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Air Conditioning Glossary

Air Conditioning Glossary / Dictionary by Suttle Air


The Air Conditioning Contractors of America, is a national trade association that represents heating, ventilation, air conditioning, and refrigeration contractors. 


Equipment with a heating element and/or cooling coil and other components in a cabinet or casing. 


Air-Conditioning, Heating and Refrigeration Institute, is a non-profit, voluntary organization composed of heating, air conditioning and refrigeration manufacturers. AHRI publishes standards for testing and rating heat pumps and air conditioners.  


Normal atmospheric conditions of temperature and pressure.

AIR BALANCING or Balancing

 Adjusting an air conditioning system so that the right amount of air is delivered to the right places in your home in order to achieve the right heating or cooling effect. 

air changes per hour

The hourly ventilation rate is divided by the volume of a space. For perfectly mixed air or laminar flow spaces, this is equal to the number of times per hour that the volume of the space is exchanged by mechanical and natural ventilation. Also called air change rate or air exchange rate. Abbreviated ACH or ac/hr.

Air Conditioner

An appliance, system, or mechanism designed to dehumidify and extract heat from an area. Usually, this term is reserved for smaller self-contained units such as a residential system.


British Thermal Unit, the measurement of heating and air conditioning capacity. A BTU is the amount of heat that must be added to one pound of water to raise its temperature by one degree Fahrenheit. 


A refrigerant mixture of two or more refrigerants.

Bubble point

The pressure at which a refrigerant liquid starts to vaporize.


 Chlorofluorocarbons, used as a refrigerant in air conditioners and heat pumps, are linked to the depletion of the ozone layer. 


Coefficient of performance, is an efficiency ratio that compares the amount of heat delivered to the amount of energy used. As with MPG on a car, the higher the score the more energy efficient the equipment is. 

centrifugal fan

A centrifugal fan is a mechanical device for moving air or other gases.


A device that removes heat from a liquid via a vapor-compression or absorption refrigeration cycle. This cooled liquid flows through pipes in a building and passes through coils in air handlers, fan-coil units, or other systems, cooling and usually dehumidifying the air in the building. Chillers are of two types; air-cooled or water-cooled. Air-cooled chillers are usually outside and consist of condenser coils cooled by fan-driven air. Water-cooled chillers are usually inside a building, and heat from these chillers is carried by recirculating water to a heat sink such as an outdoor cooling tower.


Equipment that performs heat transfer to air when mounted inside an air handling unit or ductwork. It is heated or cooled by electrical means or by circulating liquid or steam within it.


A component in the basic refrigeration cycle that ejects or removes heat from the system. The condenser is the hot side of an air conditioner or heat pump. Condensers are heat exchangers and can transfer heat to air or to an intermediate fluid (such as water or an aqueous solution of ethylene glycol) to carry heat to a distant sink, such as ground (earth sink), a body of water, or air (as with cooling towers).

constant air volume

A system designed to provide constant airflow. This term is applied to HVAC systems that have a variable supply-air temperatures but constant air flow rates. Most residential forced-air systems are small CAV systems with on/off control. Abbreviated CAV.


A device that controls the operation of part or all of a system. It may simply turn a device on and off, or it may more subtly modulate the set point of components. Most controllers are automatic but have user input such as temperature set points, e.g. a thermostat. Controls may be analog or digital.


The final step in installing a heating or air conditioning system. Every component is checked and tested for compliance with codes, ACCA manuals, manufacturer requirements, and occupant needs. After commissioning, the technician will: provide documentation of testing, provide all equipment manuals, and show the homeowner how to operate the system. 

Carbon monoxide

An odorless, colorless, tasteless, poisonous gas that’s produced when fuels like natural gas burn with insufficient air. A CO leak in your home can be extremely dangerous.

Coil cleaning

A technology that utilizes an appropriate amount of water pressure to remove the dust and debris from evaporator and condenser coils.


 A device that removes excess moisture from the air. 


The U.S. Department of Energy, is the federal agency that sets industry efficiency standards. 


Conduits are used to carry air. They can be round or rectangular, sheet metal or fiberglass or vinyl tubes. In air conditioning systems they carry air from the home to the air conditioning system or furnace and back to the home. 


A modulating device for controlling airflow rates through ductwork.

deep lake water cooling

The heat is rejected to deep lake regions to cool homes and offices, reducing the energy costs.


(Also delta T) a reference to a temperature difference. It is used to describe the difference in temperature of a heating or cooling medium as it enters and as it leaves a system.


A diffuser is placed over ductwork, and it separates air with vanes going in differing directions. It evenly distributes air flow in the desired directions.

Dry Bulb Temperature

Dry bulb temperature is the temperature of air measured by a thermometer which is freely exposed to the air while it is shielded from radiation and moisture. It is usually thought of as air temperature, and it is the true thermodynamic temperature. It is a measurement of heat intensity independent of humidity and a dry bulb thermometer is used to measure it.

Dry Bulb Thermometer

A dry bulb thermometer is a device that measures air temperature independently of humidity. It is freely exposed to the air it is measuring and is protected from radiation and moisture.





A government-supported branding is used to identify energy-efficient products. The branding was developed by the US Department of Energy and the US Environmental Protection Agency. 


The U.S. Environmental Protection Agency.  


An HVAC component that uses outside air, under suitable climate conditions, to reduce required mechanical cooling. When the outside air’s enthalpy is less than the required supply air during a call for cooling, an economizer allows a building’s mechanical ventilation system to use up to the maximum amount of outside air.


For a given sample of air, a measure of the total heat content (the sum of the heat energy of the dry air and heat energy of the water vapor within it). It is typically used to determine the amount of fresh outside air that can be added to recirculated air for the lowest cooling cost.

A component in the basic refrigeration cycle that absorbs or adds heat to the system. Evaporators can be used to absorb heat from air or from a liquid. The evaporator is the cold side of an air conditioner or heat pump.

Evaporator coil

An evaporator coil is usually located indoors as part of a split unit. Inside the coil, refrigerant evaporates as it absorbs heat from the air that passes over it.


A transfer of fluid volume per unit of time.

Fresh Air Fan

Abbreviated FAF.

Fresh Air Fraction

Fresh air fraction quantifies the action of the economizer and is defined by (Mass flow rate of fresh-air / Total air flow rate). Abbreviated faf.

Fresh Air Intake

An opening through which outside air is drawn into the building. This may be to replace the air in the building that has been exhausted by the ventilation system or to provide fresh air for the combustion of fuel. Abbreviated FAI.


A component of an HVAC system that adds heat to air or an intermediate fluid by burning fuel (natural gas, oil, propane, butane, or other flammable substances) in a heat exchanger.


The HVAC filters used in your home’s heating and cooling system filter dust, pollen, pet dander, and debris out of the air.


A transfer of fluid volume per unit of time.


Gas Appliance Manufacturing Association is a national trade association serving the interests of manufacturers of gas, oil, and electric appliances and equipment, components, and related products used in residential, commercial, and industrial applications. 

Gas Furnace Heat Exchanger

A gas furnace heat exchanger is responsible for the transfer of heat from inside the furnace into the air outside the furnace. The duct system then transfers this exchanged air to different rooms in the building or space.


A facing across a duct opening, often rectangular in shape, containing multiple parallel slots through which air may be delivered or withdrawn from a ventilated space. The grille directs the airflow in a particular direction and prevents the passage of large items.

Gauge Pressure

The reading is taken from a gauge normally zeroed at atmospheric pressure so that pressures can also be read as negative when below atmospheric pressure.

Heat Transfer

Heat transfer describes the exchange of thermal energy between two interacting media passing through a heat exchanger, such as refrigerants, air, and water.


Heating, ventilation, and air conditioning is known as the technology of indoor and vehicular environmental comfort.

heating coil

A heating coil is the part of the system that conducts heat. It allows electricity to act as fire.

Heat Exchanger

A heat exchanger is the part of the system that transfers heat from the hot parts of the machine or a system to the cold parts of the machine or system.

heat gain

heat load

heat loss

Terms for the amount of cooling (heat gain) or heating (heat loss) needed to maintain desired temperatures and humidifies in controlled air. Regardless of how well-insulated and sealed a building is, buildings gain heat from sunlight, conduction through the walls, and internal heat sources such as people and electrical equipment. Buildings lose heat through conduction during cold weather. Engineers use heat load calculations to determine the HVAC needs of the space being cooled or heated.

Heat Transfer

Heat transfer happens when heat moves from one area to another. It is an important and vital step in the process of cooling a space.

hspf – (Heating Seasonal Performance Factor)

The Heating Seasonal Performance Factor is the measurement of heat efficiency over the period of a heating season.


 Hydrochlorofluorocarbons are used as a refrigerant in air conditioners and heat pumps. HCFCs were thought to contribute to the depletion of the earth’s ozone layer. 


 A single refrigeration system designed to provide both heating and cooling. Compare to a furnace and an air conditioner, separate units that only heat or cool. 


A Heat Recovery Ventilator is a machine that brings fresh air into a home through a process that preheats the air so it has less impact on your utility bill. 


Hydrofluorocarbon is used as a refrigerant in air conditioners and heat pumps. It has little or no effect on the ozone layer. 


Heating Seasonal Performance Factor, an equipment efficiency rating. As with MPG on a car, the higher the rating the more fuel efficient the equipment is. 


 A device that adds moisture to warm air for your home. HVAC Heating, ventilating, and air conditioning 


Heating, ventilating, air conditioning, and refrigeration

Industrial Refrigerator

An industrial refrigerator is a refrigeration equipment designed for low-temperature processing of food products by creating and maintaining inside the object a specified operating cooling mode, including temperature, humidity, and speed for the cooling environment, and sometimes may include pressure and gas composition.


Intermediate Fluid

A liquid or gas is used to transfer heat between two heat exchangers. An intermediate fluid is used when the hot and cold fluids are too bulky (such as air) or difficult to handle (such as halocarbon refrigerant) to directly transfer the heat.

Indoor Coil

The part of an air conditioner or heat pump that absorbs the heat from the air in your house.


When air enters your home through holes, gaps, and cracks, (e.g., plumbing or electrical holes, the heating and air conditioning system, doors, and windows).


Indoor air quality 


 The energy that suspends moisture vapor in the air. 


  1. Components are made of multiple smaller blades, sometimes adjustable, placed in ducts or duct entries to control the volume of airflow. When used inside of ducts, their function is similar to that of a damper, but they can be manufactured to fit larger openings than a single-piece damper.
  2. Blades in a rectangular frame are placed in doors or walls to permit the movement of air.


A mathematical determination of how much cooling and heating (BTUs) an HVAC system must deliver for occupant safety and comfort. It is based on a variety of factors: square footage, building orientation, number of occupants, size and placement of rooms, number and size of windows and doors, amount of insulation, number of floors, and climate. 

Mineral oil

A lubricant commonly used with HCFC and HFC refrigerants, with a low affinity for moisture.

Minimum outside air

The lowest amount of fresh air flow can be allowed into a recirculating system.


 An ACCA procedure covering the proper design, installation, maintenance, and repair of ductwork. 


An ACCA procedure covering the method for calculating heating and cooling requirements (load calculation) for single-family detached homes and mobile homes. 


An ACCA publication covering the design, installation, and commissioning of a residential HVAC system. 


 An air conditioner or heat pump system composed of equipment that has been certified by ARI to work together to deliver the specified heating and cooling capacity at the stated efficiency rating. 


A natural byproduct of the fungi family that thrives when organic substances and water combine under certain circumstances. Mold reproduces via spores that can remain dormant, yet viable, for years. Many molds are beneficial. For example, they are the “bleu” in bleu cheese, and we use them to make wine, penicillin, and antibiotics. However, some molds can cause health problems. 

Makeup Air Unit

An air handler that conditions 100% outside air. Typically, used in industrial or commercial settings, or in “once-through” (blower sections that only blow air one-way into the building), “low flow” (air handling systems that blow air at a low flow rate), or “primary-secondary” (air handling systems that have an air handler or rooftop unit connected to an add-on makeup unit or hood) commercial HVAC systems. Abbreviated MAU.

Minimum Outside Air

The lowest amount of fresh air flow can be allowed into a recirculating system. This limit is set to ensure that the interior air remains safe and comfortable to breathe.

Natural refrigerant

Non-fluorochemical refrigerants, such as ammonia (R717).

NATE Certification

North American Technician Excellence (NATE) conducts rigorous, independent testing verifying the real-world knowledge and application of HVAC technicians and installers.

Outside unit

The air conditioner’s outside portion is often referred to as the “condenser unit.” The outside unit’s job is to release the heat to the air outside.

Outside air damper

An automatic louver or damper that controls the fresh air flow into an air handler and modulates to the most energy-efficient setting.

Outside Air Temperature

A measure of the air temperature outside a building. The temperature and humidity of air inside and outside the building are used in enthalpy calculations to determine when outside air can be used for free heating or cooling. Abbreviated OAT.


Planned maintenance agreement, which provides regular maintenance of your HVACR system. Most ACCA member contractors offer PMAs, although they may use different names for them, such as Maintenance Inspection Agreements, Planned Service Agreements, Energy Service Plans, etc. 

Packaged Terminal Air Conditioner

An air conditioner and heater combined into a single, electrically powered unit, typically installed through a wall and often found in hotels. Abbreviated PTAC.


Polyalkylene glycol is a synthetic oil used in A/C systems to lubricate the compressor. Most all newer model cars use PAG as the lubricant in the A/C system.

Packaged unit

An air-handling unit, defined as either a “recirculating” or “once-through” design, is made specifically for outdoor installation. They most often include, internally, their own heating and cooling devices. Very common in some regions, particularly in single-story commercial buildings. Also called a rooftop unit (RTU)

Plenum Space

An enclosed space inside a building or other structure, used for airflow. Often refers to the space between a dropped ceiling and the structural ceiling, or a raised floor and the hard floor. Distinct from ductwork as a plenum is part of the structure itself. Cable and piping within a plenum must be properly rated for its fire and smoke indices. See also: plenum chamber


The study of the behavior of air-water vapor mixtures. Water vapor plays an important role in energy transfer and human comfort in HVAC design.

rooftop unit (RTU)

Same as Air handler.

radiant ceiling panels

Usually, metal panels are suspended under the ceiling, and insulated from the building structure. The primary cooling/heating agent temperature is close to the room’s temperature.

radiant floor

A type of radiant heating system where the building floor contains channels or tubes through which hot fluids such as air or water are circulated. The whole floor is evenly heated. Thus, the room is heated from the bottom up. Radiant floor heating eliminates the draft and dust problems associated with forced air heating systems.


The transfer of heat directly from one surface to another (without heating the intermediate air acting as a transfer mechanism).


A chemical that produces a cooling effect while expanding or vaporizing.

Refrigerant charge

The amount of refrigerant in kg in a closed system.


A refrigerant containing chlorine is used in air conditioning systems. The EPA has mandated that R-22 cannot be manufactured after 2010 because it has been linked to the depletion of the ozone layer and global warming. Most commonly referred to by its trademarked name, Freon. 


The refrigerant replaces R-22. It does not contain chlorine and is not hazardous to the environment. 


(RH) The percent of moisture actually in the air compared to the maximum amount of moisture the air can hold at that temperature. 


The path the air takes to get to an air-handling unit or furnace so it can be cooled or heated. It is the “return” path. The return side should be “balanced” with the supply side to ensure proper airflow and comfort. 


The SEER (Seasonal Energy Efficiency Ratio) rating of a unit is the cooling output during a typical cooling season divided by the total electric energy input during the same period. The higher the unit’s SEER rating the more energy efficient it is.



By January 1, 2023, SEER2 product regulations will be in full effect. With new testing procedures, limited sell-through opportunities and a 1% SEER increase for northern and southern regions, it’s important you stay up to date with your territory’s guidelines.

smoke damper

A damper or adjustable louver is designed to augment the ventilation of a space during a fire.

split system

A split system is the combination of an outdoor unit and an indoor unit. This is the most common type of system.


The number of degrees a vapor is above its boiling point at a specific pressure.


The condition where liquid refrigerant is colder than the minimum temperature required to keep it from boiling would change it from a liquid to a gas phase. Subcooling is the difference between its saturation temperature and the actual liquid refrigerant temperature.


The general term is used to refer to the set or a subset of components that perform a specific HVAC function within a building.


Seasonal Energy Efficiency Ratio, is an equipment efficiency rating that measures how much energy it takes to cool the air. As with MPG on a car, the higher the number the more efficient the unit. 


The temperature of the air. This type of heat is measured with a thermometer. 


 A two-component heating and cooling (heat pump) or cooling-only (air conditioner) system. The condensing unit is installed outside, the air handling unit is installed inside (preferably in a conditioned space). Refrigerant lines and wiring connect them together. 


 The part of an HVAC system that takes (supplies) the conditioned air from the air handling unit or furnace to your home. The supply side should be “balanced” with the return side to ensure proper airflow and comfort. 

Scroll Compressor

A specially designed compressor that works in a circular motion.

Terminal Unit

A small component that contains a heating coil, cooling coil, automatic damper, or some combination of the three. Used to control the temperature of a single room. Abbreviated TU.

Thermal Zone

An individual space or group of neighboring indoor spaces that the HVAC designer expects will have similar thermal loads. Building codes may require zoning to save energy in commercial buildings. Zones are defined in the building to reduce the number of HVAC subsystems, and thus initial cost. For example, for perimeter offices, rather than one zone for each office, all offices facing west can be combined into one zone. Small residences typically have only one conditioned thermal zone, plus unconditioned spaces such as garages, attics, and crawlspaces, and basements.


A thermostat is a system that monitors and regulates a heating or cooling system. It can be used to set the desired temperature at which it keeps the environment either heated or cooled.

Two-Stage (cooling and heating)

A two-stage air conditioner is designed to operate on high and low settings during different weather conditions and seasons. The high setting is used during extreme weather, and the low setting is used during moderate weather. This type of air conditioner produces a balanced temperature and is in use for a longer period of time.

TXV – Thermostatic Expansion Valve

A thermostatic expansion valve is a piece of equipment that meters the flow of liquid refrigerant into the evaporator while measuring the vapor refrigerant leaving the evaporator. It thereby controls the superheating at the outlet of the evaporator.

Thermal zone

An individual space or group of neighboring indoor spaces that the HVAC designer expects will have similar thermal loads.

underfloor air distribution

A method for providing ventilation and space conditioning is by using the air plenum below a raised floor to distribute conditioned air through diffusers directly to the occupied zone. Abbreviated UFAD.

Unitary Controller

A unitary controller is a device that controls only one zone in a building.

Upflow unit

A type of air conditioning system that discharges air into the conditioned space via a top-mounted discharge plenum.

Underfloor air distribution

A method for providing ventilation and space conditioning is by using the air plenum below a raised floor.

Vacuum test

A test to check the gas tightness of a refrigeration system before charging it with refrigerant

variable air volume

An HVAC system that has a stable supply-air temperature, and varies the air flow rate to meet the temperature requirements. Compared to constant air volume systems, these systems conserve energy through lower fan speeds during times of lower temperature control demand. Most new commercial buildings have VAV systems. VAVs may be bypass-type or pressure-dependent. Pressure-dependent type VAVs save energy while both types help in maintaining the temperature of the zone that it feeds. Abbreviated VAV.


A ventilator is a device that replaces stale, recirculated air with fresh, filtered outdoor air.

Water-Cooled System

A type of refrigerant-based air conditioning system that uses water as a condensing medium.


A watt is a derived unit of power in the International System of Units (SI).


A single HVAC system that can meet different heating and cooling needs in different areas (zones). Each zone of a home has its own thermostat with which it can regulate the temperature and humidity in its area. One “zoned air conditioner” could be set for a high temperature in one zone and for a lower temperature in the other zone. Zone systems have two or more zones.