How Heat Pumps Work: How Air Conditioning Heats Your Home
The refrigerant cycle in plain English: how the air conditioning we fit heats as an air-to-air heat pump, what changes as winter gets colder, and how it differs from an air-to-water heat pump.
Ask how heat pumps work and most UK guides will describe an air-to-water heat pump: a box outside that replaces a gas boiler and heats radiators and hot water. The air conditioning we fit (wall-mounted split, multi-split, ceiling cassette and ducted systems) works on exactly the same principle, but it heats the air in your rooms directly. In heating mode it is an air-to-air heat pump.
This guide explains the cycle inside it, what happens as the weather turns colder, and how it differs from the air-to-water kind.
The short answer
- It moves heat rather than making it. A refrigerant picks up heat from the outdoor air, a compressor raises its temperature, and the indoor unit releases that heat into your room.
- The systems we fit are typically rated at 400% or more seasonal heating efficiency (SCOP 4 or more), with premium models just over 500%. In a cold UK winter, expect about three units of heat for every unit of electricity: a little less in a hard frost, more on a mild day. An electric heater gives one.
- In summer a valve reverses the flow, and the same system cools the room instead.
- It heats air, not water. Your hot water stays with your boiler, cylinder or immersion heater.
Moving heat, not making it
An electric heater turns electricity into heat, one unit for one unit. Air conditioning in heating mode does something different: it uses electricity to carry heat that is already in the outdoor air into your home. Moving heat takes far less energy than making it.
Your fridge does the same job the other way round. It takes heat out of the food compartment and lets it out at the back, which is why the back of a fridge feels warm. A reversible air conditioner can run that process in either direction.
Heat always flows from warmer to colder on its own, never the other way. So the trick is temperature. Outdoors, the refrigerant is made colder than the air, so heat flows into it, even on a frosty day. Indoors, the compressor makes it hotter than the room, so heat flows out of the indoor unit and into the room. In summer the same trick runs the other way round: the refrigerant is made colder than the room indoors and hotter than the air outside.
It works in winter because cold air is not empty of heat. 0 °C is about 273 degrees above absolute zero, and air well below freezing still holds heat that a cold enough refrigerant can pick up.
The four parts of the cycle
Inside every split air conditioner a refrigerant circulates around a sealed loop of copper pipe between the outdoor and indoor units. Four parts do the work:
The outdoor coil
In heating mode, where the refrigerant evaporates and picks up heat from the outdoor air. Engineers call it the evaporator.
The compressor
In the outdoor unit. It squeezes the refrigerant gas, raising its pressure and its temperature. This is where most of the electricity goes.
The indoor coil
In the wall, ceiling or ducted unit. The hot refrigerant condenses here and gives its heat to the room. Engineers call it the condenser.
The expansion valve
Drops the refrigerant’s pressure, and with it its temperature, so it can pick up heat outdoors again.
A fifth part, the reversing valve, decides which way round the loop runs: heating, cooling, or a short defrost.
Heating mode, step by step
- 1
The outdoor unit gathers heat
A fan draws outdoor air across a coil carrying cold liquid refrigerant. The refrigerant runs a few degrees colder than the outside air, so even on a frosty day heat flows from the air into it.
- 2
The refrigerant evaporates
As it takes in that heat, the refrigerant boils and becomes a low-pressure, low-temperature gas. R32, the refrigerant in most new systems, boils at about −52 °C at normal air pressure, which is why it can pick up heat from air that feels cold to us.
- 3
The compressor squeezes it hotter
An electric compressor in the outdoor unit squeezes the gas. As its pressure rises, so does its temperature, to well above room temperature. This is where most of the electricity goes.
- 4
The indoor unit releases the heat
The hot gas travels through insulated copper pipes to the indoor unit. A fan blows room air across the indoor coil, the refrigerant gives up its heat to the room, and it condenses back into a liquid.
- 5
The expansion valve resets it
The liquid passes through an expansion valve, which drops its pressure so that it turns colder than the outdoor air again, ready to pick up more heat. The cycle runs continuously while the system is heating.
Cooling mode: the reversing valve
In summer the reversing valve sends the refrigerant round the loop the other way. Now the indoor coil is the cold one: it takes heat out of your room, and the outdoor unit lets it out into the air outside. As room air passes over the cold coil, some of its moisture condenses too, so the room feels drier as well as cooler.
One system, one set of pipes, heating in winter and cooling in summer. That is the main difference between reversible air conditioning and most air-to-water heat pumps, which heat only.
How efficient is it?
The ratio of heat delivered to electricity used is called the coefficient of performance, or COP. An electric heater’s COP is 1. For air conditioning in heating mode, GOV.UK’s guidance on air-to-air heat pumps puts it at about three units of heat for every unit of electricity, and the Energy Saving Trust puts them at about three times as efficient as standard electric heaters. You may see the same ratio written elsewhere as “300% efficient”.
The ratio changes with the weather, so product labels quote a seasonal figure, the SCOP, measured in a standard test over a typical heating season. The systems we fit are typically rated at a SCOP of 4 or more in that test, which is 400% or more, and premium models just over 5, or over 500%. Real homes usually see noticeably less than the label, because of colder spells, defrosting, and how a system is sized and used. That is why “about three” is the honest everyday figure, and the one our cost sums use.
| Outside | Heat for each unit of electricity |
|---|---|
| Mild, 7 to 10 °C | about 4 |
| Around freezing, 0 to 5 °C | about 3 |
| Hard frost, around −5 °C | about 2 to 2.5 |
| Any electric heater, any weather | 1 |
Illustrative figures for a modern wall-mounted system. The real ratio depends on the model, how it is sized and installed, and how it is used.
What that means for your bills, room by room, is worked through in air conditioning vs electric heaters, and against a gas boiler in our heating comparison.
How it works in winter
As it gets colder
The colder the outdoor air, the harder the system has to work to lift its heat to room temperature. So as the temperature drops, each unit of electricity brings in a little less heat, and the most heat the system can deliver falls too. Modern systems keep heating well below zero; the working limit depends on the model, commonly around −15 °C to −20 °C.
That is why sizing matters. At the survey we work out how much heat each room needs on a cold winter day, not an average one, and size the system to match.
Defrost cycles
In cold, damp weather the outdoor coil runs below freezing, and moisture in the air freezes onto it as frost. Frost blocks the airflow, so the system clears it automatically. The reversing valve briefly sends warm refrigerant to the outdoor coil to melt it, and then switches back to heating.
During a defrost, which usually lasts a few minutes:
- the indoor fan stops, so it does not blow cold air into the room, and the heat output drops for a while;
- you may see steam rising from the outdoor unit, or water dripping from it;
- afterwards the unit goes back to its full heating output.
All of that is normal. Defrosting does use some energy, which is one reason real-world efficiency sits below the figure on the label.
Keep the outdoor unit clear
In heating mode the outdoor unit needs a free flow of air through its coil. A few simple habits keep it working well through the winter:
- Clear leaves, snow and plant growth from around it, especially from the back and front where the air goes in and out.
- Don’t cover it while it is in use. A cover starves it of the air it takes its heat from.
- Let meltwater drain away. Keep the ground underneath it clear so water from defrosting does not pool and refreeze.
- Don’t chip at ice or pour hot water on it; the thin fins of the coil bend easily. If ice stays on the coil, or keeps building up after a defrost, switch the system off and call us.
A yearly service keeps the coils clean and the system running efficiently, and is often a condition of the manufacturer’s warranty. See our air conditioning servicing.
Air-to-air or air-to-water: which heat pump is which?
Both take heat from the outdoor air with the same refrigerant cycle. The difference is where the heat goes.
| Air-to-air (air conditioning) | Air-to-water | |
|---|---|---|
| What it heats | The air in the room, through wall, ceiling or ducted indoor units | Water for radiators or underfloor heating |
| Hot water | No | Usually, with a hot water cylinder |
| Cooling | Yes, most systems are reversible | Usually not |
| What it replaces | Electric heaters, or works alongside a boiler | A gas or oil boiler |
Arctic AC fits air-to-air systems only: wall-mounted split, multi-split, ceiling cassette and ducted air conditioning. We don’t fit air-to-water heat pumps. For the full comparison, see air-to-air vs air-to-water heat pumps.
About the refrigerant
Most new split systems use R32. It boils at about −52 °C at normal air pressure, which is what lets it pick up heat from cold outdoor air.
It is not harmless. R32 has a global warming potential of 675: lower than the R410A many older systems use, but still a strong greenhouse gas if it escapes. That is why the circuit is sealed and pressure-tested when it is fitted, and why by law only F-gas certified engineers may install or work on it.
Video transcript
How does air conditioning heat your home? In heating mode, it’s an air-to-air heat pump. It doesn’t make heat. It moves it.
Heat always flows from warmer to colder, all on its own. So the refrigerant is made colder than the air outside, and hotter than the room inside.
Even frosty air holds heat. In the outdoor unit, the refrigerant runs colder than the air. So heat flows into it, and it boils into a gas.
Then the compressor squeezes that gas. As its pressure rises, it gets hotter: well above room temperature.
Indoors, a fan blows room air across the hot coil. The refrigerant gives up its heat to your room, and turns back into a liquid.
Then an expansion valve drops its pressure. Now it’s colder than the outdoor air again, and round it goes.
Over a typical season, the systems we fit are rated at about four units of heat for every unit of electricity. A bit less in a hard frost, more on a mild day.
In cold, damp weather, frost builds up on the outdoor coil. So now and then, it defrosts: for a few minutes, warm refrigerant goes outside to melt it. The indoor fan pauses, and you may see steam. That’s normal.
In summer, a reversing valve sends the refrigerant the other way. And the same system cools your room.
Arctic Air Conditioning. Cool in summer, warm in winter.
Frequently asked questions
How do heat pumps work?
A heat pump moves heat rather than making it. A refrigerant picks up heat from the outdoor air, a compressor raises its temperature, and an indoor unit releases that heat into your home. Reversible air conditioning works this way in heating mode, as an air-to-air heat pump. The systems we fit are typically rated at a seasonal heating efficiency of 400% or more (SCOP 4 or more). Real winter output is lower than the rating: GOV.UK puts it at about three units of heat for every unit of electricity it uses, a little less in a hard frost and more on a mild day.
Is air conditioning a heat pump?
Reversible air conditioning is an air-to-air heat pump: it heats a room by moving heat in from outdoors, and cools it by moving heat out. In the UK, “heat pump” on its own usually means an air-to-water heat pump, which replaces a boiler and heats radiators and hot water. Arctic AC fits air-to-air systems only: wall-mounted split, multi-split, ceiling cassette and ducted air conditioning.
Does air conditioning heat work in winter?
Yes. Air below freezing still holds heat that the refrigerant can pick up, and modern systems keep heating well below zero; the working limit depends on the model, commonly around −15 °C to −20 °C. Efficiency falls as it gets colder, and in frosty weather the outdoor unit pauses now and then to defrost, but it still gives more heat for each unit of electricity than an electric heater.
Why does my air conditioning stop heating for a few minutes in cold weather?
It is defrosting. In cold, damp weather, frost builds up on the outdoor coil. The system clears it automatically by briefly reversing the refrigerant flow to warm the coil. The indoor fan stops so it does not blow cold air, and you may see steam or water coming from the outdoor unit. After a few minutes it goes back to heating. That is normal.
Should I cover the outdoor unit in winter?
No. In heating mode the outdoor unit needs air flowing freely through it, so a cover stops it working. Keep snow, leaves and plant growth clear of it, and leave room for meltwater to drain away. If ice stays on the coil or keeps building up after it has defrosted, switch it off and call your installer.
Can air conditioning heat my hot water?
No. An air-to-air system heats and cools the air in the rooms it serves. Your hot water stays with your existing boiler, cylinder or immersion heater. An air-to-water heat pump is the kind that heats water.
Is R32 refrigerant environmentally friendly?
It is better than the refrigerant it replaced, but not harmless. R32 has a global warming potential of 675: lower than the R410A many older systems use, but still a strong greenhouse gas if it escapes. That is why the circuit is sealed and tested, and why only F-gas certified engineers may install or work on it.
See what it would cost in your home
Two minutes with our estimator gets you a realistic range for your rooms. A free survey turns it into a fixed quote, with no obligation and no hard sell.
Sources
- GOV.UK: Check if a heat pump is suitable for your home (air-to-air)
- Energy Saving Trust: In-depth guide to heat pumps
- Energy Saving Trust: Electric heating
- Department for Energy Security and Net Zero: The suitability of reversible air-to-air heat pumps for UK domestic use (July 2026)
- Fluorinated greenhouse gases regulation, Annex I: global warming potential of HFC-32 (R32)
- GOV.UK: Certification for companies working on equipment containing F gas
- Manufacturers’ data sheets and operation manuals for operating limits and defrost
- Manufacturers’ ErP product fiches and databooks (Mitsubishi Electric, LG, Samsung, Daikin, Midea), compiled October 2026: the published seasonal ratings of the models we fit
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