How does air conditioning actually work?

Air conditioning doesn’t make cold air. It moves heat out of a room and dumps it outside. That distinction is the whole thing, so it’s worth understanding before anything else.

The refrigerant cycle, briefly

Inside the system is a refrigerant: a fluid that absorbs heat as it turns from liquid to gas, and releases heat as it turns back from gas to liquid. A wall-mounted unit cycles that refrigerant between the indoor and outdoor units:

Warm air from your room passes over a cold coil inside the indoor unit. The refrigerant inside that coil absorbs the heat and evaporates into a gas.
That gas travels to the outdoor unit, where a compressor pressurises it, which makes it hot.
The outdoor unit’s fan blows air across the hot gas, releasing that heat outside and turning the refrigerant back into a liquid.
The now-cool liquid refrigerant returns indoors, and the cycle repeats.

The air you feel coming from the indoor unit hasn’t been “cooled” from nothing. It’s the same room air, with its heat already removed.

Why the same system can heat, too

Nearly every modern residential system sold in the UK is a reversible heat pump, which means it can run that entire cycle backwards: extracting heat from the (surprisingly still-present) warmth in outside air, and releasing it indoors. That’s what people mean when they say an air conditioning unit “also heats”: it’s not a separate heating element, it’s the same refrigerant cycle in reverse.

Why it also dehumidifies

As warm, humid room air passes over the indoor unit’s cold coil, moisture in the air condenses on that coil the same way it condenses on a cold glass on a warm day. That water drains away outside, and the air returned to the room is both cooler and drier, which is part of why a room with air conditioning running often feels more comfortable than the temperature alone would suggest.

Why sizing it correctly matters

A unit that’s too small for a room runs constantly and still struggles to keep up. One that’s too large cools the room quickly but shuts off before it’s had time to properly dehumidify the air, which can leave a room feeling cold and slightly clammy at the same time. Getting the kW rating right for your specific room is worth more to comfort than most other decisions in the process.