How does air conditioning work?
A plain-English guide to how home air conditioning works: it does not make cold, it moves heat. We explain the refrigeration cycle, the parts and the jargon.
Air conditioning works by moving heat, not by making cold. An air conditioner does not create coolness out of nothing. It collects heat from the air inside your home and carries it outside, leaving the room cooler and drier. It does this with a circulating fluid called a refrigerant and a continuous loop known as the refrigeration cycle. Because it is, in engineering terms, a heat pump, a reverse-cycle unit can run the same loop backwards to heat your home in winter too.
That one idea, "it moves heat, it does not make cold", is the key to understanding everything else. Once it clicks, the parts, the jargon and the energy bills all make sense.
Does an air conditioner make cold, or move heat?
It moves heat. Heat naturally flows from warm things to cooler ones, which is why a closed-up house heats up in summer. An air conditioner works against that natural flow: it actively pumps heat out of the room and dumps it outside, the same way a fridge keeps its inside cold by moving heat into your kitchen.
There is no "cold" being manufactured. The room feels cooler because heat has been taken away from it. This is why every split air conditioner has two parts: an indoor unit where heat is collected, and an outdoor unit (the box on the wall or the ground) where that heat is released.
It also dries the air as it works. Warm indoor air holds moisture, and when that air passes over the cold indoor coil, water condenses out of it, just like a cold drink "sweats" on a hot day. That is why an air conditioner produces water and why a treated room feels fresher, not just cooler.
How the refrigeration cycle works, step by step
The whole system runs on a refrigerant: a fluid chosen because it boils and condenses at convenient temperatures, so it can soak up heat in one place and release it in another. The refrigerant flows around a sealed loop in four repeating stages.
| Stage | Where | What happens |
|---|---|---|
| 1. Evaporator coil | Indoor unit | Cold, low-pressure liquid refrigerant absorbs heat from room air blown across it and boils into a gas. The air leaving the coil is cooler and drier. |
| 2. Compressor | Outdoor unit | Squeezes the low-pressure gas into a hot, high-pressure gas. This is the pump at the heart of the system and the main user of electricity. |
| 3. Condenser coil | Outdoor unit | The hot, high-pressure gas releases its heat to the outside air (helped by the outdoor fan) and condenses back into a liquid. This is where the heat from your room is dumped outside. |
| 4. Expansion valve | Between the units | The high-pressure liquid passes through a narrow valve, dropping sharply in pressure and temperature, becoming the cold liquid that re-enters the evaporator. The cycle then repeats. |
Round and round it goes, continuously, for as long as the unit is cooling. The refrigerant never gets used up; it just changes between liquid and gas as it carries heat from inside to outside.
What each part does
If you understand the four stages, the components almost explain themselves. Here is the plain-English version of each.
Refrigerant: the heat-carrying fluid that cycles between liquid and gas. In most new UK home units this is R32 (more on that below).
Compressor: raises the refrigerant's pressure and temperature. It sits in the outdoor unit, is the main moving part, and uses most of the electricity.
Condenser coil and outdoor fan: reject the collected heat to the outside air.
Evaporator coil and indoor fan: absorb heat and humidity from the room.
Expansion valve: drops the pressure to chill the refrigerant before it reaches the evaporator.
Reversing valve (reverse-cycle units only): swaps the jobs of the two coils so the unit heats the room instead of cooling it.
What is refrigerant, and what gas do UK units use?
Refrigerant is the fluid that does the actual heat-carrying. Without it there is no cooling, just two fans blowing air around.
The standard refrigerant in new UK domestic split systems is R32. It has replaced the older R410A. The reason matters for the environment: R32 has a global warming potential (GWP) of around 675, roughly a third of R410A's 2,088, so a leak does far less climate damage. R32 systems also need around 30% less refrigerant and are slightly more efficient. Older equipment may still use R410A, which is being phased out of new systems under F-Gas rules. The exact UK dates and equipment categories are worth checking before you rely on them, but the direction of travel is clear: new home units are R32.
Because refrigerant is a fluorinated gas, the law treats it seriously. That has real consequences for installation, which we cover further down.
Why an air conditioner is the same thing as a heat pump
This surprises a lot of people. An air conditioner and a heat pump are the same machine doing the same job: moving heat from one place to another. An air conditioner moves heat out of your home. A heat pump used for heating moves heat into it. The hardware is fundamentally the same.
That is why a reverse-cycle air conditioner (also called an air-to-air heat pump) can do both. A reversing valve flips the direction of the cycle, so the outdoor coil now collects heat from the outside air, even when it is cold out, and the indoor coil releases it into your room. The same box on your wall cools you in July and warms you in January.
This is also where a common money question comes up. The Boiler Upgrade Scheme grant of up to £7,500 is for air-to-water heat pumps, which heat your central heating water and radiators, not for typical air con. Separately, a £2,500 air-to-air heat pump grant became available in England and Wales from 28 April 2026, but it comes with real conditions: the unit must heat as well as cool, it must replace an existing fossil-fuel or electric heating system, and it must be fitted by an MCS-certified installer. It does not apply to comfort-cooling air con fitted alongside a kept gas boiler, and it does not apply to portable units. In other words, air conditioning is not "free", and most cooling-only installs get no grant at all. Always check the current GOV.UK and Ofgem pages before counting on any grant.
Inverter vs fixed-speed: why it matters for your bills
When you shop for air con you will see "inverter" almost everywhere. It refers to how the compressor runs, and it makes a real difference to comfort, noise and running cost.
| Feature | Fixed-speed (non-inverter) | Inverter |
|---|---|---|
| How it runs | Fully on or fully off | Variable speed, runs continuously at a low level |
| Temperature | Swings as it cycles on and off | Held steady |
| Noise | Louder, with surges | Quieter |
| Efficiency | Lower | Higher |
| Purchase price | Cheaper | Higher upfront |
A fixed-speed unit blasts at full power until the room hits the target, switches off, then restarts when the room drifts, giving you temperature swings, surges and more noise. An inverter ramps up quickly, then slows down and ticks over gently to hold a steady temperature, which is more efficient and quieter. Inverter technology is now found on most quality UK domestic splits, though some budget and portable units are still fixed-speed.
Efficiency is measured by SEER (a seasonal score for cooling) and SCOP (a seasonal score for heating). Higher is better, and units carry an ErP energy label from A+++ down to D, much like a fridge or washing machine. As a rough orientation, a home air conditioner might use in the region of 2,000 kWh of electricity a year, though that is a US estimate and real UK usage depends heavily on how hot your summer is and how long you run the unit. For UK figures and worked examples, see our running-costs guide.
How air conditioning works in a house: the system types
The cycle above is the same whatever the system. What changes is how the indoor and outdoor parts are arranged. UK homes are dominated by ductless splits, where refrigerant pipes link a wall unit inside to a condenser outside. The main options are:
Single split: one indoor unit paired with one outdoor unit. The default for cooling a single room.
Multi-split: several indoor units running off one outdoor unit, for cooling multiple rooms.
Portable: a plug-in unit that vents hot air out of a window through a hose. Cheaper, but noisier and less efficient.
Ducted or concealed: the indoor unit is hidden, with air delivered through grilles for a tidier finish.
You may also read about "central air", but that is far more common in the US than in UK homes, so treat it as background rather than the UK norm. To go deeper, see our guide to the types of air conditioning system.
Can I install it myself?
In practice, no. A refrigerant-charged split system contains fluorinated gas, so by law it must be installed, serviced and decommissioned by an F-Gas certified company (in the UK, typically REFCOM registered), with engineers holding a personal F-Gas qualification. As the owner you are responsible for making sure whoever works on your equipment is properly certified. That makes DIY installation effectively a no-go for any refrigerant-charged unit. A plug-in portable, which needs no refrigerant work, is the exception.
When you are ready to buy, the safest route is to compare local F-Gas registered installers and get a proper quote for your home.
Frequently asked questions
Does an air conditioner make cold or move heat? +
It moves heat. There is no "cold" being created. The unit collects heat from the room air and releases it outside, so the room is left cooler and drier. It works just like a fridge, which keeps its inside cold by moving heat out into your kitchen.
Why does my air conditioner drip water? +
Because it dehumidifies as it cools. Warm room air holds moisture, and when that air passes over the cold indoor coil the moisture condenses into water, the same way a cold glass sweats on a hot day. That water is drained away, which is why a treated room feels fresher as well as cooler.
What gas does a home air conditioner use? +
New UK domestic split systems use R32 refrigerant, which has replaced the older R410A. R32 has roughly a third of R410A's global warming potential and needs less charge, so it is the current mainstream choice. Older units may still contain R410A.
Can air conditioning heat my home too? +
Yes, if it is a reverse-cycle unit (an air-to-air heat pump). A reversing valve runs the cycle backwards, pulling heat from the outside air and releasing it indoors, so the same unit both cools in summer and heats in winter. Cooling-only units cannot do this.
What is the difference between inverter and fixed-speed air conditioning? +
A fixed-speed compressor is either fully on or fully off, which causes temperature swings, more noise and lower efficiency. An inverter compressor varies its speed and runs continuously at a low level, holding a steady temperature more quietly and efficiently, for a higher purchase price.
Can I install air conditioning myself? +
Not a refrigerant-charged unit. By law these must be installed by an F-Gas certified company, so DIY is effectively off the table. A plug-in portable unit is the only type you can set up yourself, because it needs no refrigerant work.
Now see the different types
Now you know how it works, the next step is picking a format. Our guide compares the main types of air conditioning so you can see what suits your home.
