How air conditioning works

The air from the indoor unit feels cold, so it is natural to think the machine makes cold the way a heater makes heat. It works more like a conveyor belt: heat is picked up indoors, carried through a pair of pipes and put down outside. That one idea explains why there is an outdoor unit, why it blows warm air in summer and why the same equipment can heat a room in winter.

Pushing heat the wrong way

Left to itself, heat only flows from somewhere warmer to somewhere cooler. On a hot day that means heat from outside creeps into your home through the walls, windows and roof. To cool a room, heat has to go the other way, from the cooler room to the hotter air outdoors, and that takes energy.

Air conditioning does it with a refrigerant, a fluid sealed inside the pipes. By changing the refrigerant's pressure, the system makes it boil in a coil indoors, soaking up heat as it does, and condense in a coil outdoors, where it gives that heat back.

The electricity runs the compressor that drives the refrigerant round and the fans that move air across the coils. None of it is turned into cold. It is the effort needed to push heat uphill.

The main parts

A wall-mounted split system, a common choice for fixed air conditioning in UK homes, has four main parts:

• The indoor unit blows room air across a cold coil and back into the room.
• The outdoor unit holds the compressor, plus a second coil and a fan that release the heat outside.
• The connecting pipes carry refrigerant between the two, bundled with a cable and a drain hose.
• The controls read the room temperature and decide how hard the system should work.

Each component is covered in more detail in what the indoor and outdoor units do.

What happens when you switch on cooling

1. The indoor unit compares the room temperature with your setting and works out how much cooling is needed.
2. The compressor starts and refrigerant begins to circulate. Before it reaches the indoor coil, its pressure is dropped, which makes it much colder than the room.
3. The indoor fan pulls warm air across the coil. The refrigerant absorbs heat from the air and boils, and the cooled air is blown back into the room.
4. Water vapour in the air condenses on the cold coil and runs away through the drain hose.
5. The refrigerant, now carrying the room's heat, returns to the outdoor unit. The compressor squeezes it until it is hotter than the outdoor air, and the outdoor fan blows that heat away. This is why the air leaving the outdoor unit feels warm.
6. As the room approaches your setting, most split systems slow the compressor down rather than switching it off, which holds the temperature steady.

Why pressure matters so much, and what each stage does, is explained in the refrigeration cycle, step by step.

Heating, drying and efficiency

Heating. Most wall-mounted systems can run the process in reverse. A valve in the outdoor unit changes the direction the refrigerant flows, so the outdoor coil collects heat from the outside air, even in cold weather, and the indoor coil releases it into the room. There is more on this in how air conditioning heats a room.

Drying. The indoor coil usually runs colder than the dew point of the room air, so cooling removes moisture as well as heat. Dry mode is designed to make more of this effect.

Efficiency. The heat moved can be several times the electricity used. The ratio is called COP when heating and EER when cooling, and in mild weather small air-to-air systems commonly reach a COP of 3 to 5. Efficiency falls as the gap between indoor and outdoor temperature widens, so heating is harder work on a freezing day and cooling is harder work in a heatwave.

How other kinds of cooling compare

Every refrigerant air conditioner has to get rid of the heat it collects. What differs is where that heat goes and where the noisy compressor sits.

The trade-offs between fixed systems and units you can move around are set out in fixed or portable.