Glossary

Air conditioning glossary

52 terms you will see on quotes, rating plates and spec sheets, explained in plain English.

Air-to-air heat pump

A heat pump that moves heat between outdoor air and indoor air, heating or cooling rooms. It does not heat hot water.

Air-to-water heat pump

A heat pump that heats water for radiators, underfloor heating and hot water. Usually replaces a boiler.

Anti-vibration feet

Rubber or spring mounts under an outdoor unit that reduce vibration passing into floors and walls.

Boiler Upgrade Scheme (BUS)

Government grant in England and Wales for heat pumps. Air-to-air heat pumps became eligible from 28 April 2026 for homes, with Ofgem listing £2,500.

BTU

British thermal unit. Some products quote capacity in BTU/h; 1 kW is about 3,412 BTU/h.

Capacity (kW)

How much heat a unit can move per hour, in kilowatts of cooling or heating output. Not the same as the electricity it uses.

Ceiling cassette

An indoor unit built into the ceiling with a square grille blowing air in several directions.

Charge

The amount of refrigerant in a system, in kilograms, shown on the outdoor unit's rating plate.

Commissioning

Final checks after installation: running the system, recording pressures and temperatures, and confirming it works as designed.

Compressor

The pump in the outdoor unit that circulates refrigerant and drives the heat transfer.

Condensate

Water removed from the air as it cools, collected in a tray and drained away.

Condensate pump

A small pump that lifts condensate water where it cannot drain by gravity.

Condenser

The coil that rejects heat. In cooling mode it is the outdoor coil.

COP

Coefficient of performance: heat output divided by electrical input in heating mode. A COP of 4 means four units of heat per unit of electricity.

Core drilling

Cutting a neat round hole through a wall for pipework, usually 65–100 mm.

CO₂ equivalent

A refrigerant charge expressed as the climate impact of the same amount of CO₂. Used to set F-gas leak-check thresholds.

Ducted unit

An indoor fan unit hidden in a ceiling void or loft that delivers air through ducts to grilles.

EER

Energy efficiency ratio: cooling output divided by electrical input at a test condition.

Evaporator

The coil that absorbs heat. In cooling mode it is the indoor coil.

F-gas certificate

The qualification an engineer must hold to work on refrigerant circuits in Great Britain.

F-gas regulations

Rules on fluorinated greenhouse gases covering who may handle them, leak checks, records and product bans.

Flare joint

A mechanical pipe joint made by flaring the end of copper pipe and tightening a nut. Common on split systems.

Floor console

A low-level indoor unit installed near the floor, often under a window.

GWP

Global warming potential: a refrigerant's climate impact relative to CO₂. R32 is about 675, R410A about 2,088.

Heat gain

Heat entering a room from sun, people, equipment and the outside. Sets cooling capacity.

Heat loss

Heat leaving a room in cold weather. Sets heating capacity.

Inverter

A variable-speed compressor drive that slows down once the room is at temperature, saving energy and reducing noise.

Leak check

A documented inspection of a refrigerant circuit for leaks, required at set intervals above legal thresholds.

Linear slot diffuser

A long, narrow ceiling grille used with ducted systems for a discreet finish.

MCS

Microgeneration Certification Scheme: certifies heat pump installers and products. Required for Boiler Upgrade Scheme grants.

MCS 020(a)

The noise calculation used to check whether a heat pump outdoor unit meets permitted development limits: 37 dB(A) or less at the nearest neighbouring window or door.

Multi-split

A system with one outdoor unit serving two or more indoor units.

Nitrogen pressure test

Filling new pipework with dry nitrogen at pressure to prove it is leak-free before refrigerant is added.

Outdoor unit

The external part of a split system containing the compressor, coil and fan.

Permitted development

Planning rules allowing some works without a planning application if set conditions are met.

R22

An old refrigerant. Systems can no longer be recharged with it, so leaks usually mean replacement.

R32

The common refrigerant in new UK split systems, with lower GWP than R410A. Mildly flammable (A2L).

R410A

A refrigerant widely used until around 2020. New single splits using it can no longer be placed on the GB market.

Refrigerant recovery

Removing refrigerant from a system into a cylinder before repair or removal. Venting is illegal.

Regassing

Recharging a system with refrigerant by weight after a leak has been repaired.

SCOP

Seasonal coefficient of performance: average heating efficiency over a heating season.

SEER

Seasonal energy efficiency ratio: average cooling efficiency over a cooling season.

Single split

A system with one outdoor unit and one indoor unit.

Trunking

Plastic channel that covers pipework and cables on walls.

Vacuum (evacuation)

Pumping air and moisture out of new pipework before refrigerant is released.

Wall bracket

A galvanised steel frame that holds an outdoor unit on a wall.

Defrost cycle

In heating mode in cold, damp weather the outdoor coil can ice up. The unit briefly reverses to melt it, and warm air stops for a few minutes. This is normal.

Fused spur

A switched, fused outlet wired from a ring or radial circuit. Small units can sometimes run from one; larger units need a dedicated circuit.

Night mode

A setting that limits fan speed and adjusts the temperature gradually overnight so the system stays quiet while you sleep.

Wi-Fi module

A small adapter fitted inside or beside the indoor unit so the system can be controlled from a phone app.

Zone

An area of a building controlled separately, with its own set temperature.

FAQ

Questions we get asked

What does Air-to-air heat pump mean?