Terminology 
Confused by the many terms and acronyms surrounding your indoor comfort system? Don't be! The HVAC industry is diverse, and because of its diversity and size it has a language all its own. This section provides some explanation for most of the commonly used terms and acronyms.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
A
AIR HANDLING UNIT
Equipment with a heating element and/or cooling coil and other components in a cabinet or casing.
B
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.
BTU
British Thermal Unit, the measurement of heating and air conditioning capacity. A BTU is the amount of heat that must be added to one pound of water to raise its temperature one degree Fahrenheit.
C
CFC
Chlorofluorocarbons, used as a refrigerant in air conditioners and heat pumps, linked to the depletion of the ozone layer.
COIL
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.
D
DEHUMIDIFIER
A device that removes excess moisture from the air.
DUCT
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.
E
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.
F
FURNACE
A self-contained heating unit that is designed to deliver heated air to a home.
H
HCFC
Hydro chlorofluorocarbons, used as a refrigerant in air conditioners and heat pumps. HCFC's 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 single refrigeration system designed to provide both heating and cooling. Compare to a furnace and an air conditioner, separate units that only heat or cool.
HFC
Hydroflorocarbon, used as a refrigerant in air conditioners and heat pumps. It has little or no effect on the ozone layer.
HUMIDIFIER
A device that adds moisture to warm air for your home.
HVAC
Heating, ventilating, and air conditioning
I
INFILTRATION
Air that enters your home through holes, gaps, and cracks, (e.g., plumbing or electrical holes, the heating and air conditioning system, doors, and windows).
IAQ
Indoor air quality
L
LATENT HEAT
The energy that suspends moisture vapor in the air.
LOAD CALCULATION
A mathematical determination of how much cooling and heating (BTU's) 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.
M
MATCHED SYSTEM
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.
MOLD
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.
N
NATE
North American Technician Excellence, the nonprofit organization that tests and certifies HVACR technicians.
P
PMA
Preventive maintenance agreement, which provides regular maintenance of your HVAC system.
R
R-22
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.
R-410A
The refrigerant that replaces R-22. It does not contain chlorine and is not hazardous to the environment.
REFRIGERANT
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.
RELATIVE HUMIDITY (RH)
The percent of moisture actually in the air compared to the maximum amount of moisture the air can hold at that temperature.
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.
S
SEER
Seasonal Energy Efficiency Ratio, an equipment efficiency rating that measures how much energy it takes to cool the air. As with MPG on a car, the higher the number the more efficient the unit.
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.
Z
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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