Glossary

Plain-English definitions of the terms you'll meet across these guides.

Air conditioning: A system that controls the temperature (and often humidity) of indoor air; most modern units both cool and heat.
Air handling unit (AHU): A larger unit, common in commercial buildings, that conditions air and can also bring in and treat fresh air, often using chilled or hot water from central plant.
Air-to-air heat pump: What a reverse-cycle AC unit is — it moves heat between outside air and inside air, delivering heating as well as cooling.
Air-to-water heat pump: A different technology that heats water for radiators, underfloor heating and hot water; not the same as air conditioning.
BTU (British Thermal Unit): An older unit of heat; AC capacity is sometimes quoted in BTU/h. 1 kW ≈ 3,412 BTU/h. The UK standard is kW.
Cassette: An indoor unit recessed into a ceiling, blowing air from a flush grille.
Compressor: The pump at the heart of the refrigerant cycle.
Condensate: The water a unit extracts from the air, which must be drained away.
Condenser / outdoor unit: The outside box containing the compressor; rejects or absorbs heat outdoors.
COP / SCOP: Coefficient of Performance / Seasonal COP — heating efficiency (higher is better); SCOP reflects performance across a season.
dB(A): A-weighted decibels — the standard measure of how loud a unit is.
Ducted: Indoor unit concealed in a ceiling void, distributing air through ducts and grilles.
EER / SEER: Energy Efficiency Ratio / Seasonal EER — cooling efficiency (higher is better).
ErP energy label: The energy-efficiency rating label on AC products, based on seasonal efficiency, energy use and noise. Air conditioners have used an A+++ to D scale; the simpler A to G scale is being phased in, so both are currently seen.
Evaporator / indoor unit: The inside unit that delivers conditioned air to the room.
F-gas: Fluorinated greenhouse gases (including many refrigerants), subject to handling and leak-check regulation.
Fan coil unit / AHU: Units that condition air using chilled or hot water from a central chiller or boiler; common in larger commercial systems.
Heat load: The amount of heating or cooling a space needs, based on its size, construction, glazing, occupancy and equipment.
Heat recovery: In a VRF system, the ability to move heat from zones that need cooling to zones that need heating, so a building is heated and cooled at once rather than wasting heat.
Inverter: A compressor that varies its speed to hold a steady temperature efficiently, rather than cycling on and off.
kW (kilowatt): The standard UK unit for AC capacity — how much heating or cooling a unit can deliver. Note this is output, not the electricity it consumes.
Multi-split: Several indoor units served by a single outdoor unit.
Non-inverter (fixed-speed): A compressor that runs at full speed or switches off, cycling on and off to hold a temperature.
Packaged unit (rooftop unit): A self-contained system with all components in one casing, often roof-mounted, ducting conditioned air into the space below.
Precision (close-control) cooling: Purpose-built cooling that holds temperature and humidity within tight limits continuously; used for server rooms, data centres and labs.
R32: The current market-standard refrigerant for domestic split systems (GWP ~675), chosen for its lower global-warming potential than older types such as R410A.
Refrigerant: The fluid that carries heat around the system as it changes between liquid and gas.
Reverse cycle: Running the refrigerant cycle backwards to heat instead of cool.
SCOP: See COP / SCOP — the seasonal measure of heating efficiency.
SEER: See EER / SEER — the seasonal measure of cooling efficiency.
Single-split: One indoor unit served by one outdoor unit.
Split system: An air conditioner divided into an indoor part and an outdoor part, joined by thin insulated refrigerant pipes. Almost all fixed AC is a split system.
Static pressure (high-static / medium-static): How hard a ducted unit's fan can push air against the resistance of the ductwork. A medium-static unit suits shorter runs or a single room; a high-static unit can serve a whole network of ducts. It's the key difference between one large ducted unit (high-static, zoned by dampers) and several smaller ducted units (medium-static, one per room).
TM44 inspection: A periodic inspection of larger air conditioning systems required under UK energy-performance rules.
VRF / VRV: Variable refrigerant flow — multi-split scaled up for whole buildings; can heat some zones while cooling others (heat recovery).