Air source heat pumps

What "air-to-water" means, and why it matters

Almost every air source heat pump fitted to a UK home is an air-to-water system. It produces hot water, which is then circulated through radiators or underfloor heating and stored in a cylinder for taps and showers. From inside the house, it does the same jobs a boiler did.

The other kind — air-to-air — blows warm air into rooms instead. It does not heat water at all, so it cannot serve radiators or supply hot taps. The two are often written about as if they were interchangeable. They are not, and the difference decides whether a system will meet your needs.

How it works

An outdoor unit draws air across a heat exchanger containing a refrigerant that boils at a very low temperature. Even cold air carries enough energy to evaporate it. A compressor then raises the pressure of that vapour, which raises its temperature, and indoors the heat is passed into your heating water before the refrigerant condenses and starts again.

All the electricity is doing is driving the compressor and the fans. The heat itself came from outside for nothing, which is why the system delivers several units of heat per unit of electricity used.

Efficiency, described honestly

Efficiency is expressed as a coefficient of performance: units of heat out per unit of electricity in. A well-designed domestic system running at a low flow temperature typically achieves a figure comfortably above three across a heating season, and the same hardware running at a high flow temperature achieves considerably less.

That is the whole argument for careful design in one sentence. It is also why we do not publish a headline efficiency figure or a savings estimate: the number belongs to the installation, not to the technology, and a national average would tell you nothing useful about your house.

Two things reduce performance in practice — running the system at a higher flow temperature than it was designed for, and treating it like a boiler by switching it on and off in bursts. Both are avoidable.

What a property needs

Somewhere for the outdoor unit

The unit needs clear airflow in front and behind, a solid base, and a position that suits both your house and your neighbours. Siting rules apply, and a competent installer will position it to comply rather than asking you to sort that out afterwards. It also needs to be reachable for servicing — a unit boxed into an inaccessible corner causes problems later.

Radiators sized for lower temperatures

A radiator gives off less heat when the water inside it is cooler, so some radiators are usually replaced with larger or higher-output versions. Which ones is a matter of calculation, not guesswork, and the answer is usually a handful rather than all of them.

A hot water cylinder

Air-to-water systems heat water for storage. If the property currently has a combi boiler, space has to be found for a cylinder — commonly an airing cupboard, sometimes a loft. This is worth settling early, because it is the constraint most likely to change the plan.

An electrical supply that suits

Most homes need nothing unusual beyond a dedicated circuit. Some need a consumer unit change, and occasionally the network operator is involved. It is checked at survey rather than discovered on installation day.

Which properties suit them best

Detached and semi-detached houses with a garden are the easiest, and bungalows are often excellent — good pipe runs, straightforward siting, and usually a cylinder already in place. Terraced houses work well too; the questions there are where the outdoor unit goes and how pipework reaches it.

The strongest case of all is a property off the mains gas grid currently running on oil or LPG. Those homes usually have higher running costs to begin with, and they may be able to claim a higher grant value.

Flats are the hardest, not because of the heat pump but because of permission for an external unit and space for a cylinder. Our guide on whether a home suits a heat pump goes through this property type by property type.

Installation, step by step

1. Survey and heat loss calculation. Room by room, at a design outdoor temperature. Everything else follows from this.
2. System design. Flow temperature, heat pump size, emitter schedule, cylinder, unit position, pipework and electrical work.
3. Quotation. The full scope with the grant deduction shown separately, if a grant applies.
4. Installation. Typically a few days: outdoor unit, indoor equipment, cylinder, radiator changes, controls.
5. Commissioning and handover. Setting the weather compensation curve and the controls, and showing you how to run it. Do not skip this conversation — it is where most disappointment is prevented.

Looking after it

An annual check keeps a heat pump running as designed: clearing debris and vegetation from around the outdoor unit, checking system pressure and the refrigerant circuit, inspecting the cylinder and reviewing the control settings. Between visits, the main thing you can do is keep the airflow around the unit clear.

There is no combustion and no flue, so there is no gas safety check and no annual carbon monoxide concern associated with the heat pump itself.

Grant support

In England and Wales, an eligible air-to-water heat pump installation currently attracts £7,500 under the Boiler Upgrade Scheme. Properties off the mains gas grid that are replacing oil or LPG heating may instead qualify for £9,000, which is available until 31 March 2027.

The scheme is installer-led. An MCS-certified installer assesses eligibility, submits the application and shows the grant as a deduction on your quotation and invoice. You can read the values in full on our heat pump grants page, and the criteria on the eligibility page.