How Does Air Conditioning Work? The 5-Minute Version
It doesn't make cold — it moves heat. Once that clicks, everything else (why it heats in winter, why it's cheap to run, why there are two boxes) follows.
The one idea: heat gets moved, not destroyed
Air conditioning runs on the same principle as your fridge: a fluid called refrigerant is very good at soaking up heat when it evaporates and giving it back when it condenses. Pump it round a loop — evaporate indoors, condense outdoors — and heat flows out of your room, continuously, for as long as you like.
The four-step loop, one lap at a time:
1. Absorb. Inside the wall-mounted unit, liquid refrigerant evaporates at low pressure. Evaporation drinks heat — the indoor coil gets cold, the fan pushes room air across it, air comes out the other side 10–12°C cooler.
2. Squeeze. The now-warm refrigerant vapour travels through a small pipe to the outdoor unit, where the compressor squashes it. Compression concentrates its heat — the vapour leaves hotter than a summer afternoon.
3. Dump. Outdoor air, blown across the hot coil, carries that heat away as the refrigerant condenses back to liquid. This is the warm breeze off the condenser.
4. Reset. An expansion valve drops the pressure, the liquid gets cold again, and the lap restarts. Round and round, moving kilowatts of heat with a fraction of a kilowatt of electricity.
Winter: the same machine, running backwards
Flip a reversing valve and the coils swap jobs: the outdoor unit absorbs heat from outside air — there's usable heat in air well below freezing — and the indoor unit releases it into your room. That's the whole secret of the heat pump: same cycle, opposite direction, 3–4kWh of heat per 1kWh of electricity where a plug-in heater manages exactly 1.
Why two boxes, and what's in the pipes
The split design puts the compressor — the working, humming part — outside, and leaves only a coil and a slow fan indoors, which is why a modern wall-mounted unit can run at 19–26dB in a bedroom. The two copper pipes between the boxes carry refrigerant, not air: no ducts, no fresh-air opening, just a 65mm hole in the wall. Handling that refrigerant is legally restricted to F-Gas certified engineers.
Why it's cheaper to run than people expect
Moving heat beats making it, roughly three to one. A single-room split draws about as much power as a kettle uses in short bursts — around 20p an hour of cooling at 2026 electricity prices. Modern inverter compressors idle at low speed once the room hits temperature instead of stop-starting, which is where the efficiency (and the quiet) comes from.
From theory to a cool bedroom
In practice the machinery arrives as a one-day install: unit on the wall, pipes through, system vacuumed, charged and commissioned. What it costs by room and city: 2026 installation prices.
Frequently asked questions
How does air conditioning work in simple terms?
Air conditioning does not "make cold" — it moves heat. A fluid called refrigerant absorbs heat from indoor air (making the room cooler) and releases it outside. A compressor pumps the refrigerant around this loop continuously. Your fridge works the same way; an air conditioner is the same trick at room scale.
Why does an air conditioner have an indoor and outdoor unit?
The indoor unit is where refrigerant absorbs heat from your room; the outdoor unit is where the compressor squeezes that heat out into the outside air. Splitting the system puts the quiet half (a fan and coil) in your room and the working half (the compressor) outside — which is why modern indoor units run at whisper levels.
How does air conditioning also heat in winter?
A reversing valve flips the cycle: the system absorbs heat from outdoor air (there is usable heat in air well below 0°C) and releases it indoors. Because it moves heat rather than generating it, you get 3–4kWh of heating per 1kWh of electricity — this is all a heat pump is.
What does an inverter air conditioner mean?
The compressor runs at variable speed instead of just on/off. It sprints to reach your set temperature, then idles at low power to hold it — quieter, cheaper to run and steadier than the old start-stop systems. Effectively every unit worth installing in 2026 is an inverter unit.
Does air conditioning use a lot of electricity?
Less than most people assume: moving heat is roughly three times more efficient than generating it. A typical UK single-room split draws 0.7–0.8kW while cooling — about 20p an hour at 2026 prices. The same efficiency logic is why its heating mode undercuts plug-in electric heaters three to one.
Does air conditioning bring in fresh air from outside?
No — standard splits recirculate and cool the room's own air (the pipes to the outdoor unit carry refrigerant, not air). Dehumidifying is a side benefit: moisture condenses on the cold indoor coil and drains away, which is a big part of why conditioned rooms feel better.
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