At a glance
- An air source heat pump transfers heat from outdoor air to the water in a central heating system using a refrigerant cycle.
- Real UK homes achieved a median seasonal performance factor of 2.8 in the Energy Systems Catapult Electrification of Heat trial (742 heat pumps, 2020 to 2022).
- On the coldest trial days (mean daily temperature down to minus 6°C) the median efficiency was still 2.44.
- Typical installed cost of an air source heat pump is around £12,000 (Energy Saving Trust, October 2026), before the £7,500 Boiler Upgrade Scheme grant in England and Wales.
- Heat pumps work best at low flow temperatures, usually 35°C to 50°C, so radiator size and controls matter as much as the unit itself.
- A monobloc air source heat pump is about the size of two wheelie bins side by side and sits outside the house.
How does an air source heat pump heat a home?
An air source heat pump heats a home by absorbing low-grade heat from the outdoor air and raising it to a useful temperature with an electrically driven compressor. Outdoor air always contains heat energy, even at minus 10°C, because heat is only absent at absolute zero. A fan draws that air across an outdoor coil called the evaporator, where a cold refrigerant picks up the heat and boils into a gas.
The compressor squeezes the refrigerant gas, which raises its pressure and its temperature. The hot gas then passes through a second heat exchanger, the condenser, where it gives up its heat to the water in the central heating circuit. That water flows to radiators, underfloor heating or the coil in a hot water cylinder, exactly as boiler-heated water would. The cooled refrigerant passes through an expansion valve, drops in pressure and temperature, and returns to the evaporator to collect more heat.
The practical difference from a gas boiler is temperature. A boiler sends water out at 60°C to 75°C. An air source heat pump is most efficient sending water out at 35°C to 50°C, so the whole system is designed around a lower heat pump & radiator compatibility questions answered. Flow temperature is the temperature of the water leaving the heat pump to heat the house.
What is inside a heat pump?
An air source heat pump contains four main refrigerant-circuit components plus a fan, a circulating pump and a controller. The table below lists each part and what it does. Most UK installations are monobloc units, where the whole refrigerant circuit sits in the outdoor box and only water pipes enter the house. Split systems put the condenser indoors and run refrigerant pipework through the wall, which means any work on the refrigerant circuit must be done by an F-Gas certified engineer.
| Component | What it does |
|---|---|
| Evaporator (outdoor coil) | Absorbs heat from outdoor air into the refrigerant, which boils into a gas. |
| Fan | Moves outdoor air across the evaporator so heat transfer continues. |
| Compressor | Raises the pressure and temperature of the refrigerant gas. Uses most of the electricity. |
| Condenser (plate heat exchanger) | Transfers heat from hot refrigerant into the heating water. |
| Expansion valve | Drops the refrigerant pressure and temperature so the cycle can repeat. |
| Four-way (reversing) valve | Reverses the cycle briefly to defrost the outdoor coil. |
| Controller | Runs weather compensation, hot water schedules, defrost and fault reporting. |
The refrigerant itself has changed in recent years. Older units used R410A, which has a global warming potential (GWP) of 2,088. Many current units use R32 (GWP 675) and a growing number use R290, which is propane, with a GWP below 3 on manufacturer figures. GWP compares how much heat a gas traps against carbon dioxide at 1. R290 units can reach higher flow temperatures but are flammable, so siting rules in the installation manual must be followed.
How does a heat pump work in winter?
An air source heat pump keeps working in winter because cold air still holds heat and the compressor simply works harder to lift it. Manufacturer data sheets give operating limits well below UK winter temperatures: the Vaillant aroTHERM plus operates in heating mode from minus 25°C to 46°C, and the Daikin Altherma 3 H HT is rated to operate down to minus 28°C. The Met Office long-term average UK January mean temperature is 3.9°C (1991 to 2020), so most winter days sit comfortably inside those limits.
Efficiency does fall as the air gets colder. The Energy Systems Catapult trial recorded a median performance of 2.44 on days with a mean temperature down to minus 6°C, against 2.8 across the whole year. The unit also runs defrost cycles. A defrost cycle melts frost from the outdoor unit’s evaporator by briefly reversing the refrigerant flow, which is why you may see steam and a pause in heating for a few minutes on a damp, cold morning. Our guide to Do heat pumps work in cold weather guide explains defrost, backup heaters and winter running costs in detail.
How does a heat pump heat hot water?
A heat pump heats hot water by diverting its heated water through a coil inside a hot water cylinder, rather than heating water on demand like a combi boiler. A motorised diverter valve switches the flow between the heating circuit and the cylinder coil. Because the heat pump is working at a higher temperature when it heats the cylinder, it runs less efficiently during hot water mode, so the controller usually schedules hot water once or twice a day.
A heat pump cylinder has a much larger coil than a boiler cylinder so that water at 50°C to 55°C can still transfer heat quickly. The Health and Safety Executive advises that stored hot water should be kept at 60°C or above to control legionella bacteria, and heat pump controllers include a periodic legionella programme that lifts the cylinder above 60°C, often with help from an immersion heater. Cylinder sizing, reheat times and the legionella cycle are covered in our guide to What to know about Heat Pump Hot Water Cylinders.
How efficient is a heat pump?
A heat pump is typically three to four times more efficient than a gas boiler because it moves heat instead of generating it. The Energy Saving Trust puts the figure at 300% to 400%, compared with an A-rated boiler that converts about 90% of its fuel into useful heat. Efficiency is expressed as the coefficient of performance (COP): a COP of 3 means 3 kW of heat for every 1 kW of electricity.
COP is measured at a single test point, so the more useful number is the seasonal coefficient of performance (SCOP) from the data sheet, or the seasonal performance factor (SPF) measured in a real home over a year. In the Electrification of Heat trial the median SPF for air source heat pumps was 2.8, with wide variation between homes driven by design quality and flow temperature. Manufacturer SCOP figures for the latest units are higher: Vaillant quotes a SCOP of up to 5.03 for the aroTHERM plus under laboratory conditions. The gap between the two numbers is mainly flow temperature, radiator sizing and how the system is controlled. Our Heat pump CoP explained shows how to read the data sheet and check your own figures.
What does a heat pump cost to install and run?
An air source heat pump costs around £12,000 to install in a typical home (Energy Saving Trust, October 2026), and a ground source heat pump around £15,000, rising substantially where boreholes are needed. The Boiler Upgrade Scheme provides a £7,500 grant towards an air source or ground source heat pump in England and Wales, with an extra £1,500 for homes heated by oil or LPG with no mains gas connection until March 2027 (gov.uk, checked October 2026). In Scotland the Home Energy Scotland Grant and Loan offers a grant of up to £7,500, or £9,000 with the rural uplift, plus an optional interest-free loan. Full price breakdowns are in our Heat Pump Installation Cost UK questions answered.
Running costs depend on the price gap between electricity and gas. Under the Ofgem price cap for 1 October to 31 December 2026, electricity averages 26.32p per kWh and gas 7.97p per kWh, a ratio of about 3.3 to 1. A heat pump delivering 2.8 units of heat per unit of electricity therefore costs roughly the same to run as a new gas boiler on a standard tariff, and less on a heat pump time-of-use tariff. Worked examples are in our guide to Heat Pump Running Costs UK explained.
Air source or ground source?
An air source heat pump suits most UK homes because it needs only a wall or a small plot of ground outside and costs less to install. A ground source heat pump draws heat from buried pipework instead of air, so its source temperature is more stable through winter and its efficiency across the year is usually higher, at the price of trenches or boreholes and a larger indoor unit. The two are compared room by room in our guide to air source vs ground source heat pump in detail, and the case for replacing a boiler at all is weighed in Heat Pump vs Gas Boiler questions answered.
Which heating system suits a heat pump?
A heat pump works best with heat emitters that can deliver a warm room from water at 35°C to 50°C. Guide to Underfloor Heating with a Heat Pump is the ideal pairing because a whole floor gives off plenty of heat at 35°C to 40°C. Radiators work well too if they are sized for the lower temperature, which usually means larger or double-panel radiators in some rooms.
Controls matter just as much. Weather compensation lowers the flow temperature automatically as the weather gets milder, and keeping the heat pump running with a small overnight set-back beats switching it on and off. Our guides to Guide to heat pump smart controls and more on 10 tips to improve your heat pump efficiency cover the settings that make the biggest difference, and the heat pump noise levels questions answered explains the sound figures on the data sheet before you choose a location for the outdoor unit.
When to call an engineer
An air source heat pump should be serviced once a year, ideally before winter, to clean the evaporator coil, check the refrigerant circuit, test the controls and confirm the water quality and filter in the heating circuit. UK Heat Pump Servicing provides annual servicing, fault diagnosis and repair, cover plans and new installations for heat pump owners across England, Scotland and Wales, with F-Gas certified engineers. You can book an annual heat pump service, spread the cost through a heat pump service plan, or ask about a heat pump installation service.
Call an engineer rather than investigating yourself if the unit shows a fault code you cannot clear, if the house or hot water is cold for more than a day, if you hear a hissing sound or notice an oily residue near the outdoor unit, or if there is any electrical smell. Never open the refrigerant circuit: it is sealed, pressurised and regulated under F-Gas rules. Isolate the unit at its fused spur or rotary isolator if it is tripping the electrics or smells hot, then call for help. Why a yearly check pays for itself is explained in Why Heat Pump Servicing Is So Important explained, and you can find engineers in your area on our local heat pump engineers.
Guides in this topic
Frequently asked questions
Does a heat pump work like a fridge in reverse?
Yes. A fridge pumps heat out of its cabinet into the kitchen using a refrigerant, a compressor and two heat exchangers. An air source heat pump uses the same cycle in the opposite direction, pumping heat from outdoor air into the water in your heating system. The outdoor unit gets colder as it runs, which is why it frosts up on damp winter days and needs a defrost cycle.
Does a heat pump use a lot of electricity?
A heat pump uses roughly a third of the electricity that a direct electric heater would need for the same heat, because most of the energy comes from the outdoor air. In the Energy Systems Catapult trial the median air source heat pump delivered 2.8 units of heat per unit of electricity over a year. Your bill depends on your home’s heat loss, your flow temperature and your tariff.
Do heat pumps need a backup heater?
Most air source heat pumps include a small electric backup or boost heater for the coldest days and for the legionella cycle on the hot water cylinder. A correctly sized system should rarely use it for space heating. If your heating boost runs often, the Energy Saving Trust advises checking that the radiators are large enough and the heating curve is set correctly.
Can a heat pump replace a combi boiler?
Yes, but you will need space for a hot water cylinder, because an air-to-water heat pump stores hot water rather than heating it instantly. The installer also checks radiator sizes, pipework and the electrical supply. Many combi households find the cylinder gives better shower performance than the old boiler because two outlets can run at once.
How long does a heat pump take to heat a house?
A heat pump heats a house more slowly than a boiler because it works at a lower flow temperature, so it is designed to run for long periods and hold a steady temperature rather than reheat a cold house twice a day. Using a small overnight set-back of a degree or two instead of switching off keeps the house comfortable and the running cost down.
Sources and further reading: Energy Saving Trust, Heat pumps: how they work, costs and savings; Energy Systems Catapult, Electrification of Heat performance findings; gov.uk, Boiler Upgrade Scheme: what you can get; Ofgem, energy price cap unit rates; HSE, legionella in hot and cold water systems; Vaillant aroTHERM plus technical brochure; Daikin Altherma 3 H HT product data; Met Office UK climate averages.
