Air conditioning that heats and cools
By Airva Editorial Team · Checked against published UK guidance · Updated
Nearly every air conditioning system sold in the UK today heats as well as cools, and most buyers never use the heating. That is a shame, because the winter half is where the economics are strongest — and it is also, thanks to a quirk of planning law, the half that gives you the right to install the outdoor unit in the first place. This page is about using the system in the season people forget.
The same hardware, running backwards
A cooling system moves heat from inside to outside. Reverse the refrigerant flow with a valve, and it moves heat from outside to inside. That is the whole trick, and it is why the system is properly called a reversible air-to-air heat pump rather than an air conditioner.
The counterintuitive part is that there is usable heat in cold air. Even at freezing, outdoor air contains energy a refrigerant circuit can extract and concentrate. The system is not generating heat, it is collecting and moving it — which is why it can deliver several units of warmth for each unit of electricity, where a resistance heater is stuck at one for one.
Cooling mode. The electricity does not make cold — it drives the compressor and fans that move heat that is already there. A reversing valve swaps the two coils so the same hardware heats in winter.
Why the winter case is the strong one
Take a 2.5kW unit heating a room five hours a day from October to March, against an electric heater doing the same job, at the Great Britain average electricity unit rate of 26.32p per kWh [3]:
| Heat delivered | Electricity used | Cost per season | |
|---|---|---|---|
| Reversible unit, SCOP 4.5 | 2,250 kWh | 500 kWh | £132 |
| Electric panel or fan heater | 2,250 kWh | 2,250 kWh | £592 |
The same warmth, for roughly a quarter of the running cost. The comparison that matters is against direct electric heating, which is what most of these rooms actually have — a loft conversion with a plug-in heater, a garden room, a converted garage, a home office at the end of a heating circuit that never quite arrives. Against an efficient gas boiler the picture is closer and moves with the relative price of gas and electricity.
Our running costs guide works both seasons through in full.
What happens when it gets genuinely cold
This is the honest part, and it is where a lot of marketing goes quiet.
Capacity falls as it gets colder. The colder the outside air, the less heat there is to collect and the harder the compressor works. A unit rated at 3.5kW of heat output at 7°C outdoors will deliver less at -5°C. Manufacturers publish performance at several outdoor temperatures, and for a room you intend to heat rather than just cool, those are the figures to look at — not the headline capacity.
Defrost cycles interrupt it. In cold, damp conditions — the classic British winter morning — moisture from the outdoor air freezes onto the outdoor coil. The system periodically reverses briefly to melt it off. During a defrost cycle the unit is not heating your room, and you may hear it change note and see steam from the outdoor unit. This is normal operation, not a fault. It does mean the effective output over a cold morning is lower than the steady-state figure.
SCOP already accounts for this. The seasonal figure averages performance across a realistic range of conditions including the cold end, which is why SCOP is the number to compare rather than a single flattering test point. What SCOP cannot account for is a badly sized unit, a poorly sited outdoor unit sitting in its own cold discharge air, or a draughty room.
Cold-climate models exist. Several manufacturers produce ranges specified to maintain rated output at low outdoor temperatures. They cost more. If the system is your primary heat source for that room, they are worth the conversation; if it is a supplement to existing heating, they usually are not.
The planning consequence nobody mentions
Here is the part that turns a preference into a decision. Permitted development for outdoor units in England sits in Class G, which is written for air source heat pumps, and one of its conditions is that the unit must be not used solely for cooling purposes [2].
A reversible system satisfies that. A cooling-only air conditioner does not, and generally needs planning permission. The 2025 amendments to those rights were made partly to support air-to-air heat pumps that also provide a cooling function [2].
So specifying a reversible system is not only the better buy on running cost — it is the version with permitted development rights attached. Our planning permission guide sets out the full checklist.
Where it works best as heating
| Room | Why reversible heating suits it |
|---|---|
| Loft conversion | Hot in summer, cold in winter, and usually at the end of the heating circuit. The clearest case in most houses. |
| Garden room or garage conversion | Often no wet heating at all, so the alternative is a plug-in heater at four times the running cost. |
| Home office | Needs heat fast in the morning for one room, without firing the whole house's boiler. |
| Conservatory | The extreme of both seasons. Cooling gets the attention; the winter half is why it becomes usable all year. |
| Bedroom | Gentle background warmth on low fan speed, quietly — see our quiet units guide. |
Government-commissioned research has examined the suitability of reversible air-to-air heat pumps for UK domestic use [1], and the room-by-room case above is where they fit most naturally: spaces that are poorly served by a central wet system and would otherwise be heated electrically.
What it does not replace
Be clear about scope. A reversible air conditioning system heats the rooms it serves. It does not heat your water, and it is not a boiler replacement. If you are looking to replace a gas boiler and heat the whole house including hot water, that is an air-to-water heat pump — a different product on a different budget, with different grant eligibility. Our air-to-air versus air conditioning guide maps which system suits which problem.
Frequently asked questions
Can air conditioning heat a room? Yes. Almost all modern units are reversible and heat efficiently, typically delivering several units of heat per unit of electricity.
Does air conditioning work in cold weather? Yes, with reduced capacity as outdoor temperatures fall, and with periodic defrost cycles in cold damp conditions during which it pauses heating. Cold-climate models hold output better at low temperatures.
Is it cheaper than my electric heater? Substantially. On the worked example above, about £132 a season against roughly £592 for a resistance heater delivering the same warmth [3].
Is it cheaper than gas central heating? Closer, and it depends on the gas and electricity rates at the time. Where it wins clearly is heating one room that the wet system serves badly, without heating the whole house.
Why does the outdoor unit steam in winter? That is a defrost cycle melting ice off the outdoor coil. Normal operation.
Should I buy a cooling-only unit if I only want cooling? Generally no. It is rarely cheaper, it costs you the permitted development right [2], and it removes the half of the system with the better economics.
Get it sized for both seasons
If you intend to heat with it, the survey needs to calculate the heating load as well as the cooling load. Tell us about the rooms and we will match you with one vetted installer who will do both: get an installer match.