How air conditioning works
Air conditioning doesn’t create cold — it moves heat. In cooling mode it collects heat from inside your room and releases it outside. In heating mode, most modern systems run the very same process in reverse to warm the room instead. It does this using a refrigerant: a fluid that absorbs and releases heat as it circulates between the indoor and outdoor units.
The key idea
- It moves heat — it doesn’t “make cold”.
- Cooling: heat is taken out of the room and dumped outside.
- Heating: the process runs in reverse, drawing warmth from the outside air into the room.
- The same unit usually does both — a reverse-cycle air-to-air heat pump.
- It also removes some moisture from the air as it cools.
The refrigerant cycle, step by step
A refrigerant is a fluid chosen because it boils and condenses at convenient temperatures, which lets it carry heat around the system. In cooling mode it runs through four stages:
- In the indoor unit, cold liquid refrigerant absorbs heat from the room air blown across it and boils into a gas. (This part is the evaporator.)
- The compressor in the outdoor unit squeezes that gas, raising its pressure and temperature.
- In the outdoor unit, the hot gas releases its heat to the outside air and condenses back to a liquid. (This part is the condenser.)
- The liquid passes through an expansion valve, which drops its pressure and temperature sharply, and it returns to the indoor unit cold — ready to absorb heat all over again.
Round and round it goes, shifting heat from inside to outside for as long as the unit is cooling.
Cooling and heating from one machine
Most modern units are “reverse-cycle”. A valve reverses the direction the refrigerant flows, so the outdoor unit becomes the one absorbing heat and the indoor unit becomes the one releasing it. The system then pulls warmth out of the outside air — even cold air holds usable heat — and delivers it indoors. That is exactly what an air-to-air heat pump is, and it’s why a single air conditioner can cool you in summer and warm you in winter.
(This is not the same as an air-to-water heat pump, which heats radiators and hot water rather than the air in a room — see the glossary.)
Why it's so efficient
Because the system moves heat rather than generating it, it can deliver several units of heat for every unit of electricity it uses. A plain electric heater turns 1 kW of electricity into 1 kW of heat. A heat pump uses that 1 kW to pump heat in from outside, and can deliver something like 3–4 kW of heat indoors. That ratio is its efficiency, and it’s why running costs are often lower than people expect. We put numbers to it in energy efficiency explained.
What it doesn’t do: as well as changing the temperature, AC removes some moisture as it cools, which is why a drain is needed for the condensate. What it does not do is bring in fresh air from outside — it recirculates and treats the air already in the room. Where fresh-air supply matters (it often does in commercial settings), that’s a separate job: see ventilation vs air conditioning.