HVAC Terms

The field of Heating, Ventilating & Air Conditioning, or HVAC, has it’s own technical language. In order to make it easier for you to understand the terms and acronyms we use, here are the most often used HVAC technical terms with their definitions. Feel free to ask us to help you understand any other technical words we use.


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

Air Handling Unit

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


Annual Fuel Utilization Efficiency: A measurement of the seasonal energy efficiency of gas heating equipment. It is the annual output energy of the equipment divided by its annual input energy, expressed in consistent units (i.e. Btu-out per Btu-in). Thus, the resultant value of AFUE is unitness. AFUE includes any input energy required by the pilot light but does not include any electrical energy or fans or pumps.


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

Balancing or Air 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.


British Thermal Unit/per hour. One of the two (watts-hours is the other) standard units of measure ( IP System) for the amount of energy consumed by a process, the amount of energy transferred from one location to another, or the amount of embodied energy (such as the heat contents of fuels). Specifically, it is the amount of energy required to raise the temperature of one pound of water by one degree Fahrenheit. The term ‘kbty’ stands for “kiloBtu” or 1,000 Btu. The term of ‘Mbtu’ stands for “MegaBtu” or 1,000,000 Btu. One Btu/his equal to 0.293 watt-hours (Wh).


The rate at which the heating or cooling load can be satisfied by a given equipment system designed to heat or cool and dehumidify a conditioned space or heat service hot water. Heating and cooling capacity is normally given on equipment nameplates in units of Btu/h. The air conditioning industry often uses units of “tons” to refer to equipment capacity. One ton of capacity equals 12,000 Btu/h.


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


A reciprocating or rotary pump for raising the pressure of air or another gas; this may be a single-stage or multistage unit. Reciprocating Compressor: a machine that compresses gases, composed of one or several cylinders; each cylinder contains a piston that is moved by a crankshaft through a connecting rod. Rotary Compressor: a machine having a rotating member that directly compresses fluid in an enclosed housing; the fluid pressure rises as the volume of the closed space decreases.


A heating or cooling element made of pipe or tubing, usually with plates or fins.

Condenser (Heat Exchanger)

The outside unit of a heating or air conditioning system. Here the refrigerant condenses from a gas to a liquid and hot or cold air from the building is released to the outside.


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


Conduits 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.

Energy Star®

A government supported branding 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.




A self-contained heating unit that is designed to deliver heated air to a home.


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

Heat Exchanger

1.  The part of a furnace that transfers heat from burning fuel to the air used to heat your home.  Also, from a boiler to water for hydronic heating.
2.  A device, such as a condenser or evaporator, in which heat is added or removed in order to heat or cool your home.

Heat pump

A heat pump is a heat transfer machine, collecting heat from one area (where it is not wanted) and depositing it in another (where it is wanted). Therefore, it can act as a heating as well as a cooling device.


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


Heating Season Performance Factor: A measurement of the seasonal efficiency of an electric heat pump using a standard heating load and outdoor climate profile over a standard heating season. It represents the total seasonal heating output in Btu divided by the total seasonal electric power input in watt-hours (Wh). Thus, the resultant value for HSPF has units of Btu/Wh.


Heating, ventilating, and air conditioning


Heating, ventilating, air conditioning, and refrigeration


Indoor air quality

Load Calculation

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.

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.


Preventive Maintenance Agreement, which provides regular maintenance of your heating and air conditioning system. 


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


A fluid that absorbs heat at low temperatures and rejects heat at higher temperatures.

Refrigerant Charge (or, “charging the refrigerant”)

The procedure an HVACR technician performs to ensure that the system has enough of the right kind refrigerant for peak operating performance.

Return, Return Air, Return Side

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 air flow and comfort.


(Seasonal) Energy Efficiency Ratio: A measurement of the instantaneous energy efficiency of cooling equipment. The is the new minimum efficiency standard (effective January 2006) for an air conditioner or heat pump is 13 SEER . All new units must now meet this standard. 

Is the steady-state rate of heat energy removal by the equipment in Btu/h divided by the steady-state rate of energy input to the equipment in watts. Thus, the resultant value for EER has units of Btu/Wh. The EER of a given system is always higher than the COP of that system by a factor equal to the number of Btu/h in a watt, or 3.413.

Sensible Heat

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

Split System

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 conditioned space).  Refrigerant lines and wiring connect them together.

Supply or Supply Side

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 air flow and comfort.


A control device that measures the temperature of the air in a home or the water in a hot water tank and activates the heating or cooling equipment to cause the air or water temperature to remain at a pre-specified value, normally called the set point temperature (see also the definitions of load and energy).


A term used primarily by the air conditioning industry to characterize the cooling capacity of air conditioning equipment. One ton equals 12,000 Btu/h.

Zones, Zoned System, Zoning

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

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