Please refer to this HVAC glossary when you find terms on our webpage that are not commonly used every day.

Active Cooling
HVAC term for compressor driven air conditioning.
Air Balance
HVAC term for distributing air quality through a system to precisely match the required amount.
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 coils, 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.
A substance capable of causing an allergic reaction because of an individual’s sensitivity to that substance.
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.
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.
Ceiling Plenum
Space below the flooring and above the suspended ceiling that accomodates the mechanical and electrical equipment and 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. 1 CFM equals approximately 2 liters per second (l/s).
Combustion Air
Air that provides the necessary oxygen for complete, clean combustion and maximum heating value.
The device in an air conditioner or heat pump in which the refrigerant condenses from a gas to a liquid when it is de-pressurized or cooled.
Condensor 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.
Conditioned Air
Air that has been heated, cooled, humidified or de-humidified to maintain an interior space within the “comfort zone” (sometimes referred to as “tempered” air).
Conditioned Space
The interior space of a building that is heated or cooled.
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 adjusted or part of an automated control system.
Defrost Cycle
Part of the refrigeration cycle in which evaporator frost and ice accumulation is melted. The use of electric heat strips or hot gas is the most common form. The defrost cycle also flushes any oil that is trapped in the evaporator back to the compressor. There should be a minimum of four defrost cycles per 24 hours to help with oil return.
Demand (tankless) Water Heater
A type of water heater that has no storage tank thus eliminating storage tank stand-by losses. Cold water travels through a pipe into the unit, and either a gas burner or an electric element heats the water only when needed.
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.
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.
Exhaust Ventilation
HVAC term for mechanical removal of air from a portion of a building (e.g., piece of equipment, room or general area).
Filter (air)
A device that removes contaminants, by mechanical filtration, from the fresh air stream before it 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 chlorofluorocarbon (CFC) gas that is highly stable and that has been historically used as a refrigerant.
Furnace (Residential)
In HVAC a combustion heating appliance in which heat is captured from the burning of a fuel for distribution, comprised mainly of a combustion chamber and heat exchanger.
Heat Loss
The heat that flows from the building interior, through the building envelope to the outside environment.
Heat Register
The grilled opening into a room by which the amount of warm air from a furnace can be directed or controlled; may include a damper.
Heating Load
The rate of heat flow required to maintain a specific indoor temperature; usually measured in BTU per hour.
HVAC is an acronym for heating, ventilation and air conditioning system.
Indoor Air
The air the people breathe inside a built environment.
Indoor Air Pollutant
Particles and dust, mists, bi-aerosals and gases or vapors.
Air leakage inward through cracks and interstices and through ceilings, floors and walls of a space or building.
Minimum Efficiency Reporting Value, is a measurement scale designed to rate the effectiveness of air filters. For the particles in the range of 0.3 to 10 micrometres the MERV rating is from 1 to 16. Higher MERV ratings correspond to a greater percentage of particles captured on each pass, with a MERV 16 filter capturing more that 95% of particles over the full range. For more information, go to .
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 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”.
HVAC term for air compartment to a duct or ducts.
Positive Pressures
Condition that exists when more air is supplied to a space than is exhausted, so the air pressure within that space is greater than that in surrounding areas. Under this condition, if an opening exists, air will flow from the positively pressurized space into surrounding areas.
Pressure, Static
In flowing air, the total pressure minus velocity pressure. The portion of the pressure that pushes equally in all directions.
Preventative Maintenance
Regular and systematic inspection,cleaning and replacement of worn parts, materials and systems. Preventative maintenance helps to prevent parts, materials and systems failure by ensuring that parts, materials and systems are in good working order.
Programmable Thermostat
A type of thermostat that allows the user to program into the devices’ memory a pre-set schedule of times (when certain temperatures occur) to turn on HVAC equipment.
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.
Recirculated Air
Air removed from the conditioned space and used for ventilation, heating, cooling, humidification or de-humidification.
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.
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.
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.
Tankless Water Heater
A water heater that heats water before it is directly distributed for end use as required; a demand water heater.
Temperature Zones
In HVAC individual rooms or zones in a building where temperature is controlled seperately 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.
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 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 combination of rooms in a structure according to similiar heating and cooling patterns. Zoning requires using more than one thermostat to control heating, cooling and ventilation equipment.