Air Conditioning Glossary of Terms

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Air Conditioning Glossary of Terms Written by Airway Group Team

Everyone knows what air conditioning does, in some way, at least. It makes the temperature of rooms more comfortable for us. However, it’s obviously not that simple, and there’s a whole dictionary of terms relating to air conditioning, with plenty of phrases that probably make no sense to a lot of people. Don’t worry, though, we’ve created a glossary of the most useful terms for you to read at your leisure.


AHU – Air handling unit. An air handler is usually a large metal box containing a blower, heating or cooling elements, filter racks or chambers, sound attenuators, and dampers. Air handlers usually connect to a ductwork ventilation system that distributes the conditioned air through the building and returns it to the AHU.

Air Conditioning – This is defined as the control of air quality, specifically temperature, humidity and cleanliness.

Air Source Heat Pumps – A type of heat pump that uses outside air as a heat source, cooling or heating it to change the temperature of a room.


BTU – British Thermal Unit; a measure of heat energy. It is the amount of energy required to heat one pound of water by one degree fahrenheit.


Compressor – This does most of the work in an AC unit. It compresses and pumps refrigerant around the pipework of the unit.

Condenser – This coiled fan picks up heat from the refrigerant, causing the gas to condense into a liquid. It is housed in the outdoor unit of an air conditioning system.

Cassette – This is something of a disguised ceiling unit – the main body of the cassette is hidden in the ceiling itself, and only the grill is on display.

Condensing Unit – This is another term for the aforementioned ‘outdoor unit’. It is the unit that houses the compressor, condenser, controls and fan. It uses weather-proof casing and can be installed on the roof or units up to 13kW can be mounted on wall brackets.


Evaporator – Generally a finned coil (like a car radiator) housed in the indoor unit through which refrigerant is pumped. Room air is blown through the coil by the evaporator fan.  Heat from the room air is absorbed by the refrigerant and causes the refrigerant liquid to evaporate into a gas. Having given up heat to the refrigerant the air is returned to the room 10-15ºC cooler.


FCU – An abbreviation of Fan Coil Unit which is a generic name for the internal system which creates the cooling or heating effect, whether it be wall, ceiling, void mounted or otherwise.


Ground Source Heat Pumps – This heating system pumps heat to or from shallow ground. In winter, the earth is used as a heat source, and in summer, it’s used as a coolant.


Heat Exchanger – This transfers heat from one fluid to another, but prevents them from mixing together. Where air conditioning is concerned, air is classed as a fluid.

Humidity – Moisture or water vapour in the air. When the temperature is warmer, the air can support more water in it.

HVAC – The classic acronym for Heating, Ventilation and Air Conditioning. Today, an HVAC system also includes air cleaning and moisture control.


R22 Gas – R22 is a refrigerant gas that is in the bracket of Hydrochlorofluorocarbons, commonly used across air conditioning, heat pump and refrigeration systems. HCFCs have become illegal for servicing and maintenance in 2015, meaning that many systems will need to be replaced or altered in order to comply with this legislation. In 2013, approximately 55% of the country’s air conditioning systems were using this refrigerant, but within the next two years these units will not be able to be serviced, and new HFC based inverter systems will be 30-40% more efficient.

Refrigerant – This is the fluid that courses through the unit, changing in state from gas to liquid and vice versa.


SEER – Seasonal Energy Efficiency Ratio. The total cooling of an air conditioner or heat pump in BTUs during its normal annual usage period for cooling divided by the total electrical energy input in watt-hours during the same period.

Split System Air Conditioning – This is a common type of air conditioning unit. It is called a split system because the evaporator system is inside the building, and the condensing unit is outside the building. These units are connected via copper piping and control cabling. Depending on the size of the units, they can be as much as 70m apart.


Thermostat – A temperature-control device, typically found on a wall inside the home. It consists of a series of sensors and relays that monitor and control the functions of a heating and cooling system. American Standard Heating & Air Conditioning’s programmable controls allow you to program different levels of comfort for different times of the day.


VRF Air Conditioning – Variable refrigerant flow: a type of split system air-conditioning system used in medium and large sized air-to-air applications. One or more condensing units are connected to a number of indoor units (up to 64). Each indoor unit can be selected for either cooling or heating. Variable speed compressors provide control flexibility.

VRV Air Conditioning – Variable Refrigerant Volume air conditioning is used for larger systems, changing the amount of refrigerant flowing through the system depending on the amount of cooling required. These systems can consist of literally hundreds of units, all controlled by a single control panel.

As you can see, there’s a lot of terms to get to grips with when it comes to air conditioning, and we’ve not covered all of them here (that would take forever), but these are some of the most commonly used and useful terms.

Airway Air Conditioning are experts in providing the highest quality air conditioning installation, repairs and maintenance for commercial and residential customers alike. For more information, please get in touch us with today and we’ll be happy to help.