Air-to-Water Heat Pumps: Wet Systems Explained

An air-to-water heat pump is the machine most people mean when they say "heat pump" in the UK. It sits outside your house, extracts warmth from the outdoor air, and transfers that heat into water that circulates through your existing central heating: the radiators, the underfloor pipes if you have them, and a hot water cylinder. It is, in effect, a boiler replacement that runs on electricity and outside air rather than gas or oil¹.

The headline numbers are these. A typical installation costs around £11,000, and the Boiler Upgrade Scheme knocks £7,500 off that in England and Wales³. Efficiency in normal conditions reaches 300 to 400%, meaning 3 to 4 units of heat for every unit of electricity consumed⁴. Around 250,000 UK homes, about 1% of the housing stock, already have one⁵.

How the appliance works in practice

An air-to-water heat pump takes heat from the outside air and puts it into water. In MCS's description, "the air is converted into water that is then heated, ready for use in your home"¹. Ofgem's guidance defines the same thing from the other end: the system "transfers heat drawn from surrounding air, to water in a wet central heating system"². That wet system is the network of pipes, radiators and cylinder your boiler already uses, which is why this type of heat pump is the closest thing to a like-for-like boiler swap.

The process relies on a refrigerant. The outdoor unit contains a coil of refrigerant that absorbs warmth from the air even at low temperatures, a compressor that raises the refrigerant's temperature, and a heat exchanger that dumps that heat into the water circuit. The warmed water is then pumped around the house, "warming your radiators, underfloor heating, or hot water tank, depending on your setup"⁹.

Two things follow from this that matter to a householder. First, the heat is moved rather than generated, which is why the efficiency can exceed 100%: each kilowatt-hour of electricity buys several kilowatt-hours of heat⁴. Second, the water leaving the heat pump is cooler than the water a boiler produces, typically low enough that the emitters, the radiators or underfloor loops, need enough surface area to deliver the same warmth from a lower temperature. That single fact drives most of the design decisions around a conversion, from radiator upgrades to flow temperatures, and it is covered in the sections below.

For a household's energy position, the machine replaces a fuel, gas, oil or LPG, with electricity. It removes the gas connection or the oil deliveries, but it does not remove the grid: the compressor and pumps run on mains electricity, and the system's economics depend on the electricity tariff. More detail on that sits in our page on heat pumps and household energy independence.

The reasons most British households choose the wet version

Air-to-water heat pumps are "the most common type of heat pump currently installed in UK properties"⁹, and Energy Saving Trust confirms that air source heat pumps more broadly "are the most common type of domestic heat pump in the UK and are suitable for most types of homes"¹⁰. Nesta puts the installed base at around 250,000 homes, roughly 1% of the stock⁵.

The reason is plumbing heritage. Most UK homes built since the 1960s have wet central heating: a boiler, water-filled radiators, and often a cylinder cupboard. An air-to-water heat pump slots into that infrastructure. The radiators can often stay, sometimes enlarged, and the pipework stays. Compare that with the air-to-air alternative, which blows warm air through fan units and has no connection to the existing radiator circuit at all. Air-to-air systems "are most commonly used for commercial premises" in the UK¹¹, and MCS notes that "at the moment, no installers are yet certified to install air-to-air heat pumps"¹², so the wet route is also the route with an established, certified installer base.

The dominance is also policy-made. The Boiler Upgrade Scheme lists air-to-water heat pumps as an eligible technology¹³, with a £7,500 grant⁶, while air-to-air heat pumps are not grant-eligible, a difference explained in our page on air-to-air grant eligibility. The grant, the plumbing, and the installer base reinforce each other: more air-to-water installations mean more installers trained on wet systems, which in turn makes it the default choice for the next household.

For a householder, that dominance has a practical benefit: quotes are comparable, installers are plentiful, and the technology is well understood. The trade-off is that the wet route carries the wet system's requirements, above all the hot water cylinder, which an air-to-air system sidesteps at the cost of losing the radiator circuit entirely.

Efficiency of 300 to 400 per cent and the factors that reduce it

A heat pump's efficiency is usually quoted as 300 to 400%, "meaning 3 to 4 times more heat per kilowatt" of electricity than the machine consumes⁴. Nesta's analysis of air-to-water systems gives "at least 300%"⁵. Those figures describe typical annual conditions.

Cold weather pulls the number down, for a physical reason: the smaller the temperature gap between the outside air and the water the system must produce, the less work the compressor does. "An air source heat pump will be less efficient in the winter when the air temperature is colder"¹⁴. Energy Saving Trust adds the mechanism: "During winter, the air temperature can drop by 5°C or more overnight. This will reduce the heat pump's efficiency because" the system is heating water to a high temperature at the moment the air is coldest¹⁵.

The best UK evidence on how far it falls comes from the Electrification of Heat Demonstration Project, which monitored 742 domestic installations and found them "achieving a median efficiency figure of 2.44 (244%) on the coldest days of the year"⁷. So a system that averages 300% or better across a year may be running nearer 244% on the coldest mornings. It still works, and it still beats direct electric heating by a wide margin, but the electricity drawn on those days is higher.

The other lever is flow temperature. "Lowering the heat pump's flow temperature reduces the reaction time of the heating system," because it takes longer to bring the house up to temperature, "and has a big impact on running costs"¹⁵. This is why emitter choice, covered next, is really an efficiency decision: bigger radiators or underfloor heating let the system run cooler, and cooler water means cheaper heat. Our pages on heat pump efficiency and cold weather performance go deeper.

Roughly £11,000 to install, excluding the grant

The most commonly quoted figure is around £11,000 to install, with the Boiler Upgrade Scheme providing a grant of £7,500³. Energy Saving Trust's installation guide gives the same £11,000 figure⁸. Independent estimates bracket it: £8,000 to £12,000 from the Low Carbon Hub⁴, and £7,000 to £13,000 depending on the size of the property¹⁶. Nesta's costed examples run from £10,275 for a small property down to £13,600 for a large one, both figures already including the £7,500 grant⁵.

The two grant figures differ because Ofgem's installer guidance lists both: £7,500 for a standard air-to-water heat pump, and £9,000 where the property is off the gas grid and the heat pump replaces an oil or liquefied petroleum gas system⁶. Both are current official figures and which one applies depends on the property.

The voucher has a clock on it. Air-to-water heat pump vouchers are valid for 3 months, so the installation must be completed within that window⁶. Ground source heat pump vouchers run to 6 months, because those installations take longer¹⁷.

Running costs are separate from the install. Heat pumps cost around £800 to £1,050 a year to run, "making them one of the cheapest electric heating options"³. What a household actually pays depends on the flow temperature the system runs at, the insulation of the house, and the electricity tariff, with time-of-use tariffs able to shift the bill further. Our pages on heat pump cost and running costs carry the detail.

Picking your emitters: radiators versus underfloor

Heat pumps "can work with radiators or underfloor heating, but some types can work without either"¹⁹. For an air-to-water system the choice is between the two, and it is not really about comfort, it is about water temperature.

"This type of heat pump works well in combination with large radiators and underfloor pipes"⁹. The reason is surface area. A heat pump's water is cooler than a boiler's, so to deliver the same heat into a room, the emitter needs more area: either a bigger radiator panel, or a floor's worth of pipe. Underfloor heating is the natural partner because a whole floor at a low temperature delivers a lot of heat gently, and it suits the slow, steady running a heat pump prefers.

Whether existing radiators can stay is a room-by-room question, answered by heat loss calculations rather than rules of thumb. Some existing radiators already have the surface area needed at heat pump temperatures; others need replacing with larger panels. The practical questions, including whether every radiator must go, are covered in do I need to upgrade my radiators and in our wider page on radiators and emitters.

The emitter decision feeds straight back into running costs. Because lowering the flow temperature "has a big impact on running costs"¹⁵, a system designed around underfloor heating or generously sized radiators runs cooler and cheaper than one squeezing high-temperature water through small panels. Undersizing the emitters is the classic mistake: it forces the heat pump to run hot, which cuts efficiency and raises bills for the life of the system.

The cylinder: why instant hot water is unavailable

This is the change that surprises most combi households. "Heat pumps don't provide hot water on demand like a combi boiler, so you'll need another way to provide hot water"⁸. Croydon Council's guidance puts it as a flat rule: "Unlike combi-boilers, heat pumps do not provide hot water instantly, so you will need to use a hot water cylinder"²⁰. "Usually your installer will fit a hot water cylinder to store water that's been heated by the heat pump ready for when you need it"⁸.

The cylinder changes daily life in three ways. First, space: a cylinder cupboard or equivalent is needed, which homes that removed a cylinder when they fitted a combi do not have. Second, capacity: hot water is finite, and a large bath drawn after two showers may empty the store until the heat pump refills it. Third, priority: government methodology for heat pump modelling notes that "the heat pump will not provide any space heating until water heating demand is satisfied"²¹, so while the cylinder is being reheated the radiators wait. A well-sized cylinder and sensible heating schedules make this invisible; a small cylinder and a houseful of morning showers make it obvious.

The cylinder is also where the independence story has a limit. The heat pump removes the gas or oil dependence for water heating, but the stored hot water is only as secure as the electricity supply: a power cut stops both space heating and hot water, unlike a gas combi which also needs power but for a different reason. Households wanting resilience sometimes pair the system with a battery, a combination examined in solar panels and a battery with a heat pump. Cylinder sizing, coils and mains pressure are covered in hot water cylinders for heat pumps.

Positioning, dimensions and space needed for the outside unit

The outdoor unit is the visible half of the system. "These external units vary in size but may be as much as 1 metre by 1.5 metres"⁸. For comparison, air-to-air outdoor units run "roughly one metre high, between 50cm-100cm wide, and around 50cm deep"²², and the noise from an air-to-air outdoor unit "is likely to be the same as an air-to-water heat pump"²², so the two technologies make similar demands on the space outside a wall.

Siting rules are about airflow first. "You'll need somewhere outside your home where a unit can fit to a wall or on the ground. It must have some space around it to allow a good flow of air"¹⁰. Energy Saving Trust frames the space requirement reassuringly: air source heat pumps "need less outdoor space than two wheelie bins stood side-by-side (around 1.5 metres wide)"²³. The unit cannot be boxed in, tucked under a stair or jammed against a fence, because the fan needs to pull air through the coil and trapped air both cuts efficiency and adds noise.

Noise is regulated. "In the UK, the legal noise limit for heat pumps is 42 decibels"²⁰, measured under the MCS noise assessment, which our page on MCS 020 explains. Siting affects the assessment: a unit further from the boundary and away from neighbouring windows has an easier job meeting the limit, and the rules differ between England, Scotland, Wales and Northern Ireland, each covered in our planning pages.

Sizing the unit itself is a separate exercise from siting it. The heat pump must be matched to the heat loss of the house, calculated room by room, a process described in heat pump sizing. An oversized unit short-cycles and wears; an undersized one runs constantly on the coldest days and may need a backup. The siting and the sizing are the two things a survey settles before any quote is firm.

Fitting, approvals and the required installer

For the grant, the installer matters as much as the machine. The Boiler Upgrade Scheme is delivered through certified installers, and air-to-water heat pumps are an eligible technology under it¹³. The installer applies for the voucher on the household's behalf and redeems it after completion, which is why the 3-month validity window¹⁷ is really a deadline on the whole installation.

Building law applies regardless of the grant. Under Approved Document L, a registered competent person must "install and commission the system to the standards set out in this approved document," and the certificate, or a copy of the information on it, must be given to the building control authority²⁴. Where the installer is not registered as a competent person, "before work begins the installer must either notify the local authority or arrange for a registered building control approver to oversee the work"²⁵.

Planning is usually straightforward: "Installation of an air source heat pump is permitted development" in most cases²⁰, meaning no planning application is needed, subject to conditions on size and noise. The conditions differ between the four nations, and conservation areas and listed buildings are treated differently again, as our planning permission pages set out.

Refrigerant handling is a certified trade. An installer working on the refrigerant circuit must hold the appropriate certification, the air-to-air guidance requiring "a Category 1 f-gas certificate" being the clearest statement of the principle²². For a householder the practical rule is simple: the work is not a DIY job, and the grant, the building regulations compliance and the warranty all depend on a certified installer doing it. The full sequence from survey to handover is walked through in heat pump installation.

Year-round operation and upkeep

A heat pump is closer to a boiler in maintenance than to a plug-in heater, but lighter on the wallet. "The heating system should be serviced every year"⁷, and government expert guidance echoes it: "an annual heat pump service will help it remain clean and ensure it runs efficiently and sounds as quiet as possible"²⁶. Servicing also matters for warranties, which our page on servicing and warranty covers.

Day to day, the system rewards a different habit from a boiler. Because lowering the flow temperature has "a big impact on running costs"¹⁵, the efficient pattern is to run the heating longer and cooler rather than in short hot bursts, letting weather compensation do the adjusting. Controls, including smart operation requirements, are covered in heat pump controls.

Household jobs still exist. Radiators need bleeding "about once a year, ideally before winter when you start using the heating more regularly," and "this usually only takes a few minutes per radiator"²⁷. Trapped air matters more with a heat pump than with a boiler, because the water is cooler and a half-cold radiator delivers noticeably less.

Through the seasons, the pattern is: a service before or during autumn; radiator checks before the heating season; attention to the cylinder temperature and legionella settings as the installer set them; and, in winter, acceptance that the system runs longer hours at lower output. The machine is designed for that. What it is not designed for is being switched off and on like a boiler, which wastes the slow warm-up the efficiency depends on.

Summer cooling: available subject to conditions

Some air-to-water systems can cool as well as heat. Which? notes that "you'll also need the right sort of emitters to make the most of cooling via an air-to-water heat pump system," naming heat pump convectors and underfloor heating as the emitters that work²⁸. Standard radiators are poor at cooling: they have no fan and no condensation handling, so a radiator circuit cannot deliver useful cooling.

The cooling question is where the air-to-air comparison bites. Air-to-air systems "can also offer cooling"¹, and "air-to-air heat pumps can provide cooling as well, very useful in a heatwave"³. An air-to-air heating and cooling system "can be especially cost-effective if you have a smaller house or flat with relatively few rooms, especially homes that overheat in warm weather"²⁸. For a household whose main interest is summer cooling rather than winter radiators, the air-to-air route is the one designed for it, and the trade-offs are set out in air-to-air vs air-to-water heat pumps.

For an air-to-water household, cooling is a bonus, not a given. It needs the right emitters fitted at installation, and it changes the pipework and controls specification, so it has to be decided before the system is designed, not added later. Underfloor cooling works but must be controlled carefully to avoid condensation on the floor surface. Our page on using a heat pump for cooling carries the detail.

The limitations of an air-to-water setup

The limits are worth stating as firmly as the benefits, because most of them are not fixable by buying a better model.

• No instant hot water. The cylinder is not optional: heat pumps "do not provide hot water instantly, so you will need to use a hot water cylinder"²⁰. A combi household loses the on-demand tap and gains a storage cupboard's worth of plumbing.
• Heating waits for hot water. "The heat pump will not provide any space heating until water heating demand is satisfied"²¹, so a cylinder reheat pauses the radiators.
• Efficiency falls on the coldest days. The monitored median on the coldest days was 244%⁷, well below the 300 to 400% headline⁴, and winter air temperatures cut efficiency "because" the system is heating water hardest when the air is coldest¹⁵.
• Emitters may need upgrading. The system "works well in combination with large radiators and underfloor pipes"⁹, and small radiators force higher flow temperatures, which raise running costs¹⁵.
• Outdoor space and noise rules bind. The unit needs airflow space around it¹⁰, and the legal noise limit is 42 decibels²⁰.
• The grid dependence remains. The machine moves heat using mains electricity, so a power cut stops heating and hot water together, and the running cost is tied to the electricity tariff³.
• Cooling is conditional. Radiator circuits cannot cool usefully; cooling needs convectors or underfloor heating chosen at design time²⁸.

None of these makes the technology a poor choice; they make it a different choice from a combi boiler. The household that understands the cylinder, the slower warmth and the electricity dependence before signing gets the system the headlines describe. The one that expects a quieter gas boiler is the one that writes the complaint.