energy into mechanical motion.
PNEUMATIC ATOMIZING HUMIDIFIER: Air pressure is used to break up the water into a mist of tiny droplets and to disperse them.
PNEUMATIC CONTROLS: Controls operated by low-pressure air, typically 20 psig.
PNEUMATIC TEST: A test in which a valve is tested with air – usually a seat closure test.
POLYALKYLENE GLYCOL (PAG): A popular synthetic glycol-based lubricant used with HFC refrigerants, mainly in automotive systems. This was the first generation of oil used with HFC refrigerants.
POLYBUTYLENE: A material used for the buried piping in geothermal heat pumps. 
POLYCYCLIC ORGANIC MATTER: By-products of wood combustion found in smoke and considered to be health hazards.
POLYETHYLENE: A material used for the buried piping in geothermal heat pumps.
POLYOL ESTER (POE): A very popular ester-based lubricant often used in HFC refrigerant systems.
POLYPHASE: Three or more phases.
POLYPHOSPHATE: A scale inhibitor with many phosphate molecules.
POPPET VALVE: A mushroom or tulip shaped valve made of head resisting steel, commonly used for inlet and exhaust valves.
PORCELAIN: A ceramic material.
PORT: An internal-external terminus of a passage in a component. The inlet or outlet of a valve. Sometimes used to refer to the valve seat opening.
PORTABLE DOLLY: A small platform with four wheels on which heavy objects can be placed and moved.
POSITIONER: A device used to position a valve with regard to a signal. The positioner compares the input signal with a mechanical feedback link from the actuator. It then produces the force necessary to move the actuator output until the mechanical output position feedback corresponds with the pneumatic signal value.
POSITION INDICATOR: Any external device which visually indicates the open and closed position of valve.
POSITION SWITCH: A switch that is normally fitted on the actuator to detect extremes of valve travel. The switch is normally electric.
POSITION TRANSMITTER: A device that is mechanically connected to the valve stem and will generate and transmit either a pneumatic or electric signal that represents the valve stem position.
POSITIVE DISPLACEMENT: A term used with a pumping device such as a compressor that is designed to move all matter from a volume such as a cylinder or it will stall, possibly causing failure of a part.
POSITIVE ELECTRICAL CHARGE: An atom or component that has a shortage of electrons. 
POSITIVE TEMPERATURE COEFFICIENT (PTC) START DEVICE: A thermistor used to provide start assistance to a permanent split-capacitor motor.
POTENTIAL RELAY: A switching device used with hermetic motors that breaks the circuit to the start capacitor and/or start windings after the motor has reached approximately 75% of its running speed.
POTENTIAL VOLTAGE: The voltage measured across the start winding in a single-phase motor while it is turning at full speed. This voltage is much greater than the applied voltage to the run winding. For example, the run winding may have 230 V applied to it, and a measured voltage across the start winding may be 300 V. This is created by the motor stator turning in the magnetic field of the run winding. Voltage potential is the difference in voltage between any two parts of a circuit. 
POTENTIOMETER: An instrument that controls electrical current.
POWDER ACTUATED TOOL (PAT): A tool with a powder load that forces a pin, threaded stud, or other fastener into masonry. 
POWER: The rate at which work is done.
POWER-CONSUMING DEVICES: A power-consuming device is considered the electrical load. For example, in a lightbulb circuit, the switch is a power-passing device that passes power to the lightbulb that consumes the power and produces light.
POWER DRAFTED: A fan assisted draft.
PRESSURE: Force per unit of area.
PRESSURE ACCESS PORTS: Places in a system where pressure can be taken or registered.
PRESSURE DIFFERENTIAL: The difference in pressure between any two points of a system or a component. Also known as pressure drop or ?P.
PRESSURE DIFFERENTIAL VALVE: A valve that senses a pressure differential and opens when a specific pressure differential is reached.
PRESSURE DROP: The difference in pressure between two points.
PRESSURE/ENTHALPY DIAGRAM: A chart indicating the pressure and heat content of a refrigerant and the extent to which the refrigerant is a liquid and vapor.
PRESSURE LIMITER: A device that opens when a certain pressure is reached.
PRESSURE-LIMITING TXV: A valve designed to allow the evaporator to build only to a predetermined pressure when the valve will shut off the flow of refrigerant.
PRESSURE REGULATOR: A valve capable of maintaining a constant outlet pressure when a variable inlet pressure occurs. Used for regulating fluid flow such as natural gas, refrigerant, and water.
PRESSURE SWITCH: A switch operated by a change in pressure.
PRESSURE TANK: A pressurized tank for water storage located in the water piping of an open-loop geothermal heat pump system. It prevents short cycling of the well pump. 
PRESSURE-TEMPERATURE RATINGS: The maximum allowable working pressures at specified temperatures. For steel valves, the ratings are defined by “classes” and found in ASME B16.34. For iron and bronze valves, the ratings are defined in the applicable MSS specifications.
PRESSURE/TEMPERATURE RELATIONSHIP: This refers to the pressure/ temperature relationship of a liquid and vapor in a closed container. If the temperature increases, the pressure will also increase. If the temperature is lowered, the pressure will decrease.
PRESSURE TRANSDUCER: A pressure-sensitive device located in the piping of a refrigeration system that will transform a pressure signal to an electronic signal. The electronic signal will then feed a microprocessor.
PRESSURE VESSELS AND PIPING: Piping, tubing, cylinders, drums, and other containers that have pressurized contents.
PREVENTIVE MAINTENANCE: The action of performing regularly scheduled maintenance on a unit, including inspection, cleaning, and servicing.
PRIMARY AIR: Air that is introduced to a furnace’s burner before the combustion process has taken place.
PRIMARY CONTROL: Controlling device for an oil burner to ensure ignition within a specific time span, usually 90 seconds.
PROGRAMMABLE THERMOSTAT: An electronic thermostat that can be set up to provide desired conditions at desired times. 
PROPANE: An LP (liquified petroleum) gas used for heat.
PROPELLER FAN: This fan is used in exhaust fan and condenser fan applications. It will handle large volumes of air at low-pressure differentials.
PROPORTIONAL INTEGRAL (PI) CONTROLS: Electronic controls that sense and react to the error between the actual and desired states of a system. 
PROPYLENE GLYCOL: An antifreeze fluid cooled by a primary (phase-change) refrigerant, which then is circulated by pumps throughout the refrigeration system to absorb heat.
PROPORTIONAL CONTROLLER: A modulating control mode where the controller changes or modifies its output signal in proportion to the size of the change in the error.
PROTON: That part of an atom having a positive charge.
PROTOZOA: A microscopic organism with a complex life cycle.
PSC MOTOR: See Permanent split-capacitor motor.
psi: Abbreviation for pounds per square inch. The force per unit area exerted against a resisting body.
psia: Abbreviation for pounds per square inch absolute.
psig: Abbreviation for pounds per square inch gauge.
PSYCHROMETER: An instrument for determining relative humidity.
PSYCHROMETRIC CHART: A chart that shows the relationship of temperature, pressure, and humidity in the air.
PSYCHROMETRICS: The study of air and its properties, particularly the moisture content. 
P-TYPE MATERIAL: Semiconductor material with a positive charge.
PULSE FURNACE: This furnace ignites minute quantities of gas 60 to 70 times per second. Small amounts of natural gas and air enter the combustion chamber and are ignited with a spark igniter that forces the combustion materials down a tailpipe to an exhaust decoupler. The pulse is reflected back to the combustion chamber, igniting another gas and air mixture. 
PULSE-WIDTH MODULATOR (PWM): An electronic device in a motor circuit that is used to control motor speed for variable-speed motors.
PUMP: A device that forces fluids through a system. Any of various machines which force a gas or liquid into, or draw it out of something, as by suction or pressure.
PUMP DOWN: To use a compressor to pump the refrigerant charge into the condenser and/or receiver.
PURE COMPOUND: A substance formed in definite proportions by weight with only one molecule present.
PURGE: To remove or release fluid from a system.
PVC (POLYVINYL CHLORIDE): Plastic pipe used in pressure applications for water and gas as well as for sewage and certain industrial applications.
**Q**
QUENCH: To submerge a hot object in a fluid for cooling.
QUICK-CONNECT COUPLING: A device designed for easy connecting or disconnecting of fluid lines.
**R**
R-12: Dichlorodifluoromethane, once a popular refrigerant for refrigeration systems. It can no longer be manufactured in the United States and many other countries.
R-22: Monochlorodifluoromethane, a popular HCFC refrigerant for air-conditioning systems. 
R-123: Dichlorotrifluoroethane, an HCFC refrigerant developed for low-pressure application. 
R-134a: Tetrafluoroethane, an HFC refrigerant developed for refrigeration systems and as a replacement for R-12.
R-410A: An HFC binary refrigerant blend that contains R-32 and R-125. This refrigerant has an ODP of 0.
R-502: An azeotropic mixture of R-22 and R-115, a once popular refrigerant for low-temperature refrigeration systems. It can no longer be manufactured in the United States and many other countries.
RA (SURFACE FINISH): Ra is calculated as the roughness average of a surfaces measured microscopic peaks and valleys.
RACK AND PINION: A type of actuator that comprises a pair of gears which convert rotational motion into linear motion.
RACK SYSTEM: Many compressors piped in parallel and mounted on a steel rack. The compressors are usually cycled by a microprocessor.
RADIANT HEAT: Heat that passes through air, heating solid objects that in turn heat the surrounding area.
RADIATION: Heat transfer. See Radiant heat.
RADON: A colorless, odorless, and radioactive gas. Radon can enter buildings through cracks in concrete floors and walls, floor drains, and sumps.
RAISED FACED (RF): The raised area of a flange face which is the gasket sealing surface between mating flanges. Defined in ASME B16.5. Class 150 and 300 valves have 0.06” RF and Class 600 and up have a 0.25” RF.
RANDOM OR OFF-CYCLE DEFROST: Defrost provided by the space temperature during the normal off cycle.
RANGE: The pressure or temperature settings of a control defining certain boundaries of temperature or pressure.
RANKINE: The absolute Fahrenheit scale with 0 at the point where all molecular motion stops.
RAPID OXIDATION: A reaction between the fuel, oxygen, and heat that is known as rapid oxidation or the process of burning.
RATED-LOAD AMPERAGE (RLA): A mathematical calculation used to get Underwriters Laboratory’s (UL) approval for a motor. The RLA value is used by a service technician to size wires, fuses, overloads, and contactors for the motor. It should not be used to determine the proper refrigerant charge, or to determine if a compressor motor is running properly. 
RATE OF FLOW: An expression of measurement of the medium flowing through a given space in a given time; expressed as GPM (Gallons per Minute), CFH (Cubic Feet per Hour), LBH (Pounds per Hour), etc.
REACTANCE: A type of resistance in an alternating current circuit.
REAMER: Tool to remove burrs from inside a pipe after it has been cut.
RECEIVER-DRIER: A component in a refrigeration system for storing and drying refrigerant. 
RECIPROCATING: Back-and-forth motion.
RECIPROCATING COMPRESSOR: A compressor that uses a piston in a cylinder and a back-and-forth motion to compress vapor.
RE-CIRCULATED WATER SYSTEM: A system where water is used over and over, such as a chilled water or cooling tower system.
RECOVERY CYLINDER: A cylinder into which refrigerant is transferred; should be approved by the Department of Transportation as a recovery cylinder. The color code for these cylinders is a yellow top with a gray body.
RECTIFIER: A device for changing alternating current to direct current.
REDUCED PORT: A valve port opening that is smaller than the line size or the valve end connection size.
REED VALVE: A thin steel plate used as a valve in a compressor.
REFLECTIVITY: The ability of a materials surface to reflect radiant energy. Also referred to as reflectance.
REFRIGERANT: The fluid in a refrigeration system that changes from a liquid to a vapor and back to a liquid at practical pressures.
REFRIGERANT BLEND: Two or more refrigerants blended or mixed together to make another refrigerant. Blends can combine as either azeotropic or zeotropic blends.
REFRIGERANT LOOP: The heat pump’s refrigeration system, which exchanges energy with the fluid in the ground loop and the air side of the system.
REFRIGERANT RECEIVER: A storage tank in a refrigeration system where the excess refrigerant is stored. Since many systems use different amounts of refrigerant during the season, the excess is stored in the receiver tank when not needed. The refrigerant can also be pumped to the receiver when repairs on the low-pressure side of the system are made.
REFRIGERANT RECLAIM: “To process refrigerant to new product specifications by means which may include distillation. It will require chemical analysis of the refrigerant to determine that appropriate product specifications are met. This term usually implies the use of processes or procedures available only at a reprocessing or manufacturing facility.”
REFRIGERANT RECOVERY: “To remove refrigerant in any condition from a system and store it in an external container without necessarily testing or processing it in any way.”
REFRIGERANT RECYCLING: “To clean the refrigerant by oil separation and single or multiple passes through devices, such as replaceable core filter driers, which reduce moisture, acidity, and particulate matter. This term usually applies to procedures implemented at the job site or at a local service shop.”
REFRIGERATED AIR DRIERS: A device that removes the excess moisture from compressed air.
REFRIGERATION: The process of removing heat from a place where it is not wanted and transferring that heat to a place where it makes little or no difference. 
REGISTER: A terminal device on an air-distribution system that directs air but also has a damper to adjust airflow.
REGULATOR: A valve used to control the pressure in liquid systems to some value. Many households have a water pressure regulator to reduce the pressure from the main to a more usable pressure in the house. Gas systems all have pressure regulators to stabilize the pressure to the burners.
RELATIVE HUMIDITY: The amount of moisture contained in the air as compared to the amount the air could hold at that temperature.
RELAY: A small electromagnetic device to control a switch, motor, or valve.
RELIEF VALVE: A valve designed to open and release vapors at a certain pressure.
REMOTE SYSTEM: Often called a split system where the condenser is located away from the evaporator and/or other parts of the system.
REM/RATE: The HERS index score is determined by computer modeling the home by Residential Energy Analysis and Rating Software named REM/Rate.
RESIDENTIAL ENERGY SERVICES NETWORK (RESNET): A nationally recognized energy certification for home energy auditors and contractors.
RESILIENT-MOUNT MOTOR: Electric motor that uses various materials to isolate the motor noise from metal framework. This type of motor requires a ground strap. 
RESILIENT SEAT: A valve seat containing a soft seal such as an o-ring or plastic to assure tight shut-off.
RESISTANCE: The opposition to the flow of an electrical current or a fluid.
RESISTOR: An electrical or electronic component with a specific opposition to electron flow. It is used to create voltage drop or heat.
RESTRICTOR: A device used to create a planned resistance to fluid flow. A device for producing a deliberate pressure drop or resistance in a line by reducing the cross-sectional flow area.
RETROFIT GUIDELINES: Guidelines intended to make the transition from a CFC/mineral oil system to a system containing an alternative refrigerant and its appropriate oil.
RETURN WELL: A well for return water after it has experienced the heat exchanger of the geothermal heat pump.
REVERSE CYCLE: The ability to direct the hot gas flow into the indoor or the outdoor coil in a heat pump to control the system for heating or cooling purposes.
RIGGING: The process of using a crane to set HVAC equipment to a roof.
RIGID-MOUNT MOTOR: Electric motor that is bolted metal-to-metal to a frame. This type of motor will transmit noise.
RIM PULL: The force required at the edge of the handwheel to generate the required torque at the center of the handwheel.
RING TYPE JOINT (RTJ): A flange connection using a specially shaped soft metal ring as a gasket. Generally used on high pressure valves. May be the body and bonnet connection and/or the end flange connection.
ROD AND TUBE: The rod and tube are each made of a different metal. The tube has a high expansion rate and the rod a low expansion rate.
ROOM HEATER: A gas stove or appliance considered by ANSI to be a heating appliance. This heater will have an efficiency rating.
ROOT MEAN SQUARE (RMS) VOLTAGE: The alternating current voltage effective value. This is the value measured by most voltmeters. The RMS voltage is 0.707 x the peak voltage.
ROTARY COMPRESSOR: A compressor that uses rotary motion to pump fluids. It is a positive displacement pump.
ROTARY VALVE: A valve style in which the flow closure member is rotated in the flow stream to modify the amount of fluid passing through the valve.
ROTOR: The rotating or moving component of a motor, including the shaft.
RS RISING STEM: A valve stem with threads arranged so that as the stem turns, the threads engage a stationary threaded area and lift the stem along with the closure element attached to it.
RUNNING TORQUE: The determination of how much turning force is required to keep the closure element of a rotary valve rotating at a constant angular velocity once it starts rotating.
RUNNING TIME: The time a unit operates. Also called the on time.
RUN WINDING: The electrical winding in a motor that draws current during the entire running cycle. 
RUPTURE DISK: Pressure safety device for a centrifugal low-pressure chiller.
R-VALUE: Indicates an insulation’s resistance to heat transfer, or thermal resistance. This is the inverse of a material’s U-value.
**S**
S FASTENER: A device used to connect square and rectangular sheet metal duct together, in conjunction with a drive cleat. Also referred to as a “slip”.
SADDLE VALVE: A valve that straddles a fluid line and is fastened by solder or screws. It normally contains a device to puncture the line for pressure readings.
SAFETY CONTROL: An electrical, electronic, mechanical, or electromechanical control to protect the equipment or public from harm.
SAFETY PLUG: A fusible plug that blows out when high temperature occurs.
SAFETY VALVE: A self-operated quick opening valve used for fast relief of excessive pressures.
SAIL SWITCH: A safety switch with a lightweight, sensitive sail that operates by sensing an airflow.
SALT SOLUTION: Antifreeze solution used in a closed water loop of geothermal heat pumps.
SANITARY VALVE: Valve designed for use in applications requiring clean or sterile processing. Contact surfaces are highly polished, crevice free and easy to clean. Commonly used in dairy, food, pharmaceutical, medical, and chemical applications. Valve types include globe, diaphragm, pinch, knife, needle, butterfly, ball, and plug.
SATELLITE COMPRESSOR: The compressor on a parallel compressor system that is dedicated to the coldest evaporators.
SATURATED VAPOR: The refrigerant when all of the liquid has just changed to a vapor.
SATURATION: A term used to describe a substance when it contains all of another substance it can hold.
SCALE: Dissolved minerals such as calcium carbonate (limestone) which reach a certain concentration and become solid, usually clinging to HVACR equipment and piping surfaces. 
SCAVENGER PUMP: A pump used to remove the fluid from a sump.
SCHEDULE: A system for indicating the wall thickness of pipe. The higher the schedule number, the thicker the wall for a certain pipe size.
SCHEMATIC WIRING DIAGRAM: Sometimes called a line or ladder diagram, this type of diagram shows the electrical current path to the various components.
SCHRADER VALVE: A valve similar to the valve on an auto tire that allows refrigerant to be charged or discharged from the system.
SCOTCH YOKE: A mechanism used to create reciprocating motion from the electric motor drive in very small compressors. A type of actuator that translates linear motion to rotary motion. The linear motion is defined by a pin or roller element trapped within a channel in the rotary element.
SCREW COMPRESSOR: A form of positive displacement compressor that squeezes fluid from a low-pressure area to a high-pressure area, using screw-type mechanisms. 
SCROLL COMPRESSOR: A compressor that uses two scroll-type components, one stationary and one orbiting, to compress vapor.
SEALED UNIT: The term used to describe a refrigeration system, including the compressor, that is completely welded closed. The pressures can be accessed by saddle valves.
SEASONAL ENERGY EFFICIENCY RATIO (SEER): An equipment efficiency rating that is calculated on the basis of the total amount of cooling (in BTU) the system will provide over the entire season, divided by the total number of watthours it will consume.
SEAT: The stationary part of a valve that the moving part of the valve presses against for shutoff.
SEAT LOAD: The contact force between the seat and the valve plug. When an actuator is selected for a given control valve, it must be able to generate enough force to overcome static, stem and dynamic unbalance with an allowance made for seat load.
SECONDARY AIR: Air that is introduced to a furnace after combustion takes place and that supports combustion.
SECONDARY FLUID: An antifreeze fluid cooled by a primary (phase-change) refrigerant, which is then circulated by pumps throughout the refrigeration system to absorb heat.
SELF-RELIEVING: The process by which excessive internal body cavity pressure is automatically relieved either into the upstream or downstream line – generally found in ball valves.
SEMICONDUCTOR: A component in an electronic system that is considered neither an insulator nor a conductor but a partial conductor. It conducts current in a controlled and predictable manner.
SEMI-HERMETIC COMPRESSOR: A motor compressor that can be opened or disassembled by removing bolts and flanges. Also known as a serviceable hermetic. 
SENSIBLE HEAT: Heat that causes a change in temperature.
SENSITIVITY: See Gain.
SENSORS: Devices that can measure some type of environmental parameter or controlled variable and convert this parameter to a value that the controller can understand.
SEQUENCER: A control that causes a staging of events, such as a sequencer between stages of electrical heat.
SERIES CIRCUIT: An electrical or piping circuit where all of the current or fluid flows through the entire circuit.
SERIES FLOW: A flow path in which only one path exists for fluid to flow.
SERVICE FACTOR AMPS (SFA): The current the motor is drawing at its service factor loading. 
SERVICE VALVE: A manually operated valve in a refrigeration system used for various service procedures.
SERVICEABLE HERMETIC: See Semi-hermetic compressor.
SERVO PRESSURE REGULATOR: A sensitive pressure regulator located inside a combination gas valve that senses the outlet or working pressure of the gas valve.
SESSILE BACTERIA: The bacteria that live in a biofilm and adhere to the equipment surfaces. They represent 99% of the total bacteria in a cooling tower or boiler water system.
SET POINT: The desired control point’s magnitude in a control process.
SHADED-POLE MOTOR: An alternating current motor used for very light loads.
SHELL: The floors, roof, and walls of a building, which is often referred to as the exterior envelope of the building.
SHELL AND COIL: A vessel with a coil of tubing inside that is used as a heat exchanger.
SHELL AND TUBE: A heat exchanger with straight tubes in a shell that can normally be mechanically cleaned.
SHORT CIRCUIT: A circuit that does not have the correct measurable resistance: too much current flows and will overload the conductors.
SHORT CYCLE: The term used to describe the running time (on time) of a unit when it is not running long enough.
SHORTED MOTOR WINDING: Part of an electric motor winding is shorted out because one part of the winding touches another part, where the insulation is worn or in some way defective.
SHROUD: A fan housing that ensures maximum airflow through the coil.
SIGHT GLASS: A clear window in a fluid line. 
SIGNAL: Information relayed from one point in the system to another.
SIGNAL CONVERTER: An electronic device that converts one signal type to another. 
SILICA GEL: A chemical compound often used in refrigerant driers to remove moisture from the refrigerant.
SILICON: A substance from which many semiconductors are made.
SILICON-CONTROLLED RECTIFIER (SCR): A semiconductor control device.
SILVER BRAZING: A high-temperature (above 800°F) brazing process for bonding metals.
SINE WAVE: The graph or curve used to describe the characteristics of alternating current and voltage.
SINGLE PHASE: The electrical power supplied to equipment or small motors, normally under 7 1/2 hp.
SINGLE-PHASE HERMETIC MOTOR: A sealed motor, such as with a small compressor, that operates off single-phase power.
SINGLE PHASING: The condition in a three-phase motor when one phase of the power supply is open.
SLING PSYCHROMETER: A device with two thermometers, one a wet bulb and one a dry bulb, used for checking air conditions, wet-bulb and dry-bulb.
SLINGER RING: A ring attached to the blade tips of a condenser fan. This ring throws condensate onto the condenser coil, where it is evaporated.
SLINKY LOOP: A flattened, circular coil of plastic pipe resembling a Slinky, which is used as a ground loop.
SLIP: The difference in the rated rpm of a motor and the actual operating rpm when under a load.
SLUGGING: A term used to describe the condition when large amounts of liquid enter a pumping compressor cylinder.
SMART RECOVERY: That start-up of a building air-conditioning or heating system whereby the system is started up earlier and allowed to run at a reduced capacity for the purpose of recording a low demand metering for the billing period.
SMOKE TEST: A test performed to determine the amount of unburned fuel in an oil burner fluegas sample.
SNAP-DISC: An application of the bimetal. Two different metals fastened together in the form of a disc that provides a warping condition when heated. This also provides a snap action that is beneficial in controls that start and stop current flow in electrical circuits.
SNAP OR DETENT ACTION: The quick opening of a control that is used to minimize the arc when making and breaking an electrical circuit.
SOAPSTONE: A stone used sometimes in the manufacture of woodstoves.
SOCKET WELD END (SW): A valve or fitting which slips over end of pipe and is made pressure tight by welding.
SOFTWARE: Computer programs written to give specific instructions to computers.
SOLAR COLLECTORS: Components of a solar system designed to collect the heat from the sun, using air, a liquid, or refrigerant as the medium.
SOLAR CONSTANT: The rate of solar energy reaching the outer limits of the earth’s atmosphere has been determined to be 429 BTU/ft2/h on a surface perpendicular (90°) to the sun’s rays.
SOLAR HEAT: Heat from the sun’s rays.
SOLAR INFLUENCE: The heat that the sun imposes on a structure.
SOLAR RADIANT HEAT: Solar-heated water or an antifreeze solution is piped through heating coils embedded in concrete in the floor or in plaster in ceilings or walls.
SOLDER POT: A device using a low-melting solder and an overload heater sized for the amperage of the motor it is protecting. The solder will melt, opening the circuit when there is an overload. It can be reset.
SOLDERING: Fastening two base metals together by using a third, filler metal that melts at a temperature below 800°F.
SOLDERLESS TERMINALS: Used to fasten stranded wire to various terminals or to connect two lengths of stranded wire together.
SOLENOID: A coil of wire designed to carry an electrical current producing a magnetic field. 
SOLID: Molecules of a solid are highly attracted to each other, forming a mass that exerts all of its weight downward.
SPACE COOLING AND HEATING THERMOSTAT: The device used to control the temperature of a space, such as a home thermostat that controls the temperature in a home or office.
SPDT: A single-pole, double-throw (SPDT) have three terminals: one common pin and two pins which vie for connection to the common. SPDTs are great for selecting between two power sources, swapping inputs, or whatever it is you do with two circuits trying to go one place. SPDT switches should usually have three terminals. (Sidenote: in a pinch an SPDT can actually be made into an SPST by just leaving one of the switch throws unconnected).
SPECIFICATION: A document that defines the requirements that a finished product must conform to – may include chemical and mechanical properties, tolerances, marking, shipping, etc.
SPECIFIC GRAVITY: The weight of a substance compared to the weight of an equal volume of water.
SPECIFIC HEAT: The amount of heat required to raise the temperature of 1 lb. of a substance 1°F.
SPECIFIC VOLUME: The volume occupied by 1 lb. of a fluid.
SPLASH LUBRICATION SYSTEM: A system of furnishing lubrication to a compressor by agitating the oil.
SPLASH METHOD: A method of water dropping from a higher level in a cooling tower and splashing on slats with air passing through for more efficient evaporation.
SPLIT BODY: A valve whose body is split. This design allows for easy plug and seat removal. Split-bodied valves are made in both the straight-through and angle versions.
SPLIT-PHASE MOTOR: A motor with run and start windings.
SPLIT SUCTION: When the common suction line of a parallel compressor system has been valved in such a way as to provide for multiple temperature applications in one refrigeration package.
SPLIT SYSTEM: A refrigeration or air-conditioning system that has the condensing unit remote from the indoor (evaporator) coil.
SPRAY POND: A pond with spray heads used for cooling water in water-cooled air-conditioning or refrigeration systems.
SPRING-LOADED RELIEF VALVE: A fluid (refrigerant, air, water, or steam) relief valve that can function more than one time because a spring returns the valve to a seat.
SPST: A single-pole, single-throw (SPST) switch is as simple as it gets. It’s got one output and one input. The switch will either be closed or completely disconnected. SPSTs are perfect for on-off switching. They’re also a very common form of momentary switches. SPST switches should only require two terminals.
SQUIRREL CAGE FAN: A cylindrically shaped fan assembly used to move air.
SQUIRREL CAGE ROTOR: Describes the construction of a motor rotor.
SSE: An acronym for Steady State Efficiency.
STACK DRAFT: Draft taken in the vent stack between the combustion device and the draft diverter.
STACK SWITCH: A safety device placed in the flue of an oil furnace that proves combustion within a time frame.
STACK TEMPERATURE: A combustion appliance’s flue gas temperature measured before the introduction of dilution air, which means before the draft diverter or draft hood of the combustion appliance if one is used.
STAINLESS STEEL: Any of a number of types of iron alloy with chrome, nickel, or other elements that does not oxidize in free air.
STAMPED EVAPORATOR: An evaporator that has stamped refrigerant passages in sheet steel or aluminum.
STANDARD ATMOSPHERE OR STANDARD CONDITIONS: Air at sea level at 70°F when the atmosphere’s pressure is 14.696 psia (29.92 in. Hg). Air at this condition has a volume of 13.33 ft3/lb.
STANDING PILOT: Pilot flame that remains burning continuously.
START CAPACITOR: A capacitor used to help an electric motor start.
STARTING RELAY: An electrical relay used to disconnect the start capacitor and/or start winding in a hermetic compressor.
STARTING WINDING: The winding in a motor used primarily to give the motor extra starting torque. 
STARVED COIL: The condition in an evaporator when the metering device is not feeding enough refrigerant to the evaporator.
STATIC PRESSURE: The bursting pressure or outward force in a duct system.
STATOR: The component in a motor that contains the windings: it does not turn.
STEADY-STATE CONDITION: A stabilized condition of a piece of heating or cooling equipment where not much change is taking place.
STEADY-STATE EFFICIENCY: Refers to combustion efficiency when the flue gas temperature (stack temperature) has stabilized and the flue gas samples reach equilibrium. 
STEAM: The vapor state of water.
STEM: The valve plug stem is a rod extending through the bonnet assembly to permit positioning of the plug or closure member. The actuator stem is a rod or shaft which connects to the valve stem and transmits motion or force from the actuator to the valve.
STEP MOTOR: An electric motor that moves with very small increments or “steps,” usually in either direction, and is usually controlled by a microprocessor with input and output controlling devices.
STRAINER: A fine-mesh device that allows fluid flow and holds back solid particles.
STRATIFICATION: The condition where a fluid appears in layers.
STRATOSPHERE: An atmospheric level that is located from 7 to 30 miles above the earth. Good ozone is found in the stratosphere.
STRESS CRACK: A crack in piping or other component caused by age or abnormal conditions such as vibration.
STROKE: The distance the plug or stem moves to go from a fully closed to a fully open position. Also referred to as Travel.
SUBBASE: The part of a space temperature thermostat that is mounted on the wall and to which the interconnecting wiring is attached.
SUBCOOLED: The temperature of a liquid when it is cooled below its condensing temperature. 
SUBCRITICAL CYCLE: Refrigeration cycles that take place above the refrigerants triple point, and below the refrigerant’s critical point.
SUBLIMATION: When a substance changes from the solid state to the vapor state without going through the liquid state.
SUCTION GAS: The refrigerant vapor in an operating refrigeration system found in the tubing from the evaporator to the compressor and in the compressor shell.
SUCTION LINE: The pipe that carries the heat-laden refrigerant gas from the evaporator to the compressor.
SUCTION-LINE ACCUMULATOR: A reservoir in a refrigeration system suction line that protects the compressor from liquid floodback.
SUCTION PRESSURE: The pressure created by the boiling refrigerant on the evaporator or low-pressure side of the system.
SUCTION SERVICE VALVE: A manually operated valve with front and back seats located at the compressor.
SUCTION VALVE: The valve at the compressor cylinder that allows refrigerant from the evaporator to enter the compressor cylinder and prevents it from being pumped back out to the suction line.
SUCTION-VALVE-LIFT UNLOADING: The suction valve in a reciprocating compressor cylinder is lifted, causing that cylinder to stop pumping.
SULFUR DIOXIDE: A combustion pollutant that causes eye, nose, and respiratory tract irritation and possibly breathing problems.
SUMP: A reservoir at the bottom of a cooling tower to collect the water that has passed through the tower.
SUPERCRITICAL REGION: A region above the critical point of the refrigerant. No liquid refrigerant can be produced above a refrigerant’s critical point.
SUPERHEAT: The temperature of vapor refrigerant above its saturation (change-of-state) temperature.
SUPPLY PRESSURE: The pressure at the supply port of a device such as a controller, positioner or transducer. Common values of control valves supply pressure are 20 psig. for a 3-15 psig. output and 35 psig. for a 6-30 psig. output.
SURGE: When the head pressure becomes too great or the evaporator pressure too low, refrigerant will flow from the high- to the low-pressure side of a centrifugal compressor system, making a loud sound.
SURGE-TYPE RECEIVER: A receiver that preserve the liquid subcooling made in the condenser during the cooler ambient months when free subcooling is available. A solenoid operated diverting valve is opened and the subcooled liquid coming from the condenser’s bottom is closed off to the receiver.
SUSTAINABLE DESIGN: Meets the needs of the present without compromising the ability of future generations to meet their own needs. 
SWAGED JOINT: The joining of two pieces of copper tubing by expanding or stretching the end of one piece of tubing to fit over the other piece.
SWAGING: See Swaged joint.
SWAGING TOOL: A tool used to enlarge a piece of tubing for a solder or braze connection.
SWAMP COOLER: A slang term used to describe an evaporative cooler.
SWEATING: A word used to describe moisture collection on a line or coil that is operating below the dew point temperature of the air.
SWITCH FREQUENCY: The speed at which the variable frequency drive’s power device switches the waveform’s positive and negative halves on and off is referred to as the switch frequency or carrier frequency.
SYNTHETIC REFRIGERANTS: Man-made refrigerants. 
SYSTEM CHARGE: The refrigerant in a system, both liquid and vapor. The correct charge is a balance where the system will give the most efficiency.
SYSTEM LAG: The temperature drop of the controlled space below the set point of the thermostat.
SYSTEM OVERSHOOT: The temperature rise of the controlled space above the set point of the thermostat.
SYSTEM PRESSURE-REGULATING VALVE (SPR VALVE): A valve located between the compressor and receiver of a refrigeration system. This valve controls the amount of liquid refrigerant that bypasses a parallel  liquid receiver.
**T**
TANK: A closed vessel used to contain a fluid.
TAP: A tool used to cut internal threads in a fastener or fitting.
TARE WEIGHT: The tare weight of a cylinder is the weight the cylinder would weigh if it were empty.
TECHNICIAN: A person who performs maintenance, service, testing, or repair to air-conditioning or refrigeration equipment. Note: the EPA defines this person as someone who could reasonably be expected to release CFCs or HCFCs into the atmosphere.
TEMPERATURE: A word used to describe the level of heat or molecular activity, expressed in Fahrenheit, Rankine, Celsius, or Kelvin units.
TEMPERATURE DIFFERENCE (TD): The difference between the inlet temperature and outlet temperature of a heat exchanger. For example,
an evaporator may have a 20°F TD: 75°F air in and 55°F air out.
TEMPERATURE GLIDE: When a refrigerant blend has different temperatures when it evaporates and condenses at a single given pressure.
TEMPERATURE-MEASURING INSTRUMENTS: Devices that accurately measure the level of temperature.
TEMPERATURE/PRESSURE RELATIONSHIP: This refers to the temperature/pressure relationship of a liquid and vapor in a closed container. If the temperature increases, the pressure will also increase. If the temperature is lowered, the pressure will decrease.
TEMPERATURE REFERENCE POINTS: Various points that may be used to calibrate a temperature-measuring device, such as boiling or freezing water.
TEMPERATURE-SENSING ELEMENTS: Various devices in a system that are used to detect temperature.
TEMPERATURE SWING: The temperature difference between the low and high temperatures of the controlled space.
TEST LIGHT: A lightbulb arrangement used to prove the presence of electrical power in a circuit.
TESTING, ADJUSTING AND BALANCING BUREAU (TABB): A certification bureau for individuals involved in working with ventilation.
THERM: Quantity of heat, 100,000 BTU.
THERMAL BOUNDARY: Insulation and air barrier that surrounds a conditioned space. 
THERMAL BREAK: A gasket material made of a poor thermally conductive material, installed between two conductive materials that combats and lessens thermal bridging.
THERMAL BRIDGING: Thermal bridging is rapid heat being conducted through the structure’s shell by materials in direct contact with one another that are very thermally conductive.
THERMAL BYPASS: A break in the thermal boundary where air can either infiltrate or exfiltrates.
THERMISTOR: A semiconductor electronic device that changes resistance with a change in temperature. 
THERMOCOUPLE: A device made of two unlike metals that generates electricity when there is a difference in temperature from one end to the other. Thermocouples have a hot and cold junction.
THERMOMETER: An instrument used to detect differences in the level of heat.
THERMOPILE: A group of thermocouples connected in series to increase voltage output.
THERMOSTAT: A device that senses temperature change and changes some dimension or condition within to control an operating device.
THERMOSTATIC EXPANSION VALVE (TXV): A valve used in refrigeration systems to control the superheat in an evaporator by metering the correct refrigerant flow to the evaporator. Also often abbrevaiated TEV.
THREE-PHASE POWER: A type of power supply usually used for operating heavy loads. It consists of three sine waves that are out of phase by 120° with each other.
THREE-WAY VALVE: A multi-orifice flow control valve with supply return and control ports arranged so that valve stem action in one direction opens supply to control port and reversed valve action opens the control port to return.
THROTTLING: Creating a planned or regulated restriction in a fluid line for the purpose of controlling fluid flow. The intentional restriction of flow by partially closing or opening a valve.
THRUST: The net force applied to a part in a particular direction – e.g., on the end of a valve stem.
THRUST SURFACE: A term that usually applies to bearings that have a pushing pressure to the side and that therefore need an additional surface to absorb the push. Most motor shifts cradle in their bearings because they operate in a horizontal mode, like holding a stick in the palm of your hand. When a shaft is turned to the vertical mode, a thrust surface must support the weight of the shaft along with the load the shaft may impose on the thrust surface. The action of a vertical fan shaft that pushes air up is actually pushing the shaftdownward.
TIME DELAY: A device that prevents a component from starting for a prescribed time. For example, many systems start the fans and use a time delay relay to start the compressor at a later time to prevent too much inrush current. 
TIMERS: Clock-operated devices used to time various sequences of events in circuits.
TOGGLE BOLT: Provides a secure anchoring in hollow tiles, building block, plaster over lath, and gypsum board. The toggle folds and canbe inserted through a hole, where it opens.
TON OF REFRIGERATION: The amount of heat required to melt a ton (2000 lb) of ice at 32°F in 24 hours, 288,000 BTU/24 h, 12000 BTU/h, or 200 BTU/min. 
TORQUE: The twisting or rotational force often applied to the starting power of a motor, usually expressed in foot-pounds.
TORQUE WRENCH: A wrench used to apply a prescribed amount of torque or tightening to a connector.
TOTAL EQUIVALENT WARMING IMPACT (TEWI): A global warming index that takes into account both the direct effects of chemicals emitted into the atmosphere and the indirect effects caused by system  inefficiencies.
TOTAL HEAT: The total amount of sensible heat and latent heat contained in a substance from a reference point. Also referred to as enthalpy.
TOTAL PRESSURE: The sum of the velocity and the static pressure in an air duct system. 
TOWER APPROACH: The temperature difference between the water leaving the tower and the wet bulb temperature of the air entering the cooling tower.
TOWER RANGE: The temperature difference between the entering and leaving water of the cooling tower is referred to as the tower range.
TRACE GAS: A small amount of refrigerant introduced into an empty system for electronic leak detection or as a holding charge, which is then backed by nitrogen to build pressure. CFCs cannot be used as  trace gasses.
TRANSCRITICAL CYCLE: Thermodynamic refrigeration processes that span above the subcritical region and into the supercritical region of the pressure/enthalpy diagram. 
TRANSFORMER: A coil of wire wrapped around an iron core that induces a current to another coil of wire wrapped around the same iron core. Note: A transformer can have an air core. 
TRANSISTOR: A semiconductor often used as a switch or amplifier.
TRAVEL: The distance the plug or stem moves to go from a fully closed to a fully open position. Also referred to as Stroke.
TRIAC: A semiconductor switching device. 
TRIM: Includes all the parts that are in flowing contact with the process fluid except the body, bonnet and body flanges and gaskets. The plug, seats, stem, guides, bushings and cage are some of the parts included in the term trim.
TRIPLE POINT: The triple point is where a refrigerant, like CO2, can exist as all three phases (vapor, liquid, and solid) simultaneously in equilibrium.
TROPOSPHERE: The lower atmospheric level that extends upward from ground level to about 7 miles. Global warming takes place in the troposphere.
TRUNNION: The part of a ball valve which holds the ball on a fixed vertical axis and about which the ball turns.
TUBE-WITHIN-A-TUBE COIL: A coil used for heat transfer that has a pipe in a pipe and is fastened together so that the outer tube becomes one circuit and the inner tube another.
TUBING: Pipe with a thin wall used to carry fluids.
TURBULENT FLOWTURBULENCE: Irregular flow that is characterized by tiny whirlpool regions.
TWINNING: Two furnaces connected side by side and sharing a common ducting system.
TWO-POLE, SPLIT-PHASE MOTOR: This motor runs at 3600 rpm when not loaded and at about 3450 rpm when loaded. A four-pole motor runs at about 1800 rpm not loaded and at about 1725 fully loaded.
TWO-SPEED COMPRESSOR MOTOR: Can be a fourpole motor that can be connected as a twopole motor for high speed (3450 rpm) and connected as a four-pole motor for running at 1725 rpm for low speed. This is accomplished with relays outside the compressor.
TWO-TEMPERATURE VALVE: A valve used in systems with multiple evaporators to control the evaporator pressures and maintain different temperatures in each evaporator. Sometimes called a holdback valve.
**U**
ULTRASOUND LEAK DETECTOR: Detectors that use sound from escaping refrigerant to detect leaks.
ULTRAVIOLET: Light waves that can only be seen under a special lamp.
ULTRAVIOLET LIGHT: Light frequency between 200 and 400 nanometers.
UNCONDITIONED SPACE: Areas of a structure or residence that are not heated or cooled.
UNEVEN PARALLEL SYSTEM: Parallel compressors of unequal sizes mounted on a steel rack and controlled by a micro processor.
UNINTENTIONALLY CONDITIONED SPACE: These spaces are usually warmed by waste heat and include boiler rooms or furnace rooms.
UPFLOW FURNACE: This furnace takes in air from the bottom or from sides near the bottom and discharges hot air out the top.
UPDRAFT: A draft that flows upward.
URETHANE FOAM: A foam that can be applied between two walls for insulation.
U-TUBE MERCURY MANOMETER: A U-tube containing mercury, which indicates the level of vacuum while evacuating a refrigeration system.
U-TUBE WATER MANOMETER: Indicates natural gas and propane gas pressures. It is usually calibrated in inches of water.
U-VALUE: Indicates the thermal conductivity of a material. This is the inverse of a material’s R-value.
**V**
VACUUM: The pressure range between the earth’s atmospheric pressure and no pressure, normally expressed in inches of mercury (in. Hg) vacuum.
VACUUM GAUGE: An instrument that measures the vacuum when evacuating a refrigeration, air-conditioning, or heat pump system.
VACUUM PUMP: A pump used to remove some fluids such as air and moisture from a system at a pressure below the earth’s atmosphere.
VALVE: A device used to control the flow of fluid contained in a pipe line.
VALVE PLATE: A plate of steel bolted between the head and the body of a compressor that contains the suction and discharge reed or flapper valves.
VALVE SEAT: That part of a valve that is usually stationary. The movable part comes in contact with the valve seat to stop the flow of fluids.
VALVE STEM DEPRESSOR: A service tool used to access pressure at a Schrader valve connection.
VAPOR: The gaseous state of a substance.
VAPOR BARRIER: A thin film used in construction to keep moisture from migrating through building materials.
VAPOR CHARGE BULB: A charge in a thermostatic expansion valve bulb that boils to a complete vapor. When this point is reached, an increase in temperature will not produce an increase in pressure.
VAPOR LOCK: A condition where vapor is trapped in a liquid line and impedes liquid flow.
VAPOR PRESSURE: The pressure exerted on top of a saturated liquid.
VAPOR PUMP: Another term for compressor.
VAPOR REFRIGERANT CHARGING: Adding refrigerant to a system by allowing vapor to move out of the vapor space of a refrigerant cylinder and into the low-pressure side of the refrigeration system.
VAPORIZATION: The changing of a liquid to a gas or vapor.
VARIABLE AIR VOLUME, VAV: The control strategy that varies the volume of air that is delivered to an area in the conditioned space depending on the airflow requirements for that area. 
VARIABLE-FREQUENCY DRIVE (VFD): An electrical device that varies the frequency (hertz) for the purpose of providing a variable speed.
VARIABLE PITCH PULLEY. A pulley whose diameter can be adjusted.
VARIABLE REFRIGERANT FLOW, VRF: The control strategy that varies the volume of refrigerant that is delivered to an air handler based on the heating or cooling needs of the area being served by that air handler.
VARIABLE REFRIGERANT VOLUME, VRV: See Variable Refrigerant Flow, VRF.
VARIABLE RESISTOR: A type of resistor where the resistance can be varied.
VARIABLE-SPEED MOTOR: A motor that can be controlled, with an electronic system, to operate at more than one speed.
VAV: See Variable Air Volume.
V BELT: A belt that has a V-shaped contact surface and is used to drive compressors, fans, or pumps.
VELOCITY: The speed at which a substance passes a point.
VELOCITY METER: A meter used to detect the velocity of fluids, air, or water.
VELOMETER: An instrument used to measure the air velocity in a duct system.
VENT-FREE: Certain gas stoves or gas fireplaces are not required to be vented; therefore, they are called “vent-free.”
VENTILATION: The process of supplying and removing air by natural or mechanical means to and from a particular space.
VENTING PRODUCTS OF COMBUSTION: Venting flue gases that are generated from the burning process of fossil fuels. 
VENTURI: A smoothly tapered device used in fluid flow to increase the velocity of the fluid. 
VISCOSITY: The thickness of a fluid or the friction between the molecules in a fluid.
VOLT-OHM-MILLIAMMETER (VOM): A multimeter that measures voltage, resistance, and current in milliamperes.
VOLTAGE: The potential electrical difference for electron flow from one line to another in an electrical circuit.
VOLTAGE FEEDBACK: Voltage potential that travels through a power-consuming device when it is not energized. 
VOLTMETER: An instrument used for checking electrical potential.
VOLUMETRIC EFFICIENCY: The pumping efficiency of a compressor or vacuum pump that describes the pumping capacity in relationship to the actual volume of the pump.
VORTEXING: A whirlpool action in the sump of a cooling tower.
VRF: See Variable Refrigerant Flow.
VRV: See Variable Refrigerant Volume.
**W**
WALK-IN COOLER: A large refrigerated space used for storage of refrigerated products. 
WASTEWATER SYSTEM: A refrigeration system that uses water one time and then exhausts it to a waste system.
WATER BOX: A container or reservoir at the end of a chiller where water is introduced and contained.
WATER CAPACITY: The water capacity of a cylinder is the weight of the cylinder if it were full of water. The water capacity (W.C.) of the cylinder is stamped on the outside of the cylinder and its units are in pounds.
WATER COLUMN (WC): The pressure it takes to push a column of water up vertically. One inch of water column is the amount of pressure it would take to push a column of water in a tube up one inch.
WATER-COOLED CONDENSER: A condenser used to reject heat from a refrigeration system into water.
WATER COOLER: A small refrigeration machine that is typically used to refrigerate water for drinking purposes.
WATER HAMMER: A loud pressure pulse that occurs when valves close fast, causing shock waves.
WATER LOOP: These loops of plastic pipe are buried in the ground in a closed-loop geothermal heat pump system and contain a heat transfer fluid.
WATER MANOMETER: A device that uses a column of water to measure low pressures in air or gas systems.
WATER PUMP: Used to pump water or other fluids from one pressure level to another to promote water flow in piping systems. See Centrifugal pump.
WATER-REGULATING VALVE: An operating control regulating the flow of water. 
WATT (W): A unit of power applied to electron flow. One watt equals 3.414 BTU.
WATT-HOUR: The unit of power that takes into consideration the time of consumption. It is the equivalent of a 1-watt bulb burning for 1 hour.
WEATHERIZATION: The process of improving the energy efficiency in a home or building which will have a direct effect on energy consumption and comfort within the home or building. 
WEEP HOLES: Holes that connect each cell in an ice machine’s cell-type evaporator that allow air entering from the edges of the ice to travel along the entire ice slab to relieve the suction force and allow the ice to fall off of the evaporator.
WEIGHT: The force that matter (solid, liquid, or gas) applies to a supporting surface when it is at rest.
WELDED HERMETIC COMPRESSOR: A compressor that is completely sealed by welding, versus a semi-hermetic compressor that is sealed by bolts and flanges.
WET-BULB DEPRESSION: The difference between the wet-bulb and the dry-bulb reading when readings are taken in air.
WET-BULB TEMPERATURE: A wet-bulb temperature of air is used to evaluate the humidity in the air. It is obtained with a wet thermometer bulb to record the evaporation rate with an airstream passing over the bulb to help in evaporation.
WET COMPRESSION: Saturated refrigerant vapors being compressed by the compressor, which will turn into liquid refrigerant droplets when compressed.
WET HEAT: A heating system using steam or hot water as the heating medium.
WHIPLINE: Typically the smaller of two hoisting lines on a crane.
WINDING THERMOSTAT: A safety device used in electric motor windings to detect over-temperature conditions.
WINDOW UNIT: An air conditioner installed in a window that rejects the heat outside the structure.
WIRE CONNECTORS (SCREW-ON): Used to connect two or more wires together.
WOG: An abbreviation for Water-Oil-Gas. A rating designation generally used for small valves chiefly in low ratings. Indicates maximum working pressure at ambient + 32° F to +100° F. Also called Non-shock Rating.
WORK: A force moving an object in the direction of the force. Work = Force x Distance.
WORKING PRESSURE: The pressure (pounds per square inch) at which a valve is designed to operate.
WORM GEAR: mechanical arrangement consisting of a toothed wheel worked by a short revolving cylinder (worm) bearing a screw thread.
WRT: With reference to.
WYE TRANSFORMER CONNECTION: Typically furnishes 208 V and 115 V to a customer.
**Z**
ZEOTROPIC BLEND: Two or more refrigerants mixed together that will have a range of boiling and/or condensing points for each system pressure. Noticeable fractionation and temperature glide will occur.
ZIP CODE INSULATION CALCULATOR: A tool that provides insulation levels for your new or existing home based on your zip code and other basic information about your home.
ZONE VALVE: Zone control valves are thermostatically controlled valves that control water flow in various zones in a hydronic heating system.