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Independent buying guide Updated 22 July 2026 By YourAirCon Editorial Team Sources & methodology

Air Conditioning Glossary: Read Your Quote Like an Engineer

Every term a UK installer will use — split, multi-split, F-Gas, COP, R32, commissioning — defined in plain English, two sentences each.

Split system
The standard UK setup: one indoor unit (usually wall-mounted) connected by refrigerant pipes to one outdoor condenser. "Split" because the system is divided between inside and outside — quiet half in, working half out.
Multi-split
One outdoor condenser running two to four indoor units in different rooms, each with its own remote and temperature. Saves outdoor wall space and usually beats installing separate systems once you want more than one room cooled.
Wall-mounted unit
The familiar slim indoor unit high on the wall — the format chosen in most UK domestic installs. Ceiling cassettes and floor consoles exist for rooms where walls don't cooperate.
Ceiling cassette
An indoor unit recessed into a suspended ceiling, blowing air in four directions. The default for offices, shops and restaurants; occasionally used in large open-plan homes.
Ducted system
A hidden indoor unit in the loft or ceiling void distributing air through ducts to grilles in several rooms. Invisible and near-silent in the rooms, but needs building work — mostly a new-build and major-renovation option in the UK.
VRF / VRV
Variable Refrigerant Flow (VRV is Daikin's trademark for the same idea): commercial-scale systems where one outdoor plant runs many indoor units, each metering exactly the refrigerant it needs. What your installer proposes for offices, surgeries and restaurants.
Heat pump
A machine that moves heat rather than generating it directly. Most modern fixed split air conditioners are reversible air-to-air heat pumps—cooling in one direction and heating in the other. UK grants now distinguish qualifying air-to-air systems from air-to-water systems that feed radiators and hot water.
Air-to-air / air-to-water
Air-to-air heats or cools room air directly (this is reversible air conditioning). Air-to-water heats radiators, underfloor circuits and a hot-water cylinder. Current Boiler Upgrade Scheme grants are £2,500 for qualifying residential air-to-air systems and £7,500 for qualifying air-to-water systems in England and Wales.
F-Gas certification
The legal qualification for anyone handling fluorinated refrigerants in the UK. Installing, commissioning or de-gassing a system without it is illegal and voids equipment warranties. Every installer in the YourAirCon network must hold current F-Gas certification.
Refrigerant
The working fluid pumped between indoor and outdoor units, absorbing heat when it evaporates and releasing it when it condenses. Travels in the two insulated copper pipes — no air moves between the units.
R32
The refrigerant in most current UK domestic units: better efficiency and about one-third the global-warming potential of the R410A it replaced. Mildly flammable, which is one of several reasons refrigerant work is F-Gas-restricted.
Inverter
Variable-speed compressor control: the system sprints to your set temperature, then idles at low power to hold it, instead of stop-starting at full blast. Quieter, cheaper to run, steadier temperature. Effectively universal on units worth buying in 2026.
Compressor
The pump in the outdoor unit that drives refrigerant around the circuit and concentrates heat by compression. The hardest-working component and the reason the outdoor unit hums while the indoor unit whispers.
Condenser (outdoor unit)
The outside box: compressor, coil and fan, dumping the heat collected indoors (or gathering heat in winter). Wants airflow, a stable mount and a metre from the boundary for permitted development.
kW and BTU
Cooling capacity units. UK quotes use kilowatts: 2.0–2.5kW suits a bedroom, 3.5kW a living room, 5.0kW+ open-plan spaces. Divide BTU by 3,412 to get kW — a "12,000 BTU" unit is 3.5kW.
COP and SCOP
Coefficient of Performance: heat moved per unit of electricity. COP 3.5 means 3.5kWh of heating from 1kWh of power. SCOP is the seasonal average across a real winter — the honest number for comparing heating-mode running costs.
EER and SEER
The cooling-mode equivalents of COP/SCOP: efficiency at a fixed condition (EER) and across a season (SEER). Higher is cheaper to run; EU-market units carry these on the energy label.
dB(A) / sound level
How loudness is specified. Modern indoor units: 19–26dB on night mode (quieter than a whisper); outdoor units: 40–55dB. The scale is logarithmic — 10dB more is roughly twice as loud to the ear.
Night / silent mode
Slows the indoor fan to its quietest setting and typically drifts the setpoint gently overnight. The feature that makes bedroom installs sleep-proof.
Permitted development
The planning rules letting most UK houses install one properly placed outdoor unit without a planning application. Flats, listed buildings and conservation areas are the exceptions.
Commissioning
The final install steps that make a system reliable: vacuuming the pipework, pressure testing, charging refrigerant and verifying performance. The difference between a professional install and a future breakdown — and part of every surveyed quote.
Condensate drain
Cooling wrings moisture out of the air; it condenses on the indoor coil and drains outside through a small pipe. The dehumidifying is why conditioned rooms feel better than their thermometer suggests.
Dry mode
Dehumidifies with minimal cooling — useful on muggy 24°C days when the air, not the temperature, is the problem. Cheaper to run than full cooling.
TM44 inspection
The legally required efficiency inspection for commercial air conditioning over 12kW, repeated five-yearly. Commercial owners: if you can't find your TM44 certificate, you probably need one.

Terms decoded, next step priced: what installation costs in 2026, air-to-air heat-pump costs and grant rules, what running it costs, and how the machine actually works.

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