Air Conditioning Glossary

What is Air Conditioning? What is a Compressor? What is an Evaporator? How do I fix my air conditioning?


Air conditioning is ubiquitous in modern life. Businesses, homes, schools and hospitals all benefit from perfectly temperate air thanks to the many different kinds of air conditioning available. Everyone knows what it does, but air conditioning is a complex topic once you look beneath its simple exterior.

Here’s our glossary of terms related to air conditioning, so you’ll never be left out in the cold when reading your owner’s manual or talking to an air conditioning professional. At FM Mechanical Ltd, we always explain our work in terms that are easy to understand, but not all companies are as thoughtful as us.


AHU – Air handling unit. A large metal box that contains a blower, filter racks, heating or cooling elements, sound attenuators and dampers. It connects to ductwork and delivers conditioned air throughout a building.

Air Conditioning – The process of controlling air quality: its temperature, humidity and cleanliness.

Air Source Heat Pumps – A heat pump that uses outside air as a heat source. The air is cooled or heated to change the temperature of a room.


BTU – British Thermal Unit. A measure of heat energy. It is the energy required to heat one pound (lb) of water by one degree fahrenheit.


Compressor – A vital component of an AC unit. It compresses and pumps refrigerant gas around the unit.

Condenser – A coiled fan that picks up heat from the refrigerant. This causes the gas to condense into a liquid.

Cassette – A disguised ceiling unit. The main part of the cassette is hidden in the ceiling and only the grille is visible.

Condensing Unit – Another term for an outdoor unit. It houses the compressor, condenser, control unit and fan. Can be rack mounted or mounted on the roof.


Evaporator – Similar to a car radiator, this is a finned coil housed within the indoor unit. Refrigerant is pumped through it and air is blown through the coil by the evaporator fan.


FCU – Fan Coil Unit. A generic name for the system which cools or heats the air, whether it’s ceiling, portable or wall mounted.


Ground Source Heat Pump – This system pumps heat to or from shallow ground. In winter, the ground is a heat source. In summer, it’s a coolant.


Heat Exchanger – Transfers heat from one fluid to another without mixing them together. In the air conditioning industry, air is classed as a liquid to simplify the methodology.

Humidity – Level of moisture or water vapour in the air. Warmer air can support more water, whereas cold air is naturally dryer.

HVAC – The acronym for Heating, Ventilation and Air Conditioning.


R22 Gas – A hydrofluorocarbon commonly used in all air conditioning units, heat pumps and refrigeration systems. In 2015, R22 was made illegal for servicing and maintenance. This means many systems require replacement or alteration to meet the new standard.

Refrigerant – The fluid that flows around the unit changing from gas to liquid and back again.


Split System Air Conditioning – A common type of air conditioning unit. It is split into two parts, with the evaporator inside the building and the condensing unit outside the building, hence the name. They can be installed up to 70 metres apart.


Thermostat – Controls the temperature via an interface. Typically found mounted on the wall of a property. Sensors and relays monitor the temperature of the room and adjust the air conditioning settings appropriately.


VRF Air Conditioning – Variable refrigerant flow. This is a split air system used in medium and large applications. Condensing units can be attached to up to 64 indoor units. This is a flexible system to cater for an entire building.

VRV Air Conditioning – Variable Refrigerant Volume air conditioning. Used for the largest systems. The amount of refrigerant is increased or decreased depending on the amount of cooling required. VRV can take hundreds of units and can be controlled by a single panel.

FM Mechanical Ltd supply and install air conditioning systems of all sizes and types, in buildings of all sizes and types, for customers of all sizes and types. Our fully qualified engineers provide a swift and safe installation and have the knowledge and experience to service and maintain AC units from all major brands. Contact us today to find out more.