Air conditioning glossary
Air conditioning spec sheets and reviews are full of jargon. This glossary explains the key terms in plain English so you can compare units with confidence. Where relevant, each definition links to a calculator or guide that puts the term in context.
- Air changes per hour (ACH)
- A measure of how many times the entire volume of air in a room is replaced or circulated in one hour. Higher ACH values indicate more thorough ventilation or air treatment.
- BTU (British Thermal Unit)
- The unit of heat energy used to rate air conditioner cooling capacity in the UK. One BTU is the energy needed to raise one pound of water by one degree Fahrenheit; in air conditioning, BTU/hr describes how much heat the unit can remove per hour.
- Condenser
- The component (usually located outdoors in a split system) where refrigerant releases the heat absorbed from the room. The condenser coil and fan expel that heat into the outside air.
- COP (Coefficient of Performance)
- The ratio of useful heating or cooling output to the electrical energy consumed. A heat pump with a COP of 3 delivers three units of heat energy for every one unit of electricity used.
- Dehumidify mode
- A dedicated operating mode on many air conditioners that removes excess moisture from the air without significantly lowering the temperature. Useful on humid days when the room feels clammy rather than hot.
- Dual-hose
- A portable air conditioner design that uses one hose to expel hot exhaust air and a second hose to draw in outside air for the condenser. This avoids negative pressure in the room and makes the unit more efficient than a single-hose design.
- EER (Energy Efficiency Ratio)
- A measure of cooling output (in watts) divided by power input (in watts) tested at a fixed set of conditions. A higher EER indicates a more efficient unit at peak operating conditions.
- Evaporator
- The indoor coil in an air conditioning system where refrigerant absorbs heat from the room air, causing the refrigerant to evaporate. This is the process that cools and dehumidifies the indoor air.
- F-Gas (Fluorinated greenhouse gases)
- A group of synthetic refrigerants regulated under UK and EU F-Gas regulations because of their high global warming potential. Only certified engineers are permitted to handle, recover, or recharge F-Gas refrigerants in air conditioning systems.
- Inverter
- A technology that allows a compressor motor to vary its speed rather than simply switching on and off. Inverter air conditioners are more energy-efficient, maintain a steadier room temperature, and tend to be quieter than non-inverter (fixed-speed) models.
- kW (kilowatt)
- A unit of power equal to 1,000 watts, used alongside BTU to express cooling or heating capacity. One kilowatt of cooling capacity equals approximately 3,412 BTU/hr.
- Noise level (dB(A))
- Air conditioner noise is measured in decibels using the A-weighting scale (dB(A)), which reflects how the human ear perceives sound. A quiet split system indoor unit typically measures 19–26 dB(A); most portable units range from 48–55 dB(A).
- Refrigerant
- The fluid that circulates through an air conditioning system, absorbing heat in the evaporator and releasing it in the condenser. Modern systems use lower-GWP refrigerants such as R-32 or R-290 (propane) to reduce environmental impact.
- SEER (Seasonal Energy Efficiency Ratio)
- A measure of cooling efficiency averaged across a range of outdoor temperatures that better represents real-world seasonal performance than EER. A higher SEER indicates lower running costs over a typical cooling season.
- Single-hose
- The most common portable air conditioner design, which uses a single hose to vent hot exhaust air outside. Because it expels room air, it can create slight negative pressure that draws warm air in through gaps — making it slightly less efficient than a dual-hose design.