A substance with the ability to absorb another substance.
In physics, the taking up of light, heat, or other energy by molecules. The absorbed energy is converted into heat. Absorption in chemistry is the taking up of one substance by another. For example, a gas such as oxygen may be absorbed, or dissolved, in water. In the HVAC industry heat energy is absorbed from the medium being cooled and transferred in the refrigerant.
A shell device installed in the suction line of a HVAC system to prevent liquids from entering the compressor.
Activated Carbon
A processed carbon commonly used in air filters to clean the air.
Active Cooling
HVAC term for compressor driven air conditioning.


Air-Conditioning and Refrigeration Institute

AHRI is an industry trade association that develops standards for measuring and certifying product performance. For instance, ARI Standard 270 provides guidelines for establishing sound levels for outdoor air-conditioning equipment.

Air Balance
HVAC term for distributing air through a system to precisely match the required amount.
Air Cleaning
In HVAC an IAQ control strategy to remove various airborne particulates and/or gases from the air. The three types of air cleaning most commonly used are particulate filtration, electrostatic precipitation, and gas sorption.
Air Conditioner
A device used to control temperature and humidity of the air.
Air Conditioning
In HVAC the control of the quality, quantity, and temperature-humidity of the air in an interior space.
Air Diffuser
HVAC term for an air distribution outlet, typically located in the ceiling, which mixes conditioned air with room air.
Air Exchange Rate
In HVAC the rate at which outside air replaces indoor air in a space. Expressed in one of two ways: the number of changes of outside air per unit of time - air changes per hour (ACH); or the rate at which a volume of outside air enters per unit of time - cubic feet per minute (CFM).
Air Handler
HVAC term for a fan-blower, heat transfer coil, and housing parts of a system.
Air Handling Unit (AHU)
In HVAC refers to equipment that includes a blower or fan, heating and/or cooling coils, and related equipment such as controls, condensate drain pans, and air filters. Does not include ductwork, registers or grilles, or boilers and chillers.
Air Infiltration
The unwanted entrance of air due to leakage, temperature difference, or wind.
Air Passages
Openings through or within walls, through floors and ceilings, and around chimney flues and plumbing chases, that permit air to move out of the conditioned spaces of the building.
Air Retarder/Barrier
A material or structural element that inhibits air flow into and out of a building's envelope or shell. This is a continuous sheet composed of polyethylene, polypropylene, or extruded polystyrene. The sheet is wrapped around the outside of a house during construction to reduce air in-and exfiltration, yet allow water to easily diffuse through it.
Air Standard
HVAC term for air having a temperature of 68 degrees F (20 degrees C) and a relative humidity of 50% at 14.7 psia.
Air to Air
Where both the condensing and evaporating mediums are air.
Air Vent
HVAC term for a valve, either manual or automatic, that is used to remove unwanted air from the highest point of a piping system.
A substance capable of causing an allergic reaction because of an individual's sensitivity to that substance.
Ambient Air
The air external to a building or device.
The usually unfinished space above a ceiling and below a roof.
Attic Fan
A fan mounted on an attic wall used to exhaust warm attic air to the outside.
Attic Vent
HVAC term for a passive or mechanical device used to ventilate an attic space, primarily to reduce heat buildup and moisture condensation.
Available Heat
The amount of heat energy that may be converted into useful energy from a fuel.