How Does Air Conditioning Work?

Air conditioning does not make cold air. It moves heat. A sealed loop of refrigerant picks up heat from the air in your room and carries it outside, where it is released. Run the same loop in reverse and it brings heat in from outside, which is how the systems we fit for homes heat in winter.

The Short Version

1. A fan in the indoor unit draws warm air from the room across a cold coil.

2. The refrigerant inside the coil absorbs heat from the air, and the cooled air is blown back into the room.

3. The refrigerant, now carrying that heat, travels through copper pipes to the outdoor unit.

4. A compressor raises its pressure, which makes it hotter than the outside air.

5. A fan blows outside air across the outdoor coil, and the heat passes into that air.

6. The refrigerant passes through an expansion valve, which drops its pressure and temperature, and it flows back to the indoor unit to collect more heat.

The loop runs continuously while the system is cooling. The refrigerant is never used up. It circulates in a sealed circuit for the life of the system.

Why Refrigerant Can Move Heat

A refrigerant is a fluid that boils at a very low temperature. When a liquid boils and turns into a gas, it absorbs a large amount of heat. When the gas condenses back into a liquid, it gives that heat out again. Air conditioning uses pressure to control where each of these happens.

In the indoor coil, the refrigerant is at low pressure, so it boils at a temperature well below the room temperature and soaks up heat. In the outdoor coil, the compressor has raised the pressure, so the refrigerant condenses at a temperature above the outside air and gives the heat away. Your fridge works the same way, moving heat from inside the cabinet out through the back.

The Four Main Parts

• Evaporator. The coil in the indoor unit, where the refrigerant absorbs heat from the room.

• Compressor. The pump in the outdoor unit that raises the pressure and temperature of the refrigerant. It uses most of the electricity.

• Condenser. The coil in the outdoor unit, where the heat is released outside.

• Expansion valve. A valve that lowers the pressure of the refrigerant before it returns to the indoor coil.

Most new split systems in the UK use a refrigerant called R32. Our guide to R32 refrigerant explains why.

Why Air Conditioning Dries the Air

The indoor coil is colder than the dew point of the room air, so moisture condenses on it, just as water forms on a cold glass. The water runs into a tray and drains away outside through a small pipe. This is why air conditioning makes a warm, humid room feel more comfortable, and why most systems have a dry mode that concentrates on removing moisture.

If water starts dripping from the indoor unit, our guide to an air conditioner leaking water explains the usual causes.

How the Same System Heats

Systems that heat have a reversing valve, which changes the direction the refrigerant flows. In heating mode, the outdoor coil collects heat from the outside air, even on a cold day, and the indoor coil releases it into the room. Because the system moves heat rather than making it, it delivers several units of heat for each unit of electricity it uses.

In cold, damp weather, ice can build up on the outdoor coil. The system clears it with a short defrost cycle, and the indoor fan pauses for a few minutes while it does. This is normal. Read more about air conditioning as a form of heating.

What an Inverter Does

Older systems switched the compressor fully on and off to hold the temperature. Modern inverter systems vary the compressor speed instead. They run hard to bring the room to temperature, then slow down to hold it there. This uses less electricity and keeps the temperature steadier.

Split Systems, Portable Units and Larger Systems

In a split system, the compressor and the hot condenser sit outside, so the heat goes straight outdoors and the room stays quiet. A portable unit puts all four parts in one box in the room and pushes the heat out through a hose, which is why it is noisier and less effective. See do portable air conditioners work?

For a side-by-side look at each type, see our guide to the different types of air conditioning systems. Larger buildings use the same cycle on a bigger scale, with one outdoor system serving many indoor units. See our page on VRF and VRV systems.

How Efficient Is It?

Efficiency is shown as seasonal figures. SEER measures cooling and SCOP measures heating. Both compare the heating or cooling delivered over a typical season with the electricity used, so a higher number means lower running costs. Our guide to air conditioning costs covers running costs.

Frequently Asked Questions

Does air conditioning bring in fresh air from outside?

No. A split system treats the air already in the room. The pipes between the indoor and outdoor units carry refrigerant, not air. For fresh air, open a window or use your existing ventilation.

Does air conditioning use gas?

Not mains gas. It runs on electricity. The refrigerant in the pipes is sometimes called gas, but it stays sealed inside the system.

Why does the outdoor unit blow warm air?

In cooling mode, the warm air coming from the outdoor unit is the heat that has been taken out of your room.

Can I top up the refrigerant myself?

No. Refrigerant is not used up, so a system that is low on refrigerant has a leak that needs finding and fixing. By law, work on the refrigerant circuit must be carried out by an F-Gas certified engineer.

Call 01772 288651 or use the form below to book a free survey. We will measure the room, recommend the right system and give you a clear written quote.

Last updated: November 2026.