Ultimate List of HVAC Terminology


There are lots of fancy terms associated with your air conditioning system. Some seem pretty obvious, but others may be confusing for people who aren’t air conditioner professionals. We’ve compiled a list of the most common air conditioning terms you’re likely to run into while shopping for a new unit or having your air conditioner serviced.


A-coil — See evaporator coil.

accumulator — A device installed in the suction line of an air conditioner to prevent liquids from entering the compressor.

air balance — A term describing the distribution of air in an HVAC system. A balanced system has enough air distributed to match the amount required by all parts of the system at any given time.

air changes per hour — The number of times the entire volume of air in a building is replaced via natural ventilation or air conditioning during one hour.

air diffuser — A distribution point where conditioned air is mixed with room air. Typically these are covered with some type of grille and located in the floor or ceiling.

air exchange rate — A measure of how quickly the air indoors is replaced by outside or conditioned air. The air exchange rate is often measured in air changes per hour.

air flow — The free movement of air.

air flow volume — The amount of air your air conditioner circulates through your home, typically measured in cubic feet per minute.

air handler — A moving part inside your home that propels air through the ductwork. A furnace or blower coil often serves this function.

air handling unit — See Air Handler.

air infiltration — The unintended entrance of air into a space via leakage, temperature differentials or wind.

air passages — Spaces through or within walls, floors and ceilings that allow conditioned air to move out of the structure.

all-in-one system — See Packaged Unit.

ambient air — Outdoor, unconditioned air.


blower — See Air Handler.

British thermal unit (BTU) — A measure of heat equivalent to the amount of energy required to raise one pound of water one degree Fahrenheit. The ability of an air conditioner to extract heat is often expressed in BTUs.

British thermal units per hour (BTUH) — The number of BTUs per hour that an air conditioner can extract from a conditioned area.

building envelope — All the elements of the building that enclose the interior space, including walls, windows, doors and other materials.


capacity — The ability of an air conditioner to remove heat from an area.

charge — See Refrigerant Charge.

coil — See Condenser Coil.
compressor — The component of an air conditioner system that pressurizes the refrigerant, allowing it to flow through the system.

condensate pan — A pan located below the evaporator coil that catches the excess moisture that drops from conditioned air.

condenser — A unit that houses all the parts required to remove heat from the system. It is often placed outdoors.

condenser coil — The specific area inside an air conditioner where refrigerant is condensed into a liquid, releasing heat from the system.

condenser fan — The fan that circulates air over the condenser coil.

condensing unit — See Condenser.

conditioned air — Air that has been circulated through an air conditioner to remove heat and humidity.

conditioned space — The area of a home where an air conditioner cools the air.

constant air volume system — An air conditioning system designed to provide constant air flow.

controller — A device controlling the air conditioner, such as a thermostat.

crankcase heater — A small heater installed on compressors to boil off liquid refrigerant that has combined with compressor oil. Not all air conditioners require a crankcase heater, but most heat pumps do.

cycling — The process of an air conditioner turning on, processing some amount of air, and turning off.


damper — A physical plate that controls airflow through a duct or air diffuser.

dehumidifier — A device separate from an air conditioner that removes excess moisture from the air.

diffuser — See Air Diffuser.

drain pan — See Condensate Pan.

dry-charged unit — An air conditioner that is shipped without any refrigerant and charged after installation.

dual duct system — A climate control system with two ducts, one containing heated air and the other containing cooled air. Specific room temperatures are achieved by mixing air from both ducts.

duct fan — A fan mounted inside the duct in order to increase conditioned air movement.

duct — A section of specially designed metal, fiberglass or plastic tubing that connects the air handler to the air diffusers throughout your home.

ductwork — A series of ducts that make up an entire household system.


Energy Star — A designation given by the Environmental Protection Agency to high-efficiency air conditioners that perform above and beyond the government’s minimum standards.

evaporator coil — An a-shaped coil located inside the furnace or air handler that absorbs the heat from air passing over it.

expansion valve — A valve that deals out small amounts of refrigerants, allowing them to expand into a gas while releasing heat.


Freon — A refrigerant composed of a stable chlorofluorocarbon gas.


heat gain — The amount of cooled air required to maintain a desired indoor temperature, despite heat collected by the structure through external and internal sources.

heat pump — An outdoor unit that can both heat and cool indoor air in mild climates.

heat register — See Air Diffuser.

horizontal flow — An air handler installed horizontally so that air moves from one side to the other and out into the ductwork. Horizontal flow units are typically installed in attics or crawl spaces.

HVAC — An abbreviation meaning “heating, ventilation and air conditioning.”


indoor coil — See Evaporator Coil.

indoor / outdoor system — See Split System Air Conditioner


matched system — A system where all parts are matched for capacity and efficiency, creating a system that performs more efficiently, longer.

minimum outside air — A low limit on the amount of fresh air allowed into an air conditioning system.


outdoor coil — See Condenser.

outdoor unit — See Condenser.


packaged system — See Packaged Unit.

packaged unit — A unit that houses all the parts of your air conditioner. They’re typically installed on top of or beside a home.

plenum space — An air compartment that is part of the building’s structure and is connected to the ductwork.

programmable thermostat — A thermostat that has the ability to change the temperature in your home without your input, based on a pre-designed program of your choosing.


R-22 refrigerant — The old standard for refrigerant. This type of refrigerant is being phased out in favor of R-410A.

R-410A refrigerant — A more environmentally friendly, chlorine-free refrigerant.

refrigerant charge — The act of adding refrigerant to a closed air conditioning system.

refrigerant lines — The copper tubing that connects the condensing unit to the evaporator coil.

refrigeration capacity — A measure of the cooling capacity of an air conditioner in BTUs or Tons.

return air — The air that is returned to the evaporator coil from inside the building.

return duct — The duct through which warm air is drawn into the air conditioning system to be cooled and blown out through the air diffusers.

rooftop unit (RTU) — See Packaged Unit.


Seasonal Energy Efficiency Ratio (SEER) — A measure of efficiency that describes the average number of BTUs of cooling created for every watt-hour of electricity used by the air conditioning unit.

self-contained system — See Packaged Unit.

setback thermostat — See Programmable Thermostat.

single package — See Packaged Unit.

split system air conditioner — An air conditioner system where the compressor unit is located outside and the evaporator coil is located inside the home.

supply duct — The duct that returns conditioned air to individual rooms through the air diffusers.


temperature zones — Individual areas in a building where temperature can be controlled independently of other areas.

thermidistat — A unit that monitors both temperature and humidity and adjusts your cooling unit accordingly to maintain pre-determined levels.

thermostat — A small device typically located on a central wall that is used to control the indoor temperature.

ton — A unit of measurement for air conditioner units equivalent to 12,000 BTUs per hour.

two-stage cooling — An efficient method of cooling with two levels of operation — an energy saving level for most days and a higher energy-consumption level for very warm days.


variable speed motor — The fan motor used to drive a two-stage cooling system.

vent — See Air Diffuser.


zone — See Temperature Zone.

zoning — An air conditioning system dividing the home into various areas to increase efficiency. For example — one zone is designated for the first floor and another for the second floor of a home.

Precision August 2, 2017

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