Facts last checked 17 September 2026
The short answer
A heat pump is a fridge working in reverse: it collects low-grade heat from the outside air (or the ground), concentrates it with a compressor and pumps it into your radiators and hot water cylinder. It runs on electricity, but it moves more heat than the electricity it uses. In the government's Electrification of Heat trial, the typical air source heat pump in a British home delivered 2.8 units of heat for every unit of electricity, with the middle half of homes between 2.5 and 3.1. Whether that makes it cheaper to run than your boiler depends on your tariff and on how well the system is designed.
How does a heat pump actually work?
The fridge in your kitchen takes heat out of a cold box and dumps it out of the grille at the back. A heat pump does the same job pointed the other way: the cold box is your garden, and the warm grille is your heating system. Even air at 0°C holds a useful amount of heat. The trick is to move it from somewhere cold to somewhere warm, which is exactly what a compressor and a refrigerant are good at.
- A fan draws outside air across a coil filled with refrigerant, a fluid that boils at a very low temperature. The refrigerant absorbs heat from the air and turns into a gas.
- An electric compressor squeezes that gas. Compressing a gas raises its temperature sharply, just as a bicycle pump gets warm.
- The hot gas passes through a heat exchanger, where it hands its heat to the water circulating through your radiators, underfloor pipes or hot water cylinder.
- Having given up its heat, the refrigerant passes through an expansion valve, cools right down and goes back outside to collect more. The loop runs continuously.
The electricity powers the compressor, fan and pumps. It is not turned into heat directly, the way an immersion heater or a fan heater works. That is why a heat pump can deliver more heat than the energy you pay for.
How efficient is a heat pump in a real British home?
Efficiency is described with a few related numbers, and it is worth knowing which is which because sales material often quotes the most flattering one.
- COP (coefficient of performance): heat out divided by electricity in, at one moment and one set of conditions. It changes minute by minute with the weather.
- SCOP (seasonal coefficient of performance): a calculated average across a heating season for a given design. The Boiler Upgrade Scheme requires the designed system to reach a SCOP of at least 2.8, worked out with the MCS calculator.
- SPF (seasonal performance factor): the measured version of SCOP, taken from meters in a real home over a real year. This is the number that decides your bills.
The best UK evidence comes from the Electrification of Heat demonstration project, funded by the government, which installed 742 heat pumps in a broad mix of British homes and metered them. Its published analysis of 291 air source systems found a median SPF of 2.80, with the middle half of homes between 2.53 and 3.09. That was a clear improvement on the earlier Renewable Heat Premium Payment trial, where the median was 2.44.
The same analysis found something just as important: the spread between the best and worst installations stayed wide, and systems running at lower water temperatures performed better. Two identical heat pumps can give very different bills depending on how they were designed and set up.
What types of heat pump are there?
| Type | Where the heat comes from | What it heats | BUS grant |
|---|---|---|---|
| Air source, air-to-water | Outside air, via a unit on a wall or the ground | Radiators or underfloor heating, plus a hot water cylinder | £7,500 (£9,000 for eligible off-gas oil or LPG homes until 31 March 2027) |
| Ground source | Pipes buried in trenches or boreholes | Radiators or underfloor heating, plus a hot water cylinder | £7,500 (£9,000 for eligible off-gas oil or LPG homes until 31 March 2027) |
| Air-to-air | Outside air | Blows warm air into rooms, like air conditioning in reverse. No hot water. | £2,500 |
| Hybrid | Outside air, alongside a gas or oil boiler | Shares the heating load with the boiler | Not eligible when paired with a fossil fuel boiler |
Most UK homes with radiators end up with an air-to-water air source heat pump, because it works with a wet heating system you already have and needs only outdoor wall or ground space. Ground source systems are more efficient in principle but need land and groundworks, and cost far more. Our air source heat pump service covers what a typical install involves.
What changes inside your home?
Less than people expect, but not nothing. A typical air-to-water system replaces a gas combi boiler with four things.
- An outdoor unit, roughly the size of a large suitcase or a small fridge, on brackets or a base near an outside wall.
- A hot water cylinder, because a heat pump heats water gradually rather than instantly like a combi. You need cupboard space for it.
- Some bigger radiators, in the rooms where the existing ones are too small to heat the room at a lower water temperature.
- Controls that adjust the water temperature to the weather outside, called weather compensation.
The key idea is flow temperature: the temperature of the water leaving the heat pump for your radiators. Gas boilers are usually set up to send out much hotter water. A heat pump is most efficient when that water is cooler, so the system is designed to keep your rooms warm with a lower flow temperature, running steadily for longer. MCS design rules say a high temperature heat pump should only be chosen when a home genuinely needs water hotter than 55°C. Radiators that feel warm rather than hot are normal and intended.
Is a heat pump cheaper to run than a gas boiler?
It can be, but only by a margin that depends on your tariff and the system's real efficiency. Under the Ofgem price cap for 1 October to 31 December 2026, electricity costs 26.32p per kWh and gas 7.97p per kWh (Direct Debit averages; VAT has been removed from electricity for October 2026 to March 2027). Electricity is 3.3 times the price of gas per unit.
If your boiler turns 85% of its gas into heat, a unit of heat from gas costs about 9.4p. A heat pump with an SPF of 2.8 produces the same unit of heat for 26.32 ÷ 2.8 = 9.4p. So at the price cap, a heat pump performing at the trial median breaks roughly even on unit costs, pulls ahead as efficiency rises, and saves you the gas standing charge (about £108 a year) if you close your gas account. A heat pump tariff with cheaper hours widens the gap. We run the full numbers, with every assumption shown, in heat pump running costs vs gas.
When is a heat pump not the right choice?
A heat pump suits most homes, but not every home right now. We would tell you to wait, or to choose something else, in these situations.
- Your home loses heat very fast (draughty, uninsulated loft, unfilled cavity walls). Fix the fabric first, or the heat pump has to be larger and run hotter, which costs more to buy and to run. Our insulation service is often the better first step.
- Your boiler is fairly new and working well, and you have no budget for fabric work. A well set up boiler now and a heat pump later can be the honest answer.
- There is nowhere suitable for the outdoor unit that meets the noise rules, or no space for a hot water cylinder.
- Nobody has done a room-by-room heat loss calculation. MCS rules require one. If a quote arrives without it, the sizing is a guess.
What does it cost, and what help is there?
Government statistics put the median cost of an air-to-water heat pump installed under the Boiler Upgrade Scheme at £12,908 for April to June 2026, before the grant is taken off. In England and Wales the grant is £7,500, or £9,000 for eligible off-gas homes replacing oil or LPG until 31 March 2027, and your installer deducts it from the quote. Installing a heat pump in a home in Great Britain is also zero-rated for VAT until 31 March 2027.
The full rules are in the Boiler Upgrade Scheme explained, and you can check your eligibility in a couple of minutes. If you would like a written plan for your own home, the retrofit planner is the place to start.
Common questions
Does a heat pump work like air conditioning?
The machinery is the same kind: a refrigerant loop, a compressor and two heat exchangers. An air-to-air heat pump can heat and cool a room much like an air conditioner. An air-to-water heat pump, the usual type in UK homes, heats water for your radiators and taps instead.
Can a heat pump provide hot water?
Yes. An air-to-water or ground source heat pump heats a stored hot water cylinder. It will not give you instant hot water like a combi boiler, so the cylinder is sized to your household's use.
Do heat pumps work in winter?
Yes. Efficiency falls as it gets colder, but in the Electrification of Heat trial the median air source heat pump still delivered 2.44 units of heat per unit of electricity on the coldest day monitored. We cover this in detail in do heat pumps work in cold weather.
Will I need to replace all my radiators?
Rarely all of them. The heat loss calculation shows which rooms need more radiator area at the design flow temperature. Often it is a few rooms, not the whole house.
Is a heat pump the same as a ground source heat pump?
Ground source is one type. Most UK installs are air source because they need no digging. Both qualify for the same £7,500 Boiler Upgrade Scheme grant in England and Wales.