Air conditioning glossary
Every air conditioning term you’ll meet, in plain English — no jargon left unexplained.
Air conditioning jargon, explained
From split and multi-split to VRF, inverter, SCOP, BTU, F-Gas and TM44 — this glossary explains the air conditioning terms Surrey homeowners and businesses come across, so you can compare quotes and systems with confidence.
- Air conditioning
- A system that cools (and, in reverse-cycle units, heats) indoor air by moving heat with a refrigerant cycle rather than generating it directly.
- Air-to-air heat pump
- The correct technical name for a reverse-cycle air conditioner — it extracts heat from outdoor air to warm a room in winter and reverses the cycle to cool it in summer.
- BTU (British Thermal Unit)
- A unit of heat used to rate cooling capacity. 1 kW is roughly 3,412 BTU/hr; a typical bedroom unit is around 9,000 BTU (2.5 kW).
- Cooling capacity (kW)
- How much heat a unit can remove, measured in kilowatts. As a rough guide, allow about 0.1 kW per square metre of floor area for a standard room.
- COP (Coefficient of Performance)
- The ratio of heating or cooling output to electricity input at a given moment. A COP of 4 means four units of heat moved per unit of electricity used.
- SCOP (Seasonal COP)
- COP averaged across a whole heating season, giving a more realistic efficiency figure. Leading systems achieve an SCOP above 4.
- Inverter
- A compressor that varies its speed to match demand instead of switching fully on and off. Inverter units are quieter, more efficient and hold temperature more steadily.
- Single split system
- One indoor unit connected to one outdoor condenser, conditioning a single room — the most common and affordable domestic setup.
- Multi-split system
- Several indoor units (typically two to five) running from one shared outdoor condenser, each controlled independently. Ideal for cooling several rooms efficiently.
- VRF / VRV
- Variable Refrigerant Flow (Daikin call it VRV) — a scalable commercial system that conditions many zones from one outdoor system, common in offices and multi-tenant buildings.
- Wall-mounted unit
- The most common indoor unit, fixed high on a wall. Discreet, efficient and suitable for most rooms.
- Cassette unit
- An indoor unit recessed into a suspended ceiling, distributing air in multiple directions. Popular in offices, retail and open-plan spaces.
- Ducted system
- A concealed system that delivers conditioned air through ductwork and ceiling grilles, leaving rooms free of visible indoor units. Often used for whole-home cooling.
- Floor-mounted unit
- An indoor unit fitted at low level, useful where wall space is limited or for period rooms where a high-wall unit would be intrusive.
- Condenser (outdoor unit)
- The outdoor part of the system that releases or absorbs heat. Its siting matters for noise, efficiency and planning, especially in conservation areas.
- Evaporator (indoor unit)
- The indoor part that delivers conditioned air to the room, containing the heat-exchange coil and fan.
- Refrigerant
- The fluid that carries heat around the system as it changes between liquid and gas. Handling it is legally restricted to F-Gas-certified engineers.
- F-Gas
- UK and EU-derived regulations governing fluorinated greenhouse gases. Any business installing or servicing refrigerant equipment must hold F-Gas company certification.
- REFCOM
- A leading UK F-Gas certification scheme for refrigeration and air conditioning businesses, evidencing they are competent and legally registered to handle refrigerant.
- MCS 020
- The noise standard for air-source units under permitted development. Broadly, the outdoor unit must not exceed 42 dB(A) at the nearest neighbour’s window.
- TM44
- A CIBSE inspection regime, legally required for air conditioning systems of 12 kW and above, carried out at least every five years to check efficiency and sizing.
- Condensate
- The water that forms as the indoor unit dehumidifies the air. It is drained away via a small pipe; a blocked drain is a common cause of leaks.
- Reverse cycle
- The mode that lets an air conditioner run "backwards" to provide heating as well as cooling — what makes it an air-to-air heat pump.
- Heat load
- The amount of heat that must be removed from a space to keep it comfortable, based on size, glazing, occupancy and equipment. It determines the kW capacity needed.
- Permitted development
- Works allowed without a full planning application, subject to conditions. Many domestic AC installs qualify, but conservation areas, listed buildings and flats often do not.
- Dehumidification
- The removal of moisture from the air, a useful by-product of cooling that improves comfort. Correctly-sized units dehumidify well; oversized ones do not.