HVAC Glossary of Terms


Absolute Humidity:

The amount of water vapor per square foot.

Absorption Cycle:

Absorption cycle utilizes a thermal or chemical process to produce the refrigeration effect.


An accumeter is a flow control device within a carrier centrifugal chiller. It feeds the cooler with liquid refrigerant, which has a greater cooling capacity than a gas refrigerant.


An accurator is a metering device that provides the correct amount of liquid refrigerant into the evaporator.

Air Change:

The volume of air necessary to completely replace the air in an enclosed space.

Air Conditioner:

Equipment that controls air temperature, relative humidity, purity, and motion.

Air Cooled:

Removes indoor heat by moving heat from a condenser coil to the outdoors with the use of a fan.

Air-Cooled System:

Utilizes refrigerant and air as a condensing medium.

Air Diffuser:

Equipment that directs airflow into a desired pattern.

Air Flow:

Movement of air.

Air Handler:

The part of a central air conditioner that moves cooled air through the duct system.

Annual Fuel Utilization Efficiency (AFUE):

A rating that measures the amount of heat per dollar that the heating equipment provides. This rating is calculated in accordance with the Department of Energy test procedures.

ARI (Air-Conditioning and Refrigeration Institute):

Non-profit organization for heating, air conditioning, and refrigeration manufacturers. They publish standards for testing and rating heat pumps and air conditioners, standardizing a measure of comparison.

Atmospheric Pressure:

The measure in weight of a 1 square inch column of the atmosphere.

Auto Changeover:

Automatic switch to a back up air conditioning system in the event of a failure.


Balance Point:

Between 30 and 40 degrees, an outdoor temperature that equals the heating needs of the home without the need for supplemental electric resistance heat.


Used to open or close a switch or valve, bimetal is comprised of two metals with different rates of expansion that are secured together. When heated or cooled, they will warp and open the valve/switch.


Device that moves air in a distribution system.

Boiling Point:

Temperature in which a liquid will turn to a vapor.

BTU (British Thermal Unit):

The rate that measures the amount of heat needed to raise the temperature of a pound of water by one degree Fahrenheit.


The rate of measurement that raises the temperature of one pound of water by one degree Fahrenheit per hour.



The amount of space a heating or cooling system can heat or cool. Heating ability is expressed in BTUs and cooling ability is expressed in tons.

Carbon Monoxide:

Colorless, odorless gas produced from burning any fuel.

Central Air Conditioner System:

Air is cooled at a central point and distributed through fans and ducts.

Centrifugal Compressor:

A compressor that uses a rotating impeller to compress the refrigerant vapor. The vapor is drawn into the impeller axially, energy is added to the vapor, and then it is discharged.

Centigrade (Measure of Temperature):

A temperature scale that measures the boiling point at 100 degrees and the freezing point at 0 degrees (sea level).

CFM (Cubic Feet per Minute):

Cubic feet per minute - the measurement for the rate of air flow in an air conditioner.


The charge is the total refrigerant in a system.

Chilled Water System:

Utilizes a separate chiller that houses the refrigerant. It cools the water and pipes it to the air conditioner.

Comfort Air Conditioning:

Designed for the comfort of people. People generate humid heat. Comfort air conditioning systems remove 60-70% dry/sensible heat and 30-40% of humidity.


Cycles refrigerant from the indoor evaporator to the outdoor condenser and back to the evaporator.


Mechanically reduces the volume of vapor or gas.

Compression Ratio:

Discharge pressure divided by suction pressure.


Through heat removal, gas is changed into a liquid.

Condensation Point:

If the temperature were reduced below this point it would change the vapor to a liquid.


A condenser disposes of unwanted heat. The three types of condensers are air-cooled, water-cooled, and evaporative.

Condenser Coil:

A network of tubes filled with refrigerant. It removes heat to make the gaseous refrigerant a liquid again.

Condensing Unit:

A refrigerant mechanism that pumps vaporized refrigerant from the evaporator, compresses it, liquefies it, and returns it to the refrigerant control.

COP (Coefficient of Performance):

The COP measures how energy efficient a chiller operates.

Cooling Anticipator:

A heater that is parallel to the cooling circuit and shortens the off cycle.

Cooling Capacity:

Measures the ability of a unit to remove heat from a room or building.

Cooling Load:

Based on an engineering space analysis, the amount of conditioned air needed to cool a space.



Movable plates within the ductwork that are used in zoning to regulate airflow to certain rooms.

Defrost Cycle:

During the heating season, the procedure to remove frost/ice build up from the outdoor coil.


Removal of water vapor from the air by either cooling the air below dew point or by chemical means.

Department of Energy (DOE):

A government department dedicated to monitoring energy use and setting industry efficiency standards.

Direct Expansion Systems:

A type of basic cooling media that uses Freon for cooling and dehumidification; the three most common methods of heat rejection are air cooled, water cooled, and glycol cooled.

Direct Vent:

Uses outside air for combustion and then vents the resulting gases back outside.


Used most often in computer rooms and modern office spaces, a type of precision air conditioning system that discharges air directly beneath a raised floor.

Downflow Furnace:

A furnace that inhales return air from the top and exhales warm air at the bottom.


A closed, sheet metal, fiberglass board, or other material conduit used for conducting air to and from an air handling unit.


Channels used to move air throughout a building.



In terms of HVAC, an economizer is a mechanical device that is used to reduce energy consumption.


The cooling and heating equipment rating that judges the cost-efficiency of the system.

Electronic Air Cleaner:

Equipment that filters particles and contaminants from indoor air.

Emergency Heat (Supplemental or Auxillary Heat):

Supplementary heat in a heat pump system, used in an emergency situation.

Energy Efficiency Ratio (EER):

Ratio of the cooling capacity of an air conditioner in BTUs to electrical input in watts (API-specified test conditions).


The total heat contained in a refrigerant at any given temperature.


Absorbs heat and moves it outside the refrigerated area.

Evaporator Coil:

A series of refrigerant filled tubes that take heat and moisture out of indoor air.


Fahrenheit (Represented as degrees "F"):

United States of America's unit of temperature measurement.


Removes dust/particles from the air to reduce the load on the respiratory systems and HVAC equipment.

Free Cooling System:

Water cooled or glycol cooled system with an added coil that provides chilled water cooling when the outdoor temperature is cold.

Free Delivery:

A system that doesn't use ducts.


Describes a group of halogenated simple hydrocarbons containing fluorine, chlorine, or bromine (used as refrigerants).


Equipment that convert gas, oil, electricity, or other fuel into heat.



The container in which the lithium bromide solution is boiled and re-concentrated.

Glycol-Cooled System:

An air conditioning system that uses Freon as a refrigerant and water/glycol as a condensing medium.

Ground Water-Source:

Underground well water is used as a heat source for a heat pump.


Heat Exchanger:

Transfers heat energy from the source to the medium.

Heat Gain:

The amount of heat that enters an indoor area during the air conditioning season, measured in BTUs.

Heat Loss:

The amount of heat that is lost from an indoor area during the heating season, measured in BTUs.

Heat Pump:

A system that has a valve that allows it to alternate between heating and cooling.

Heat Source:

The heat source that the heating system uses to warm an indoor area.

Heat Transfer:

The movement of heat from one area to another.

Heating Capacity:

The rating that judges how much heat a device can provide.

Heating Seasonal Performance Factor (HSPF):

Total heat pump output in BTUs divided by total electrical input in watt-hours during normal usage period.

Horizontal Furnace:

A furnace that pulls return air in on one side and expels warm air on the other side.


Heating, Ventilation, and Air Conditioning.


The method of adding moisture to the air.


Equipment that regulates humidity input by reacting to moisture changes in the air.


Humidity is the amount of moisture in the air.


Indoor Unit:

Contains the indoor coil, fan, motor, and filtering device.

Indoor Coil:

Tubing containing refrigerant.


Air flow leaks into a living space.


Slows down the transfer of heat.

Integrally Controlled Motor (ICM):

An efficient, variable-speed motor.

Isolation Valves:

Valves that transfer and isolate refrigerate charge in the cooler or condenser.


No Definitions at this time.


(K) Factor:

(k) Factor is the insulating value.

Kilowatt (kW):

A unit of electrical consumption that equals 1,000 watts.


Latent Cooling Capacity:

The capability of an air conditioner to remove moisture from the air.

Latent Heat:

The amount of heat energy needed to change a substance from a liquid to a gas.


Liquid Crystal Display

Load Calculation:

A design tool that measures the heat gain and loss in a building; it helps determine the proper air conditioning/heating equipment size.


Matched System:

A heating/cooling system that is equipped with products that have been certified to perform optimally together.

Microprocessor Controls:

Uses computer logic to operate and monitor air conditioning systems.


Natural-Draft Furnace:

A furnace that utilizes the natural flow of air around the furnace to provide air to support combustion.


National Fire Protection Association.


Operating Cost:

The cost of running the air conditioner/heater in a space.

Outdoor Coil/Condensing Unit:

Collects heat and moves it to the outdoor air.


Package System:

A system where all air conditioning and heating equipment is located in one cabinet.

Packaged Unit:

A self-contained heating and/or air conditioning system.

Payback Analysis:

It is a measure of the value of a home comfort system; combining your purchase price and operating cost, the payback analysis determines how many years it takes to offset the purchase price.

Precision Air Conditioning:

Pre-packaged systems designed to cool electronic equipment.

Purge Device:

Equipment that removes air and water vapor from the refrigerant of a negative pressure designed chiller.


No Definitions at this time.



A substance that provides a cooling effect by expanding or vaporizing.

Refrigerant Lines:

The connection between an outdoor and indoor unit made of two copper lines.


Covers an air opening or the end of an air duct; it is a combination of a grille and damper.

Relative Humidity:

Given a certain temperature, the amount of vapor in the air divided by the largest amount of vapor that could be contained in the air.

Return Air:

The air drawn into the heating unit after circulating from the heater's output supply.

Reversing Valve:

The device within a heat pump that reverses the refrigerant flow and switches the equipment from cooling to heating.


SEER (Seasonal Energy Efficiency Ratio):

The Seasonal Energy Efficiency Ratio measures the energy efficiency of air conditioning equipment.

Sensible Cooling Capacity:

The air conditioner's ability to remove heat from an area.

Sensible Heat:

Heat energy that changes the temperature of a gas, liquid, or solid when added or removed.


The ideal thermostat temperature.

Single Package:

A year-round heating and air conditioning system that is contained in a single package.

Split System:

The most common system installed in homes, it is a central air conditioner that contains a compressor and condenser installed outdoors and a non-compressor installed inside the building.

Storage Tank:

The refrigerant temporarily is stored here while being serviced.

Supercooled Liquid:

Liquid refrigerant cooled below its saturation point.


Some condensers contain this section, which reduces the temperature of the condensed refrigerant liquid, improving energy efficiency.


Removes sensible heat from a refrigerant liquid by dropping the temperature.

Superheated Vapor:

Refrigerant vapor heated beyond its saturation point.


Adds heat energy to a refrigeration vapor by creating a rise in temperature.

Supplementary Heat:

Emergency heat, available at temperatures below a heat pump's balance point.


Equipment that switches the heat pump from cooling to heating by changing the flow of refrigerant.



The measure of heat matter contains.


A sophisticated thermostat that measures the outdoor temperature, indoor air temperature, and indoor relative humidity. A microprocessor communicates with the temperature control equipment to determine the most efficient method of achieving an optimal temperature.


A temperature control device that monitors the control functions of temperature control equipment.


In cooling systems, the unit of measurement that describes the cooling capacity.



An air conditioner that discharges cool air through a top-mounted discharge plenum or overhead duct.

Upflow Furnace:

A type of furnace that expels warm air at the top and expels return air at the bottom.


Vacuum Pump:

Removes air and water vapor from a refrigeration system below atmospheric pressure.

Vapor Barrier:

A moisture-proof layer, preventing the travel of water vapor.

Vapor Seal:

A vapor seal is a critical component of sealing moisture out of a sensitive room. The seal barricades air, moisture, and contaminants from moving through small cracks or pores in the walls, floor, or ceiling into the contaminant-sensitive area.


Adding or removing air by either natural or mechanical means to or from any space.


Removes stale indoor air and transfers it to fresh incoming air.


Water Cooled System:

An indoor cooling unit that utilizes Freon and water as a condenser. Water is piped to the unit from a suitable source.

Water Source:

Water is a heat source for the heat pump.


A unit of power.


No Definitions at this time.



A gas or oil fueled air conditioner.


Zone System:

A system that divides a space into zones and provides the ability to control the temperature for each zone separately.


The act of providing independent heating and cooling options to different zones of a structure. This practice uses a system controller, zoning dampers, and a bypass damper.

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