How air conditioning works (and why it can heat your home too)
A simple explanation of the refrigerant cycle, heating mode, efficiency ratings and what to expect from a home AC system in the UK.
It moves heat, it does not make cold
An air conditioner is a heat pump. It uses a refrigerant that boils at a very low temperature to pick up heat in one place and release it somewhere else. In summer it collects heat from your rooms and releases it outside. In winter a reversing valve swaps the direction, so it collects heat from the outdoor air, even at minus temperatures, and releases it into your rooms.
The four steps of the cycle
- Evaporator: cold liquid refrigerant absorbs heat from the air and boils into a gas.
- Compressor: the gas is squeezed, which makes it much hotter.
- Condenser: the hot gas gives its heat away and turns back into a liquid.
- Expansion valve: the pressure drops, the liquid gets very cold and the cycle starts again.
Why it is so efficient
Because it moves heat instead of burning fuel, a modern system delivers 3 to 5 units of heat for every unit of electricity it uses. That ratio over a whole season is called SCOP for heating and SEER for cooling. Look for A++ or A+++ ratings and a SCOP of 4 or more.
What gets installed
- An outdoor unit with the compressor and a fan, usually on a wall bracket or ground stand.
- One or more indoor wall units, one per room (a multi-split runs several from one outdoor unit).
- Insulated copper pipes and a condensate drain between them, normally through a single 65 mm hole.
- A dedicated electrical supply. Most installs take one to two days.
Rules you should know
Only an F-Gas certified company may install or service equipment containing refrigerant. Since 28 April 2026, an air-to-air heat pump that replaces gas, oil, LPG or electric heating in England or Wales can qualify for a £2,500 Boiler Upgrade Scheme grant, but only through an MCS-certified installer.