For those who are new to the world of commercial HVAC maintenance, there are several terms that you are going to run into on a regular basis. In this post, we’ll take a look at some of these terms that describe various HVAC system types.

A shell device installed in the suction line of a HVAC system to prevent liquids from entering the compressor.

An agent added to methyl chloride to make you aware of refrigerant leaks.

Activated Alumina
A type of aluminum oxide that absorbs moisture (used in refrigerant driers).

Activated Carbon
A processed carbon used in filter driers and commonly used in air filters to clean the air.

Active Cooling
HVAC term for compressor driven air conditioning.

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.

Ambient Air
The air external to a building or device.

In HVAC the device in an air conditioner that distributes the filtered air from the return duct over the coil/heat exchanger. This circulated air is cooled/heated and then sent through the supply duct, past dampers, and through supply diffusers to the living/working space.

A vessel or tank where heat produced from the combustion of fuels such as natural gas, fuel oil, or coal is used to generate hot water or steam for applications ranging from building space heating to electric power production or industrial process heat.

Boiler Pressure
The pressure of the steam or water in a boiler as measured, usually expressed in pounds per square inch gauge (psig).

Boiler Rating
The heating capacity of a steam boiler expressed in BTU per hour (BTU/H), horsepower, or pounds of steam per hour.

British Thermal Unit (BTU)
The amount of heat required to raise the temperature of one pound of water one degree Fahrenheit, equal to 252 calories.

Building Envelope
Elements of the building, including all external building materials, windows, and walls, that enclose the internal space.

Burner Capacity
The maximum heat output (in BTU per hour) released by a burner with a stable flame and satisfactory combustion.

Carbon Monoxide (CO)
A colorless, odorless but poisonous, combustible gas with the formula CO. Carbon monoxide is produced in the incomplete combustion of carbon and carbon compounds such as fossil fuels (i.e. coal, petroleum) and their products (e.g. liquefied petroleum gas, gasoline), and biomass.

Ceiling Plenum
Space below the flooring and above the suspended ceiling that accommodates the mechanical and electrical equipment that is used as part of the air distribution system. The space is kept under negative pressure.

Central Heating System
In HVAC, a system where heat is supplied to areas of a building from a single appliance through a network of ducts or pipes.

CFM Cubic feet per minute
HVAC term for the amount of air, in cubic feet, that flows through a given space in one minute. One CFM equals approximately two liters per second (I/s).

A device used to compress air for mechanical or electrical power production, air conditioners, heat pumps and refrigerators, to pressurize the refrigerant and enabling it to flow through the system.

The device in an air conditioner or heat pump in which the refrigerant condenses from a gas to a liquid when it is depressurized or cooled.

Condenser Coil
The device in an air conditioner or heat pump through which the refrigerant is circulated and releases heat to the surroundings when a fan blows outside air over the coils. This will return the hot vapor that entered the coil into a hot liquid upon exiting the coil.

Condensing Unit
The component of a central air conditioner that is designed to remove heat absorbed by the refrigerant and transfer it outside the conditioned space.

Conditioned Air
Air that has been heated, cooled, humidified, or dehumidified to maintain an interior space within the “comfort zone.” (Sometimes referred to as “tempered” air.)

Constant Air Volume Systems
Air handling system that provides a constant air flow while varying the temperature to meet heating and cooling needs.

Cooling Capacity
The quantity of heat that a cooling appliance is capable of removing from a room in one hour.

HVAC term for controls that vary airflow through an air outlet, inlet, or duct. A damper position may be immovable, manually adjustable, or part of an automated control system.

Diffusers and Grilles
Components of the ventilation system that distribute and return air to promote air circulation in the occupied space. As used in this document, supply air enters a space through a diffuser or vent and return air leaves a space through a grille.

Dual Duct System
An air conditioning system that has two ducts, one is heated and the other is cooled, so that air of the correct temperature is provided by mixing varying amounts of air from each duct.

Duct Fan
HVAC term for an axial flow fan mounted in a section of duct to move conditioned air.

The round or rectangular tube(s), generally constructed of sheet metal, fiberglass board, or a flexible plastic-and-wire composite, located within a wall, floor, and ceiling that distributes heated or cooled air in buildings.

Environmental Agents
Conditions other than indoor air contaminants that cause stress, comfort, and/or health problems (e.g., humidity extremes, drafts, lack of air circulation, noise, and over-crowding).

Filter (air)
A device that removes contaminants, by mechanical filtration, from the fresh air stream before the air enters the living space. Filters can be installed as part of a heating/cooling system through which air flows for the purpose of removing particulates before or after the air enters the mechanical components.

Forced Air System or Furnace
HVAC term for a type of heating system in which heated air is blown by a fan through air channels or ducts to rooms.

A registered trademark for a cholorfluorocarbon (CFC) gas that is highly stable and that has been historically used as a refrigerant.

A form of thermal energy resulting from combustion, chemical reaction, friction, or movement of electricity. As a thermodynamic condition, heat, at a constant pressure, is equal to internal or intrinsic energy plus pressure times volume.

Heat Loss
The heat that flows from the building interior, through the building envelope, to the outside environment.

Heating Capacity (Also Specific Heat)
The quantity of heat necessary to raise the temperature of a specific mass of a substance by one degree.

Heating Load
The rate of heat flow required to maintain a specific indoor temperature; usually measured in BTU per hour.

A measure of the moisture content of air; may be expressed as absolute, mixing ratio, saturation deficit, relative, or specific.

HVAC is an acronym for heating, ventilation, and air-conditioning system.

Indoor air pollution.

Indoor air quality.

Indoor Air
The air that people breathe inside a built environment.

Indoor Air Pollutant
Particles and dust, fibers, mists, bio-aerosols, and gases or vapors.

Air leakage inward through cracks, ceilings, floors, and walls of a space or building.

Integrated Heating Systems
HVAC term for a type of heating appliance that performs more than one function, for example space and water heating.

An electrically charged atom or group of atoms that has lost or gained electrons; a loss makes the resulting particle positively charged; a gain makes the particle negatively charged.

A device that removes airborne particles from breathable air. Negative ions are produced and give up their negative charge to the particles. These new negative particles are then attracted to the positive particles surrounding them. This accumulation process continues until the particles become heavy enough to fall to the ground.

Natural Ventilation
In HVAC, the movement of outdoor air into a space through intentionally provided openings, such as windows and doors, or through non-powered ventilators, or by infiltration.

Negative Pressure
Condition that exists when less air is supplied to a space than is exhausted from the space, so the air pressure within that space is less than that in surrounding areas. Under this condition, if an opening exists, air will flow from surrounding areas into the negatively pressurized space.

Outdoor Air
Air taken from the external atmosphere and, therefore, not previously circulated through the system.

Outdoor Air Supply
HVAC term for air brought into a building from the outdoors (often through the ventilation system) that has not been previously circulated through the system. Also known as “Make-Up Air”.

A measure of the capacity of a material to resist heat transfer. The R-Value is the reciprocal of the conductivity of a material (U-Value). The larger the R-Value of a material, the greater its insulating properties.

Re-circulated Air
Air removed from the conditioned space and used for ventilation, heating, cooling, humidification, or dehumidification.

The compound (working fluid) used in air conditioners, heat pumps, and refrigerators to transfer heat into or out of an interior space. This fluid boils at a very low temperature enabling it to evaporate and absorb heat.

The process of the absorption of heat from one location and its transfer to another for rejection or recuperation.

Relative Humidity
A measure of the percent of moisture actually in the air compared with what would be in it if it were fully saturated at that temperature. When the air is fully saturated, its relative humidity is 100 percent.

Return Air
Air that is returned to a heating or cooling appliance from a heated or cooled space.

Return Duct
The central heating or cooling system contains a fan that gets its air supply through these ducts, which ideally should be installed in every room of the house. The air from a room will move towards the lower pressure of the return duct.

Seasonal Energy Efficiency Ratio (SEER)
A measure of seasonal or annual efficiency of a central air conditioner or air conditioning heat pump. It takes into account the variations in temperature that can occur within a season and is the average number of BTU of cooling delivered for every watt-hour of electricity used by the heat pump over a cooling season.

Setback Thermostat
A thermostat that can be set to automatically lower temperatures in an unoccupied area and raise them again before the occupant returns.

Split System Air Conditioner
HVAC term for an air conditioning system that comes in two to five pieces: one piece contains the compressor, condenser, and a fan; the others have an evaporator and a fan. The condenser, installed outside the building, connects to several evaporators, one in each room to be cooled, mounted inside the building. Each evaporator is individually controlled, allowing different rooms or zones to be cooled to varying degrees.

Static Pressure
Condition that exists when an equal amount of air is supplied to and exhausted from
a space. At static pressure, equilibrium has been reached.

Supply Duct
HVAC term for the duct(s) of a forced air heating/cooling system through which heated or cooled air is supplied to rooms by the action of the fan of the central heating or cooling unit.

Temperature Zone
In HVAC, individual rooms or zones in a building where temperature is controlled separately from other rooms or zones.

A unit of heat containing 100,000 British thermal units (BTU).

Ton (Air Conditioning)
A unit of air cooling capacity; 12,000 BTU per hour.

Unit Ventilator
HVAC term for a fan-coil unit package device for applications in which the use of outdoor- and return-air mixing is intended to satisfy tempering requirements and ventilation needs.

Variable Air Volume System (VAV)
Air handling system that conditions the air to constant temperature and varies the outside airflow to ensure thermal comfort.

A component of a heating or ventilation appliance used to conduct fresh air into, or waste air or combustion gases out of, an interior space.

Vent Damper
HVAC term for a device mounted in the vent connector that closes the vent when the heating unit is not firing. This traps heat inside the heating system and house rather than letting it draft up and out of the vent system.

Vent Pipe
A tube in which combustion gases from a combustion appliance are vented out of the appliance to the outdoors.

The process of moving air (changing) into and out of an interior space either by natural or mechanically induced (forced) means.

Ventilation Air
Defined as the total air, which is a combination of the air brought inside from outdoors and the air that is being re-circulated within the building. Sometimes, however, used in reference only to the air brought into the system from the outdoors; this document defines this air as “outdoor air ventilation.”

Ventilation Rate
The rate at which indoor air enters and leaves a building. Expressed in one of two ways: the number of changes of outdoor air per unit of time (air changes per hour, or “ach”) or the rate at which a volume of outdoor air enters per unit of time (cubic feet per minute, or “cfm”).

In HVAC, caulking and weather-stripping to reduce air infiltration and exfiltration into/out of a building.

A material used to seal gaps around windows and exterior doors.

In HVAC, an area within the interior space of a building, such as an individual room(s), to be cooled, heated, or ventilated. A zone has its own thermostat to control the flow of conditioned air into the space.

The combining of rooms in a structure according to similar heating and cooling patterns. Zoning requires using more than one thermostat to control heating, cooling, and ventilation equipment.


For a more comprehensive glossary of commercial HVAC maintenance terms, we’d recommend that you review this comprehensive collection of HVAC definitions compiled by the HVAC Source.

Have a question about a term that you didn’t see listed in our post or the glossary? Post a comment below and we’ll be happy to get back to you with an answer.

Does your company need assistance with commercial HVAC maintenance? Don’t hesitate to contact CLS Facilities Services at 800-548-3542 for all of your facility management solutions.