Glossary of HVAC Terms


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Absolute Pressure Gauge pressure plus atmospheric pressure (14.7 lb. per sq. in.) equals absolute pressure.
Absolute Temperature Temperature measured from absolute zero.
Absolute Zero Temperature Temperature at which all molecular motion ceases(-460 F. and ­-273 C).
Absorbent Substance with the ability to take up or absorb another substance.
Absorption Refrigerator Refrigerator which creates low temperature by using the cooling effect formed when a refrigerant is absorbed by chemical substance.
ACCA A leading HVAC/R Association -
Accessible Hermetic Assembly of a motor and compressor inside a single bolted housing unit.
Accumulator Storage tank which receives liquid refrigerant from evaporator and prevents it from flowing into suction line before vaporizing.
Acid Condition In System Condition in which refrigerant or oil in system is mixed with fluids that are acid in nature.
ACR Tubing Tubing used in air conditioning and refrigeration. Ends are sealed to keep tubing clean and dry.
Activated Alumina Chemical which is a form of aluminum oxide. It is used as a drier or desiccant.
Activated Carbon Specially processed carbon used as a filter drier ; commonly used to clean air.
Actuator That portion of a regulating valve which converts mechanical fluid, thermal energy or electrical energy into mechanical motion to open or close the valve seats.
Add On Heat Pump Installing a heat pump in conjunction with an existing fossil fuel furnace. The result is a dual fuel system.
Adiabatic Compression Compressing refrigerant gas without removing or adding heat.
Adjustable Grille A grille with linear blades which can be adjusted to vary the direction of the discharged air. The linear blades are normally either vertical or horizontal, or both horizontal and vertical.
Adsorbent Substance with the property to hold molecules of fluids without causing a chemical or physical damage.
Aeration Act of combining substance with air.
Aero seal A patented sealing process; the most effective, affordable, and viable method of sealing the central heating and cooling ductwork in your home.
AFLU (Annual Fuel Utilization Efficiency) A rating that reflects the efficiency of a gas furnace in converting fuel to energy. A rating of 90 means that approximately 90% of the fuel is utilized to provide warmth to your home, while the remaining 10% escapes as exhaust.
AFUE (Annual Fuel Utilization Efficiency ) The ratio of annual output of useful energy or heat to the annual energy input to the furnace. The higher the AFUE, the more efficient the furnace -- higher efficiency translates to more savings on fuel bills. This will range from 80% to 95%.
Agitator Device used to cause motion in confined fluid.
AHU (Air Handler Unit) The inside part of the A/C system that contains the blower, cooling (evaporator) coil, and heater.
Air Change The amount of air required to completely replace the air in a room or building; not to be confused with recirculated air.
Air Cleaner Device used for removal of airborne impurities.
Air Coil Coil on some types of heat pumps used either as an evaporator or condenser.
Air Conditioner Device used to control temperature, humidity, cleanliness and movement of air in a confined space.
Air Conditioning Control of the temperature, humidity, air movement and cleaning of air in a confined space.
Air Cooler Mechanism designed to lower temperature of air passing through it.
Air Core Solenoid Solenoid that has a hollow core instead of a solid core.
Air Diffuser Supply air terminal device, designed to direct airflow into desired patterns, usually placed in the ceiling, generally of circular, square or rectangular shape, and composed of divergent deflecting members.
Air Diffusion  Distribution of the air in a space, called the treated space, by means of devices, called air terminal devices, in a manner so as to meet certain specified conditions, such as air change rate, pressure, cleanliness, temperature, humidity, air velocity and noise level.
Air Distribution The transportation of a specified air flow to or from the treated space or spaces, generally by means of ductwork.
Air Gap The space between magnetic poles or between rotating and stationary assemblies in a motor or generator.
Air Handler Fan-blower, filter, heat transfer coil, and housing parts of a system. Also known as the blower section and part of the split system, this unit is commonly in your home or attic and blows the air through your house. This has to be matched with the condenser properly to assure maximum efficiency. This will contain your heating and evaporator coil.
Air Infiltration Leakage of air into rooms through cracks, windows doors and other openings.
Air Source Equipment Heat pumps or air conditioners that uses the outdoor air to transfer heat to and from the refrigerant in the unit.
Air Terminal Device A device located in an opening provided at the boundaries of the treated space to ensure a predetermined motion of air in this space.
Air Terminal Device, Lighting Troffer An air terminal device, usually in the form of a slot or combination of slots, designed for use with a troffer unit.
Air Terminal Device, Linear  Air terminal device (grille) with an aspect ratio of 10: 1 or more.
Air Terminal Device, Slot  A device with one or several slots with an aspect ratio of 10: 1 or more for each slot (the aspect ratio is the ratio of the length to the width of the closed rectangular opening). A slot may or may not have an adjustable member to vary the direction of the air jet(s) or air flow rate.
Air Terminal Device, Supply An air terminal device through which air enters a treated space. It usually consists of one or several deflecting members which ensure reduction of the air velocity in the occupied zone as well as efficient mixing of the supply air with the air in the treated space. Moreover supply air terminal devices usually determine the direction of the air jet(s).
Air-Cooled Condenser Heat of compression, plus the heat of absorption, is transferred from refrigerant within coil to surrounding air, either by convection or fan or blower.
Airflow The distribution or movement of air.
Ak value (of an air terminal device) Quotient obtained by dividing a measured air flow rate by a measured air velocity according to a specific process and a specific instrument.
Algaecide Normally in tablet form, placed in evaporator drain pan. Used to help slow the growth of bacteria.
ARI (Air-Conditioning and Refrigeration Institute) Air-Conditioning and Refrigeration Institute is a nonprofit, voluntary organization comprised of heating, air conditioning and refrigeration manufacturers. ARI publishes standards for testing and rating heat pumps and air conditioners to provide you with a standardized measure of comparison. So, ARI ensures a level of performance within the industry.
ASHRAE A leading HVAC/R Association - American Society of Heating, Refrigerating and Air Conditioning Engineers -

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Balance Point The lowest outdoor temperature at which the refrigeration cycle of a heat pump will supply the heating requirements without the aid of a supplementary heat source.
Balancing The process of adjusting the flow of air in duct systems, or water flow in hot-water heating systems. Proper balancing is performed using accurate instrumentation to deliver the right amount of heating or cooling to each area or room of the home.
Blower (Fan) An air handling device for moving air in a distribution system.
BTU (British Thermal Unit) The amount of heat that will raise or lower one pound of water 1 degree F. at 39.2 degrees F. One BTU is the equivalent of the heat given off by a single wooden kitchen match. The British Thermal Unit is a standard of measure for cooling and heating capacities. This is how the capacity of air conditioning is measured. For your home, it represents the measure of heat givens off when fuel is burned for heating or the measure of heat extracted from your home for cooling.
BTUH (British Thermal Unit Per Hour) Establishes a time reference to btu input or output rates. A BTUH is how many BTUs are used per hour.

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Capacitor A device used to start a motor or compressor (or to keep it running after start up.)
Capacity The output or producing ability of a piece of cooling or heating equipment. Cooling and heating capacity are normally referred to in BTUs.
Central Forced-Air Heating System A piece of equipment that produces heat in a centralized area, then distributes it throughout the home through a duct system.
CFC (Chlorofluorocarbon) A class of refrigerants. Generally refers to the Chlorofluorocarbon family of refrigerants. Sometimes called Freon
CFM (Cubic Feet per Minute) A standard of airflow measurement. A typical system produces 400 CFM per ton of air conditioning.
Charge Amount of refrigerant placed in a refrigerating unit.
Comfort Zone The range of temperatures, humidities and air velocities at which the greatest percentage of people feel comfortable.
Compressor The heart of an air conditioning or heat pump system. The large (usually black) part in the condenser (outdoor unit) that pumps refrigerant. The pump of a refrigerating mechanism which draws a low pressure on cooling side of refrigerant cycle and squeezes or compresses the gas into the high pressure or condensing side of the cycle. The compressor maintains adequate pressure to cause refrigerant to flow in sufficient quantities to meet the cooling requirements of the system.
Condenser This is the unit that will sit outside and is part of a split system, it contains the compressor which is the heart of your air conditioner or Heat Pump, it pumps the refrigerant through your system. Some people call the condenser the compressor, but the compressor is a component of the condenser along with the fan motor and condenser coil. Coil or outdoor coil dissipates heat from the refrigerant, changing the refrigerant from vapor to liquid.
Condensing Unit Part of a refrigerating mechanism which pumps vaporized refrigerant from the evaporator, compresses it, liquefies it in the condenser and returns it to the refrigerant control. The outdoor portion of a split system air conditioner contains the compressor and outdoor coil ignoring the reverse cycle operation, also the outdoor in a heat pump system.
Configuration This describes the direction in which a furnace outputs heat. A furnace may have an up flow, down flow or cross flow (horizontal) configuration.
Contactor In the condenser, the main switch that turns the condenser on.
COP (Coefficient Of Performance) COP compares the heating capacity of a heat pump to the amount of electricity required to operate the heat pump in the heating mode. COPs vary with the outside temperature: as the temperature falls, the COP falls also, since the heat pump is less efficient at lower temperatures. ARI standards compare equipment at two temperatures, 47 F and 17 F, to give you an idea of the COP in both mild and colder temperatures. Geothermal equipment is compared at 32 F enter water temperature. COP & HSPF can not be compared equally. Air Source Equipment is rated by HSPF or COP and Geothermal equipment is rated by COP.

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Damper A device that is located in ductwork to adjust air flow. This movable plate opens and closes to control airflow. Dampers are used effectively in zoning to regulate airflow to certain rooms. There are basically two types of dampers: Manual and motorized. A manual damper generally consists of a sheet metal (or similar material) flap, shaped to fit the inside of a round or rectangular duct. By rotating a handle located outside of the duct a technician can adjust (see Balancing) air flow to match the needs of a particular area or room. A motorized damper is generally used in a zoned system (see Zoning) to automatically deliver conditioned air to specific rooms or zones. In particular, the following types, can be distinguished:
DB Dry Bulb
db (Decibel) A decibel describes the relative loudness of a sound. Some common sounds are fairly close to a typical air conditioner or heat pump's sound level: human voice, 7.0 decibels; blender, 8.8 decibels.
DDC (Direct Digital Control) Direct Digital Control
Defrost Cycle The process of removing ice or frost buildup from the outdoor coil during the heating season.
Dehumidification The reduction of water vapor in air by cooling the air below the dew point; removal of water vapor from air by chemical means, refrigeration, etc.
DOE (Department of Energy) The Department of Energy is a federal agency in charge of setting industry efficiency standards and monitoring the consumption of energy sources.
Down flow Furnace A furnace that pulls in return air from the top and expels warm air at the bottom.
Drier Sometimes called filter/drier, it removes moisture and keeps the refrigerant clean.
Dual Fuel System A dual heating system, for example a heat pump and a fossil fuel furnace.
Duct A pipe or closed conduit made of sheet metal, fiberglass board, or other suitable material used for conducting air to and from an air handling unit.
Ductwork Pipes or channels that carry air throughout your home. The delivery system through which warm air from the furnace is brought to where it's needed. Ductwork is made of sheet metal, fiberglass, or flexible plastic, and can be round or rectangular in shape.

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EER - (Energy Efficiency Ratio) A ratio calculated by dividing the cooling capacity in Btu's per hour (BTUH) by the power input in watts at any given set of rating conditions, expressed in BTUH per watt (BTUH/watt). EER & SEER can not be compared equally. Air source equipment is rated by SEER and geothermal equipment is rated by EER.
Effective area (of an air terminal device)  The smallest net area of an air terminal device used by the air stream in passing through the air terminal device.
Efficiency A rating on comfort equipment is similar to the miles per gallon rating on your car. The higher the rating number, the more efficient the system and the lower your fuel consumption will be. You can save a lot of money with a high efficiency unit. Depending on your local climate, lifestyle and electricity rates, savings will vary. For furnaces. it is the rate at which a furnace maximizes fuel use. This rate is numerically described as a ratio called AFUE (see AFUE). As of January, 1991, no furnaces can be manufactured with efficiencies lower than 78% AFUE. High efficiency furnaces will be rated 85 to 95% AFUE.
Electronic Air Cleaner An electronic device that filters out particles and contaminants in indoor air.
Emergency Heat (Supplementary Electric Heat) The back up electric heat built into a heat pump system. The same as an auxiliary heater, except it is used exclusively as the heat source when the heat pump needs repair.
Envelope The geometrical surface of the points of an air jet, corresponding to a determined value of the measured air velocity. This velocity is generally called «terminal velocity».
EPA (Environmental Protection Agency) Environmental Protection Agency -
ERV (Energy Recovery Ventilator) This device preheats incoming outside air during the winter and pre-cools incoming air during the summer to reduce the impact of heating and or cooling the indoor air. This means that smaller capacity heating and cooling systems can be used in homes, which results in lower installation costs, lower peak demand for energy, and lower operating costs.
Evaporator Coil  The evaporator coil is located inside your house in a split system in the air handler, or above the gas furnace. This will produce cooling in the air conditioning mode and heating in a Heat Pump mode. This coil section in the evaporator is where refrigerant evaporates and absorbs heat from air passed over the coil. This is also very important in removing humidity from your home.
Exhaust The air flow leaving the treated space. Exhaust may be accomplished by one or more of the following methods:
a. Extraction: exhaust in such a manner that the air is discharged into the atmosphere.
b. Relief: exhaust in such a manner that the air is allowed to escape from the treated space if the pressure in that space rises above a specified level.
c. Recirculation: exhaust in which the air is returned to the air treatment system.
d. Transfer: exhaust in which air passes from the treated space to another treated space.
Exhaust Air Flow Rate  Volume of air leaving an exhaust air terminal device within a time unit.
Exhaust Air Terminal Device Air terminal device through which air leaves the treated space.

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Filter A device for removing dust particles from air or unwanted elements from liquids.
Fire Valves (or Fire Dampers) Components which are installed in an air distribution system between two fire separating compartments and are designed to prevent propagation of fire and/or smoke. Generally are kept open by mechanical restraint, whose effect is canceled under specific conditions. The valve is then closed automatically.
Forced Air This describes a type of heating system that uses a blower motor to move air through the furnace and into the ductwork.
Furnace That part of an environmental system which converts gas, oil, electricity or other fuel into heat for distribution within a structure.

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Geothermal Equipment Heat pumps that uses the ground to transfer heat to and from the refrigerant in the unit. The unit circulates water through a heat exchanger in the to a closed loop buried in the ground or by pumping water from a well through the unit.
Grille  An air terminal device with multiple passages for the air.

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HCFC (Hydro chlorofluorocarbon) A class of refrigerants. Generally refers to Halogenated Chlorofluorocarbon family of refrigerants.
Heat Exchanger This is a device that enables furnaces to transfer heat from combustion safely into breathable air. The primary heat exchanger transfers heat from combustion gases to the air blowing through the ductwork. It's vital that none of the combustion gas itself gets into the air stream. The primary heat exchanger handles the hottest gases. In high efficiency furnaces, secondary heat exchangers recover heat that used to be vented up the chimney with the exhaust gases. By recovering this heat, the furnace becomes more efficient. Part of the heat recovered here causes the water and acid to condense out of the exhaust gas. Because this liquid is corrosive, secondary heat exchangers must be designed to prevent deterioration. Usually this means they are made of stainless steel or some derivative of it.
Heat Gain The amount of heat gained, measured in BTU's, from a space to be conditioned, at the local summer outdoor design temperature and a specified indoor design condition.
Heat Loss The amount of heat lost, measured in BTU's from a space to be conditioned, at the local winter outdoor design temperature and a specified indoor design condition.
Heat Pump A Heat Pump is a reverse cycle air conditioner. The Heat Pump uses a compression cycle system to supply heat or remove heat remove a temperature controlled space. When you run your air conditioner, your outdoor unit will be blowing hot air, (in other words, removing the heat from your home and sending it outside). When you run your heat pump, you reverse the flow of refrigerant and remove the heat from the atmosphere outside and blow it inside. When the temperature dips below 40 degrees outside, the Heat Pump labors in producing heat so they install a back up or auxiliary electric heat strip to supplement the Heat Pump. Electric Heat strips are very expensive to operate. In southern climates where it rarely dips below 40 degrees the heat pump is very efficient. A 3 to 1 savings in heating compared to electric heat strips.
Heat Pump Cooling Mode In the cooling cycle of a Heat Pump , you are removing hot air from inside the house and sending it outside. You can feel the hot air outside, over the condenser fan motor.
Heat Pump Heating Mode In the heating mode of a Heat Pump the refrigerant cycle is reversed and you are now removing the heat from the outside and sending it inside the house. You can feel the cool air outside, over the condenser fan motor. When temperatures go below 40 degrees Heat Pumps labor in producing heat and must use back up electric heat strips. Heat strips cost 3 times as much to operate as a Heat Pump when producing heat.
HFC (Hydro fluorocarbon) A class of refrigerants. Generally refers to Hydro fluorocarbon family of refrigerants
Horizontal Furnace A furnace that lies on its side, pulling in return air from one side and expelling warm air from the other.
HRV (Heat Recovery Ventilator) This device bring fresh, outside air into a home while simultaneously exhausting stale indoor air outside. In the process of doing this, an HRV removes heat from the exhaust air and transfer it to the incoming air, pre-heating it. This allows for the reclamation of much of the energy that otherwise would simply be vented outside. The end result: home comfort systems operate more efficiently.
HSPF (Heating Seasonal Performance Factor) Heating seasonal performance factor is similar to SEER, but it measures the efficiency of the heating portion of your heat pump. Like SEER, industry minimums have been raised recently, and the minimum is now 6.80 HSPF. The total heating output of a heat pump during its normal annual usage period for heating divided by the total electric power input in watt-hours during the same period. COP & HSPF can not be compared equally. Air Source equipment is rated by HSPF or COP and Geothermal equipment is rated by COP. ARI standards compares air source equipment at two temperatures, 47 F and 17 F. Geothermal equipment is compared at 32 F enter water temperature.
Humidifier A device that adds moisture to warm air being circulated or directed into a space.
Humidistat A device designed to regulate humidity input by reacting to changes in the moisture content of the air. Much like a thermostat but turns the system on & off by sensing the humidity level.
Humidity The amount of moisture in the air. Air conditioners remove moisture for added comfort.
HVAC Heating, Ventilating and Air Conditioning
HVAC/R Heating, Ventilating, Air Conditioning, & Refrigeration

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IAQ Indoor Air Quality
ICM (Integrally Controlled Motor) A specially engineered, variable-speed motor used in top-of-the-line indoor units. ICM motors are more than 90 percent efficient versus 60 percent efficiency for conventional motors. Continuous comfort, quiet operation and u
Indoor Coil Refrigerant containing portion of a fan coil unit similar to a car radiator, typically made of several rows of copper tubing with aluminum fins.
Indoor Coils A homes comfort system consist of two components: the outdoor unit (air conditioner or heat pump) and the indoor unit (coil or blower coil). Combinations of various units will result in vastly different efficiency ratings. Unreasonably high efficiency ratings can be created by using unrealistic indoor and out door equipment combinations. The term "most popular coil" indicates the actual tested combinations; other ratings may be simulated and unrealistic. Be sure that the efficiency ratings you are comparing are for "most popular coil." You'll know the ratings are attainable and close to reality.
Indoor Unit This is usually located inside the house and contains the indoor coil, fan, motor, and filtering device, sometimes called the air handler.
Induction Process by which the primary air sets into motion an air volume, called secondary air, in the room.
Induction ratio (i)  Ratio of the total air flow rate to the primary air flow rate.
Infiltration Air flow inward into a space through walls, leaks around doors and windows or through the building materials used in the structure.
IWC (Inches of water column) Commonly used in the USA

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kW (kilowatt) A kilowatt equals 1,000 watts.
kWh (kilowatt hour) A kilowatt hour (kWh) is the amount of kilowatts of electricity used in one hour of operation of any equipment.

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Latent Heat

Heat, that when added or removed, causes a change in state - but no change in temperature.

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The Mobile Home Package Unit is an air conditioner with electric or gas heat or can be a Heat Pump. This unit has the condenser and air handler all-in-one package. Commonly used in mobile homes and sits outside. (Self-contained unit)

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An air terminal device designed to generate a low energy loss and thus produce a maximum throw by minimum entertainment.

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Outdoor Coil

Refrigerant containing portion of a fan coil unit similar to a car radiator, typically made of several rows of copper tubing with aluminum fins.

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Package Unit or Package System A self-contained unit or system that has the Air Handler & Condenser in same unit. Normally placed outside the home and connected to a duct system by a penetration through the homes foundation. Except for geothermal which is a self-contained indoor unit that is place in a closet, attached garage, basement, or mechanical room.
Primary Air Flow Rate  Volume of air entering a supply air terminal device within a time unit.

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Refrigerant Substance used in refrigerating mechanism. A substance that produces a refrigerating effect while expanding or vaporizing. It absorbs heat in evaporator by change of state from a liquid to a gas, and releases its heat in a condenser as the substance returns from the gaseous state back to a liquid state.
Refrigerant Lines Set of two copper lines connecting the outdoor unit and the indoor unit
Register Combination grille and damper assembly covering an air opening or end of an air duct.
Return Air Air drawn into a heating unit after having been circulated from the heater's output supply to a room.
RH Relative Humidity
RSES A leading HVAC/R Association - Refrigeration Service Engineers Society -

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Saturation Temperature Also referred to as the boiling point or the condensing temperature. This is the temperature at which a refrigerant will change state from a liquid to a vapor or visa versa.
SEER (Seasonal Energy Efficiency Ratio) The amount of cooling your equipment delivers per every dollar spent on electricity. The higher the number the lower the operating cost (not more cooling.) SEER applies to air conditioners and heat pumps. In the past, a unit with a SEER of 8.00 was considered standard efficiency, and a unit with a 10.00 SEER was considered high efficiency. After January 1, 1992, the minimum SEER required by the DOE is 10.00 and 15.00+ SEER is considered high efficiency. EER & SEER can not be compared equally. Air source equipment is rated by SEER and geothermal equipment is rated by EER. The total cooling of a central unitary air conditioner or unitary heat pump in Btu's during its normal annual usage period for cooling divided by the total electric energy input in watt-hours during the same period.
Sensible Heat Heat, that when added or removed, causes a change in temperature but not in state.
Set point The temperature to which a thermostat is set to result in a desired heated space temperature.
Single Package An outdoor unit that contains both a heating and a cooling system.
Sizing Refers to the procedure a heating contractor goes through to determine how large a furnace (measured in BTUH) is needed to heat a house efficiently. Too small a furnace won't deliver enough heating; too large a furnace increases energy costs and can have an adverse effect on comfort. Sizing depends on the square-footage of the home, the amount of ceiling and wall insulation, the window area, use of storm doors, storm windows, and more.
Sound Attenuators Components which are inserted into the air distribution system and designed to reduce airborne noise which is propagated along the ducts.
Split System Refrigeration or air conditioning installation, which places condensing unit outside or away from evaporator. These unit are connected together by a supply and return refrigerant lines. Also applicable to heat pump installations.
Spread (LS) (for a supply air terminal device) Maximum distance between two vertical planes tangent to a specified envelope and perpendicular to a plane through the core center. The spread are generally referred to the envelope corresponding to 0.25 m/s for zero supply temperature differential (i.e., under isothermal conditions).
SRN (Sound Rating Number) Sound is measured in bels (a bel equals 10 decibels). The SRN of a unit is based on ARI test, performed at ARI standard rating conditions. Average sound rating range from 7.0 to 8.0 decibels. The lower the SRN rating, the quieter the unit.
Straight Cool This is an air conditioner that uses different forms of heating such as Natural Gas, LP Gas, Electric Resistance heat and oil.
Sub cooled Liquid Liquid refrigerant which is cooled below its saturation temperature.
Superheated Vapor Refrigerant vapor which is heated above its saturation temperature. If a refrigerant is superheated, there is no liquid present.
Supply  The air flow entering the treated space.
Switchover Valve A device in a heat pump that reverses the flow of refrigerant as the system is switched from cooling to heating. Also called a reversing valve or four-way valve.

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Therm Another measurement of heat. One therm equals 100,000 BTUH.
Thermostat A temperature sensitive switch for controlling the operation of a heater or furnace. Typically found on a wall inside the home, that consists of a series of sensors and relays that monitor and control the functions of a heating and cooling system.
Thermostat, Air-Sensing  Thermostat unit in which sensing element is located in refrigerated space.
Throw (Lt)  The maximum distance between the center of the core and a plane which is tangent to a specified envelope and perpendicular to the intended direction of flow The throw is generally referred to as the envelope corresponding to 0.25 m/s for zero supply temperature differential (i.e., under isothermal conditions).
Time Delay Usually refers to a device that will not allow the condenser to restart for an average of 5 minutes.
Ton A cooling unit of measure. Each ton equals 12,000 BTUH. Heat pumps and air conditioners are generally sized in tons. Typical sizes for single family residences are between two and five tons. It is important to note that actual capacity is not constant and will change based on outdoor or indoor temperatures. The published capacity rating of air conditioners and heat pumps is based on performance at the ARI standard temperature levels of 95 F outside, 80 F inside. The number of tons a system has is the total BTU capacity of the system. The size of the area to be cooled will determine the correct size of the system in tons.
Total air flow rate (QL)  Sum of the primary and secondary air flow rates which are moved in the treated space.

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Up flow Furnace A furnace that pulls return air in from the bottom and expels warm air from the top.
Valves  Components inserted into air ducts or devices which permit modification of the air resistance of the system and consequently a complete shut-off of the air flow (control valves).
VAV (Variable Air Volume) Variable Air Volume.
VFD (Variable Frequency Drive) Electronic speed control for motors.
W (Watt) A watt is a unit of electricity.
WB Wet Bulb
WC (Water Column) Common measure of air pressure used in HVAC systems.
Zoning A system in which living areas or groups of rooms are divided into separate spaces and each space's heating/air conditioning is controlled independently. This can be accomplished by using either multiple independent systems, or a single system using electronic controls and motorized dampers (see Damper).

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