Air-to-Water Heat Pumps: Wet Systems Explained

An air-to-water heat pump takes heat from the outside air and moves it into the water of a wet central heating system, so that the same radiators, underfloor pipes and hot water cylinder that a boiler would have served are instead fed by electricity¹. Heat drawn from the air is converted into heated water ready for use in the home², and that water is then distributed through the existing wet circuit³. This is the form of heat pump most people mean when they say "heat pump" in a British context.

It is also the dominant domestic technology here. Air source, meaning air-to-water, heat pumps are the most common type of domestic heat pump in the UK and are suitable for most types of home⁴. Around 1 per cent of UK homes, roughly 250,000, have one, making them the most widely installed efficient low-carbon heating technology in the country⁵. Their popularity follows directly from the hydraulics: in UK residential use, air-to-water units are the most popular because they are compatible with wet central heating systems⁶.

Performance and cost figures sit in fairly consistent ranges. Heat pump efficiency can reach 300 to 400 per cent, against a boiler's combustion efficiency⁷, meaning one unit of electricity is turned into three to four units of heat. Installed cost is quoted at around £11,000 by the Centre for Sustainable Energy⁸ and the same figure by the Energy Saving Trust⁹, while Which? gives an average of £13,000¹⁰: the published averages differ, and the spread reflects property size and the pipework and emitter changes each home needs. The Boiler Upgrade Scheme grant of £7,500 is deducted from that by the installer in England and Wales⁸.

What the machine actually does

The outdoor unit contains a fan, an evaporator, a compressor and a heat exchanger. Air is drawn across the evaporator, where a refrigerant absorbs low-grade heat and boils; the compressor raises its pressure and therefore its temperature; the heat exchanger passes that heat to the water in the central heating circuit². The refrigerant then expands and the cycle repeats. Electricity drives the compressor and fans, not the heat itself, which is why the output is several times the input.

From the householder's point of view, what changes is the temperature the water leaves at. A boiler is happy sending water out at 70 or 80 degrees; a heat pump is at its best much lower. The lower the flow temperature, the better the efficiency of the heat pump, and the lower the household's electricity use¹⁴. The consequence is that the heat pump warms the home gradually and steadily rather than in short hot bursts.

The distribution side is unchanged in principle: the system warms radiators, underfloor heating, or the hot water tank, depending on the setup¹⁵. This is the key practical difference from an air-to-air heat pump, which circulates warm air through fan units and does not normally serve a wet circuit at all.

Why the wet type dominates UK homes

Britain's housing stock is plumbed for water. Most homes already have a boiler, radiators and pipework, so an air-to-water unit replaces one heat source with another and leaves the distribution system in place, adapted rather than ripped out. That compatibility is the reason air-to-water units are the most popular residential type⁶. NICEIC describes them as the most common type of heat pump currently installed in UK properties¹⁵, and Smart Energy GB says the same of air source, or air-to-water, systems across Great Britain.

The second reason is hot water. A wet heat pump can heat a cylinder as well as the rooms, so one appliance covers the whole heating load. Air-to-air systems, by contrast, are most commonly used for commercial premises in the UK and do not provide hot water⁵.

The third reason is policy. Air-to-water heat pumps are an eligible technology under the Boiler Upgrade Scheme¹¹, which has channelled grant money into this form for years. Air-to-air has only recently been brought into the frame, with a £2,500 grant announced under the Warm Homes Plan¹⁶, and certification bodies still need to be ready to assess and certify air-to-air products and installations.

For independence, the wet heat pump moves a household off gas or oil entirely and onto electricity, which can be generated on site or bought from any supplier. The dependence that remains is real: grid electricity, a supplier tariff, and a manufacturer for parts and refrigerant servicing. It is a change of fuel and of supply chain, not the removal of one.

Efficiency: 300 to 400 per cent, and what pulls it down

Heat pumps are 300 to 400 per cent efficient, converting one unit of electricity into three to four units of heat¹¹. Nesta puts the floor for air-to-water at efficiencies of at least 300 per cent⁵. That figure is not fixed: it varies with outside air temperature, with the flow temperature demanded by the emitters, and with how well the system was designed and commissioned.

Cold weather is the main variable. An air source heat pump will be less efficient in the winter when the air temperature is colder¹⁷, and efficiency can drop in extremely cold conditions⁷. Overnight in winter the air temperature can drop by 5 degrees Celsius or more, which reduces efficiency, particularly when heating water to a high temperature¹⁸. The best independent evidence of how far it falls comes from monitored British homes: across 742 domestic heat pump installations in the Electrification of Heat Demonstration project, air-to-water systems achieved a median efficiency of 2.44, or 244 per cent, on the coldest days of the year¹². That is a real drop, and it still means two and a half units of heat per unit of electricity on the worst days.

The second variable is under the household's control at design stage. Flow temperature governs efficiency directly¹⁴, but lowering it reduces the reaction time of the heating system, so it takes longer to warm rooms, and it has a big impact on running costs¹⁸. Nesta found that the running cost gap against gas narrows to 9 to 12 per cent for low flow temperature installations, or for exceptionally efficient heat pumps in a high-quality installation¹⁹. Those low flow temperature installations may require radiator or pipework changes, which is where emitter sizing and cost meet.

More on the measurement conventions is on the page for COP, SCOP and SPF, and on winter behaviour in heat pumps in cold weather.

Cost: around £11,000 installed, before the grant

Nesta's figures show the shape clearly: £10,275 for a small property and £13,600 for a large one, falling to £2,775 and £6,100 respectively once the £7,500 Boiler Upgrade Scheme grant is applied⁵. Home Energy Scotland funding lists air, ground and water to water source heat pumps at £7,500 per improvement²⁰.

Running costs are estimated at around £800 to £1,050 a year, which the Centre for Sustainable Energy describes as one of the cheapest electric heating options⁸. See heat pump running costs and heat pump grants for the detail.

Radiators or underfloor heating: choosing the emitters

An air-to-water system can work with radiators or underfloor heating, and some types of heat pump can work without either²¹. What matters is emitter surface area relative to room heat loss, because the water arriving is cooler than a boiler would deliver.

• Existing radiators. May need upgrading or resizing rather than wholesale replacement²². The wet heat pump works well with large radiators¹⁵.
• Underfloor heating. Works particularly well with heat pumps because it provides consistent warmth at lower temperatures²². Air or ground source heat pumps are particularly effective when used with wet underfloor heating, because heat pumps are designed to be left on for longer periods and to operate at lower temperatures²³.
• Mixed systems. Underfloor downstairs and resized radiators upstairs is a common hydraulic compromise, with the circuit balanced so both run from one flow temperature.

The design decision is a trade: larger emitters cost more up front but allow a lower flow temperature and therefore better seasonal efficiency and lower bills¹⁴. Further detail sits on radiators, underfloor heating and emitters for a heat pump.

No hot water on demand: the cylinder explained

Unlike combi boilers, heat pumps do not provide hot water instantly, so a hot water cylinder is needed¹³. The installer will usually fit a cylinder to store water heated by the heat pump ready for when it is wanted⁹. The Energy Saving Trust puts the same point plainly: a standard heat pump does not provide hot water on demand like a combi boiler, so there needs to be a way of storing hot water²⁴.

There is an engineering reason as well as a capacity one. CIBSE notes that most heat pump manufacturers require the use of a cylinder, as it enables the heat pump to run more efficiently by heating the water over a longer period²⁵, and that low-temperature, low-carbon solutions such as heat pumps generally require a cylinder while many homes with gas combination boilers do not²⁵.

That priority rule matters in practice: while the cylinder is reheating, the radiators are not being fed. A well sized cylinder and a sensible reheat schedule keep those periods short and out of the coldest hours. Space for the cylinder, usually an airing cupboard or a loft, is the most common constraint in homes converting from a combi. See hot water cylinders for heat pumps and heat batteries used with heat pumps.

Sizing, siting and clearance for the outdoor unit

The outdoor unit needs a position on a wall or on the ground with space around it to allow a good flow of air⁴. External units vary in size but may be as much as 1 metre by 1.5 metres⁹. For comparison, the Energy Saving Trust notes that air source heat pumps need less outdoor space than two wheelie bins stood side by side, around 1.5 metres wide²⁴.

Clearance is not cosmetic. The unit must draw in a large volume of air and discharge it colder; if that discharged air is recirculated back into the intake by a nearby wall or fence, measured efficiency drops. Noise is the other siting constraint: in the UK the legal noise limit for heat pumps is 42 decibels¹³.

Sizing is a separate exercise from siting and should follow a room-by-room heat loss calculation rather than the output of the old boiler. Oversizing raises cost and causes short cycling; undersizing leaves the home cold on design days. The detail is on heat pump sizing, where a heat pump outdoor unit can go and how noisy is a heat pump.

Installation, permits and who must do the work

Installation of an air source heat pump is permitted development in the usual case¹³, meaning no planning application, but the permitted development right carries conditions including the noise limit. Rules differ between the nations: see planning permission in England, Scotland, Wales and Northern Ireland.

Building Regulations apply regardless of planning. Approved Document L requires a registered competent person to install and commission the system to the standards set out in the approved document, and to give the compliance certificate, or a copy of the information on it, to the building control authority²⁷. Where the installer is not registered as a competent person, the Welsh consultation version of the same document states that before work begins the installer must either notify the local authority or arrange for a registered building control approver to oversee the work²⁸.

Grant funding brings its own timetable. Vouchers for air-to-water heat pumps, air-to-air heat pumps and biomass boilers are valid for 3 months, against 6 months for ground source heat pumps²⁹. Ofgem's July 2026 statistics repeat the three-month validity for air-to-water and air-to-air vouchers³⁰.

The process from survey to handover is set out on heat pump installation.

Running and maintaining the system through the year

Two things determine whether an installed system performs as designed: how it is controlled, and whether it is serviced. On controls, weather compensation varies the flow temperature according to the season³¹, so the system sends out cooler water in mild weather and hotter water only when the outside temperature demands it. Because efficiency rises as flow temperature falls¹⁴, this is where a large part of the real-world saving is won or lost.

On maintenance, BEAMA states that the heating system should be serviced every year¹², and the Department for Energy Security and Net Zero notes that an annual heat pump service will help it remain clean and ensure it runs efficiently and sounds as quiet as possible³². Annual servicing is also required for warranties and for checking refrigerant¹². Within the house, the wet side needs the same attention any radiator circuit does: as a general rule radiators should be bled about once a year, ideally before winter when the heating starts to be used more regularly, which takes only a few minutes per radiator³³.

Seasonal behaviour differs from a boiler. The system is designed to run for longer periods at lower temperatures, so turning the heating off during the day and blasting it on in the evening works against it. More on this at heat pump controls and servicing and maintenance.

Cooling in summer: possible, but conditional

Some heat pumps can also offer cooling². For a wet system this is not automatic: the right sort of emitters are needed to make the most of cooling via an air-to-water heat pump system, meaning heat pump convectors and underfloor heating³⁴. Standard radiators are poor cooling emitters, and condensation control has to be handled in the design.

The contrast with air-to-air is sharp. Air-to-air heat pumps can provide cooling as well, which the Centre for Sustainable Energy describes as very useful in a heatwave⁸, and Which? notes the added benefit of supplying air conditioning to keep a home cool during summer³⁴. Where a household's priority is cooling in a smaller house or flat with relatively few rooms, especially a home that overheats in warm weather, an air-to-air system can be especially cost-effective³⁴, though it will not heat the hot water. A comparison is set out at air-to-air vs air-to-water heat pumps and on using a heat pump for cooling.

Where an air-to-water system falls short

It is worth naming the limits plainly alongside the benefits.

• No instant hot water. The cylinder must be sized, sited and reheated, and space heating pauses while water heating is satisfied²⁶.
• Emitter work. Existing radiators may need upgrading or resizing²², and low flow temperature installations may require radiator or pipework changes¹⁹.
• Cold-weather derating. Median efficiency fell to 2.44 on the coldest days across 742 monitored homes¹², though heat can still be extracted at -15°C³.
• Outdoor space and noise. A unit up to 1 metre by 1.5 metres⁹ with clear air flow, within a 42 decibel limit¹³.
• Continuing dependence. The household still buys electricity from the grid and a supplier, and still depends on a manufacturer for parts, refrigerant and warranty servicing¹².

None of these rules the technology out; each changes what a given house needs to spend and to accommodate. Which homes suit the technology is covered on which homes suit a heat pump, and the wider picture on the heat pumps pillar.