How air conditioning actually works (and why it is a heat pump)

There is no such thing as cold

Cold is the absence of heat, not a substance. Nothing can produce cold – it can only remove heat from somewhere and put it somewhere else.

That is what an air conditioner does. It takes heat out of the air in your room and discharges it outside. The outdoor unit is hot because your room’s heat is coming out of it. The total amount of heat in the world goes up slightly, because the electricity used to run the process also ends up as heat – but it ends up outside rather than in your room.

The cycle

The trick is a physical property: when a liquid evaporates it absorbs a large amount of heat, and when a gas condenses it releases the same amount. The refrigerant circuit is an arrangement for controlling where each of those happens.

1. Evaporator – indoorsCold low-pressure liquid refrigerant enters the indoor unit’s coil. Room air is blown across it. The refrigerant boils, absorbing heat from that air. The air leaving the unit is cooler – and drier, because moisture condenses out of it onto the cold coil.
2. Compressor – outdoorsThe now-gaseous refrigerant is drawn to the outdoor unit and compressed. Compressing a gas raises its pressure and its temperature substantially – it is now hotter than the outside air, which is the whole point.
3. Condenser – outdoorsThe hot high-pressure gas passes through the outdoor coil with outside air blown across it. Because the refrigerant is hotter than the outside air, heat flows out of it, and the refrigerant condenses back to a liquid.
4. Expansion deviceThe high-pressure liquid passes through a restriction. Its pressure drops sharply and with it its temperature, so it emerges cold and ready to evaporate again.
5. Round againContinuously, while the system runs.

Why it is a heat pump

Notice that the cycle simply moves heat from the evaporator to the condenser. If you could swap which coil is which, the same machine would move heat from outside to inside – which is heating.

That is exactly what a reversing valve does. Nearly every modern split air conditioner has one, which is why almost all of them can heat as well as cool. It is not an added feature bolted on – it is the same machine running the other way.

So an air conditioner is an air source heat pump. The differences between a split system and what the industry calls a heat pump are about what they are optimised for and what they deliver heat into, not about the underlying physics – see air conditioning vs a heat pump.

Why it can be more than 100% efficient

This confuses people reasonably, because it sounds impossible.

An electric heater converts electricity to heat at essentially 100 per cent – one unit in, one unit of heat out. It cannot do better, because it is creating heat from electricity.

A heat pump does not create heat. It uses electricity to move heat that already exists. One unit of electricity can move several units of heat, because the energy is being spent on running a compressor and a fan rather than on generating the heat itself.

That ratio is the coefficient of performance, and it is not fixed. It depends strongly on the temperature difference the system is working across:

• Cooling on a mild day – a small difference between indoors and outdoors. Efficient.
• Cooling in a heatwave – a large difference. Less efficient, and it is working harder at exactly the time you want it most.
• Heating in mild weather – efficient.
• Heating on a very cold day – a large difference. Less efficient.

This is why sizing and siting matter so much, and why a system asked to work across a large temperature difference costs disproportionately more to run.

Inverter control, which is why modern systems are better

An older system’s compressor was either on or off. To hold a room at temperature it ran flat out, overshot, stopped, drifted, and started again. Every start is a surge of current and a thermal shock, and the temperature in the room swings.

An inverter varies the compressor speed continuously. It runs hard to pull the room down initially, then slows to exactly match the rate at which heat is entering the room, and holds there. The benefits are substantial:

• Much better efficiency, because a compressor running slowly is disproportionately more efficient than one cycling on and off.
• Stable temperature, with no swing.
• Quieter, because it spends most of its time running slowly.
• Longer compressor life, because starts are what wear compressors.

It is also why oversizing matters. An oversized inverter system cannot slow down enough to match a small load, so it reverts to cycling and loses the advantage – see sizing air conditioning.

The dehumidification you did not ask for

When warm room air meets the cold evaporator coil, moisture condenses out of it – exactly as it does on a cold drink on a summer day. That water runs into a tray and out through the condensate drain.

This matters for three reasons:

1. Comfort. Removing humidity makes a room feel considerably cooler than the temperature alone suggests. Much of what air conditioning actually delivers in comfort terms is dehumidification.
2. Oversizing ruins it. An oversized unit reaches setpoint quickly and stops, so it has not run long enough to remove much moisture. The room is cool and clammy – which is worse, not better.
3. The water has to go somewhere. Condensate drainage is the single most common cause of visible damage from air conditioning, and gravity drainage is always preferable to a pump – see air conditioning servicing.