How does air conditioning work?

An air conditioner does not make cold. There is no such thing as "making cold" — cold is just the absence of heat. What an air conditioner actually does is collect heat from the air inside your room and dump it outside, over and over, until the room reaches the temperature you asked for. It moves heat the way a pump moves water: from where you don't want it to where you don't mind it.

The machinery that does this is the vapour-compression refrigeration cycle — the same cycle running in your fridge, your freezer and, increasingly, your heating [1]. Once you see how the four stages fit together, everything else about air conditioning makes sense: why the outdoor unit blows hot air, why a reversible system can heat your home in winter, and why a portable unit can never match a fitted system.

The refrigeration cycle in plain English

The cycle relies on two facts about liquids and gases. First, when a liquid evaporates into a gas, it absorbs a large amount of heat from its surroundings — which is why sweat cools your skin. Second, when a gas is compressed and condenses back into a liquid, it releases that heat again. An air conditioner exploits this by pumping a special fluid — the refrigerant — around a sealed loop, forcing it to evaporate indoors and condense outdoors.

Follow one lap of the loop:

1. Evaporation — heat is absorbed indoors. Cold, low-pressure liquid refrigerant flows through the indoor coil (the evaporator). A fan draws warm room air across the coil. Because the refrigerant is colder than the air, heat flows into it, and the refrigerant boils into a gas — carrying the room's heat away with it. The air leaving the coil is cooler and drier, which is also why air conditioning dehumidifies as a side effect.

2. Compression — the heat is concentrated. The gas travels to the outdoor unit, where the compressor — the heart of the system and its main electricity consumer — squeezes it to high pressure. Compressing a gas raises its temperature, so the refrigerant leaves the compressor hotter than the outside air. That step matters: heat only flows from hot to cold, so the refrigerant must be made hotter than a summer afternoon before it can give its heat away outdoors.

3. Condensation — heat is rejected outdoors. The hot, high-pressure gas passes through the outdoor coil (the condenser), where a fan blows outside air across it. Because the refrigerant is now hotter than the outdoor air, heat flows out of it, and it condenses back into a liquid. This is the warm draught you feel from the outdoor unit — it is literally your room's heat being thrown away.

4. Expansion — the reset. The warm liquid passes through an expansion valve, which abruptly drops its pressure. Lower pressure means a lower boiling point, so the refrigerant becomes cold again — ready to re-enter the indoor coil and absorb another load of heat. The lap is complete, and the cycle repeats continuously while cooling is called for [1].

Run it backwards and you have a heater

Reverse the direction of the refrigerant flow and the two coils swap jobs: the outdoor coil absorbs heat from the outside air — there is useful heat in air even on a cold day — and the indoor coil releases it into your room. The same box is now a heater. This is exactly what an air-to-air heat pump is, and it is why almost every modern wall-mounted system sold in the UK is reversible [2].

In heating mode the arithmetic gets even better: because the system moves heat rather than generating it, it can deliver several units of heat for each unit of electricity — typically around three, compared with exactly one for a plug-in electric heater [2]. See air-to-air heat pumps for the heating side in full, and heating and cooling for how one system covers both seasons.

COP and SEER in one paragraph

Efficiency ratings simply measure how much heat a system moves per unit of electricity. COP (coefficient of performance) is the snapshot figure — a COP of 3 means 3 kW of heating or cooling for 1 kW of electricity. SEER and SCOP (seasonal energy efficiency ratio, and its heating equivalent) average that performance across a whole season of realistic conditions, so they are the fairer numbers to compare when choosing a unit. A higher SEER means a lower electricity bill for the same comfort — the practical consequences are covered in our running costs guide.

Refrigerants and the R-32 transition

The refrigerant is the working fluid that carries heat around the loop, and it is regulated. Most fluorinated refrigerants (F gases) are potent greenhouse gases if released, so Great Britain's F-gas rules control who may handle them, require leak-checking on larger systems, and are phasing down the total quantity of high-global-warming-potential refrigerant that can be placed on the market [3]. In practice this has driven the domestic market from the older R-410A towards R-32, which has roughly one third of the global warming potential and slightly better thermodynamic performance. Two things follow for you as a buyer: a new system today should be R-32, and installation is not a DIY job — refrigerant work is legally restricted to F-gas certified engineers [3].

The parts of a split system

A fitted split system puts the cycle's four stages in two boxes, connected by slim insulated pipes.

Putting the compressor outside is also why fitted systems are quiet indoors — the noisy part is in the garden. One outdoor unit can serve several indoor units in a multi-split system, or feed concealed ceiling units in a ducted system; the cycle is identical in each case. Fitting the pipework and commissioning the refrigerant circuit is a one-to-two-day job for most single rooms.

Why portable and fitted units perform so differently

A portable air conditioner runs the same refrigeration cycle, but with all four stages in one box inside the room — so it must blow its rejected heat out through a hose in an open window. That creates two structural problems: the hose and window gap leak some of the heat straight back in, and the unit pressurises its exhaust with room air, pulling warm outside air into the house through every crack to replace it. A fitted split system has neither problem, because the heat rejection happens entirely outside the sealed envelope of the room. The result is a large real-world efficiency gap that has nothing to do with build quality — it is baked into the physics. The full comparison is in portable air conditioner vs fitted air conditioning.

How does air conditioning work — FAQ

Does air conditioning bring in fresh air from outside? No — a standard split system recirculates and cools the air already in the room. The refrigerant crosses the wall; the air does not. Ventilation is a separate job.

Why does my air conditioner produce water? Cooling air below its dew point condenses moisture out of it, just like a cold drink on a summer day. The indoor unit collects this condensate and drains it away — it is a sign of normal operation.

Is an air conditioner the same as a heat pump? Mechanically, yes. A heat pump is an air conditioner with a reversing valve, so the same cycle can move heat in either direction [2]. Nearly all modern domestic units are reversible.

Why does the outdoor unit need clear space around it? The condenser can only reject heat into air that carries it away. Boxed in, it re-breathes its own hot exhaust, pressure rises and efficiency falls — which is why installers insist on clearances when siting it.

How cold can air conditioning make a room? Domestic systems typically hold a set point down to around 17–18°C, but a sensible summer set point is 21–24°C — every degree lower costs more electricity for little comfort gain.

Next steps

Understanding the cycle is the easy half; matching the right system and capacity to your rooms is the half worth getting professional help with. Our system finder turns a few questions about your home into a shortlist of suitable system types. When you are ready for real numbers, request a quote and we will introduce you to one vetted installer where we can.