The short answer
Refrigerant boils in a cold coil, taking in heat; a compressor squeezes the vapour, making it hot; it condenses in a second coil, giving the heat away; and an expansion valve drops its pressure so it can boil again.
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Air conditioners, heat pumps and most fridges all rely on the same loop, repeated for as long as the compressor runs. The refrigerant is never used up. It simply changes back and forth between liquid and vapour, carrying heat from one coil to the other.
Why pressure is the key
A liquid's boiling point depends on the pressure around it. Water boils at 100°C in a pan at sea level, at a lower temperature up a mountain and at a higher one inside a pressure cooker.
Refrigerants are chosen because they boil at very low temperatures. At normal atmospheric pressure R32, which many home systems use, boils at about −52°C. Sealed inside the system, the refrigerant can be made to boil or condense at a useful temperature simply by setting its pressure:
- At low pressure, it boils well below room temperature, so it can absorb heat from room air.
- At high pressure, it condenses above the outdoor temperature, so it can release that heat outside.
Boiling and condensing move a lot of heat with little change in temperature, which is why a modest flow through narrow pipes can carry the heat from a whole room. Refrigerants differ in their pressures and environmental effects, as refrigerants explained sets out.
The four stages
The stages below describe cooling. In heating, the two coils swap jobs.
Evaporation in the indoor coil
The refrigerant arrives at the indoor coil as a cold, low-pressure mix of mostly liquid and some vapour, typically somewhere around 5 to 10°C when cooling. Warm room air blown across the fins gives up its heat, and the refrigerant boils as it absorbs it.
By the time it leaves the coil it should be entirely vapour, a few degrees warmer than its boiling point. This margin, called superheat, protects the compressor, which liquid can damage. Moisture from the room condenses on the cold coil too, which is how air conditioning removes humidity.
Compression in the outdoor unit
The compressor draws in the low-pressure vapour and squeezes it. Its pressure rises several-fold and its temperature climbs with it, making the gas leaving the compressor the hottest point in the circuit. Most of the system's electricity goes into this stage, and the energy the compressor adds ends up as extra heat in the refrigerant.
Condensation in the outdoor coil
The hot, high-pressure gas flows into the outdoor coil. Even on a hot day it is hotter than the outdoor air, so heat flows out of it as the fan pulls air across the fins, and the refrigerant condenses back into a liquid.
The liquid usually cools a few degrees more before it leaves, which engineers call subcooling, so that it reaches the valve with no bubbles of vapour in it. Because the refrigerant carries the compressor's energy as well as the room's heat, the outdoor coil gets rid of more heat than the indoor coil collected.
Expansion at the valve
The warm liquid, still at high pressure, is forced through a narrow expansion valve. On the other side the pressure is much lower, so some of the liquid instantly boils. That boiling draws heat from the rest of the refrigerant, and the mixture leaves the valve cold, ready for the indoor coil.
The same effect makes an aerosol can feel cold after you spray it for a while. Most modern systems use an electronic expansion valve that opens and closes in tiny steps to control the flow, while some simpler ones use a fixed length of very narrow tube.
The cycle at a glance
| Stage | Where, when cooling | What the refrigerant does | Pressure |
|---|---|---|---|
| Evaporation | Indoor coil | Boils, absorbing heat from the room | Low |
| Compression | Compressor | Heats up as it is squeezed | Low to high |
| Condensation | Outdoor coil | Condenses, releasing heat outside | High |
| Expansion | Expansion valve | Partly boils and turns cold | High to low |
Keeping the loop in balance
As the room cools, the sun moves round and the outdoor temperature changes, the system adjusts all the time. Sensors measure temperatures at several points, and the controls respond by changing the compressor's speed and how far the expansion valve is open. In an inverter system the compressor can run anywhere from a gentle tick-over to full speed.
The amount of refrigerant matters too. With too little, capacity falls, the compressor can run hot and ice may form where it shouldn't. With too much, pressures rise and efficiency suffers.
Because refrigerant is never used up, a system that is short of it has either leaked or was not filled correctly, and topping it up without finding the cause is not a repair. The circuit is sealed and under high pressure, so work on it is for a qualified engineer whatever the refrigerant. With an F-gas refrigerant such as R32, that means someone holding the right F-gas qualification.
The same loop, run in reverse
For heating, a reversing valve in the outdoor unit changes where the hot gas from the compressor goes. The compressor still pumps in one direction, but the refrigerant now travels round the loop the other way. The indoor coil becomes the condenser, releasing heat into the room, and the outdoor coil becomes the evaporator, collecting heat from the outside air.
Because the refrigerant in the outdoor coil can be kept colder than the air around it, even air below freezing gives up heat. What that means in practice is covered in how air conditioning heats a room.
Related guides
- How air conditioning worksAir conditioning cools a room by moving heat out of it, not by making cold. In a typical split system, a refrigerant picks up heat from the room at the indoor unit and a compressor pumps it outside, where the outdoor unit releases it.
- What an inverter doesAn inverter lets the compressor change speed. Instead of switching on and off, the system runs hard to reach the setting, then slows down to match what the room needs, holding a steadier temperature with less noise.
- Refrigerants explainedA refrigerant is the fluid that carries heat around an air conditioner, boiling to absorb it and condensing to release it. Refrigerants differ in their pressures, efficiency, flammability and effect on the climate, which is why the choice matters.
- How air conditioning heats a roomTo heat, the system runs its refrigerant loop in reverse. The outdoor coil turns cold and collects heat from outside air, even below freezing, while the indoor coil turns hot and warms the air blown into the room.