Glossary of Applied Air HVAC Terminology

Absolute Pressure

The total of the indicated gage pressure plus the atmospheric pressure. Abbreviated "psia" for pounds per square inch absolute.

ACH, Air Changes Per Hour

The number of times that air in a building is completely replaced with outdoor air in one hour.


Act of combining substance with air.

AHU (Air Handling Unit)

The 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 conditioner

Equipment that conditions air by cleaning, cooling, heating, humidifying, or dehumidifying it. A term often applied to comfort cooling equipment.

Air conditioning

A process that maintains comfort conditions in a defined area.

Air Cooler

Mechanism designed to lower temperature of air passing through it.

Air Diffuser

Air distribution outlet or grille designed to direct airflow into desired patterns.

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 Handler

Fan/blower, filter and housing parts of a system.

Air Infiltration

Leakage of air into rooms through cracks, windows doors and other openings.

Air Sensor

A device that registers changes in air conditions such as pressure, velocity, temperature, or moisture content.

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 Turnover Rate

The amount of air required to completely recirculate the air in a room or building one time would equal an “air turnover rate” of 1.0.  This term is also called the “air change rate” in some cases.

Air Turnover Unit

Equipment typically located in the space to be conditioned that uses a high sidewall supply and low sidewall return concept to heat or cool the space.  These units are typically made up of several sections stacked vertically and can include filters, heating sections, cooling coil sections, and air distribution sections as required by the application.

Air Vent

A fitting used to vent air manually or automatically from a system.

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.


The volume of air moving through a blower or duct. Units of measure are cubic feet per minute (CFM), liters per second (LPS) or cubic meters per hour (m3h).


An instrument used to measure air velocities.


American National Standards Institute is a nonprofit organization that publishes safety standards for the HVAC industry.  These standards are used by other organizations and local code authorities to create test protocols that prove equipment can be operated safely when installed properly.

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.


A leading HVAC/R Association - American Society of Heating, Refrigerating and Air Conditioning Engineers -


American Society for Testing and Materials.

Atmospheric Pressure

The pressure exerted upon the earth's surface by the air because of the gravitational attraction of the earth. Standard atmosphere pressure at sea level is 14.7 pounds per square inch (psi). Measured with a barometer.

Automatic Control

Controls that react to a change in conditions to cause the condition to stabilize.

Automatic Expansion Valve

A refrigerant control valve that maintains a constant pressure in an evaporator.

Axial Fan

A device that propels air in an axial direction.