Air-to-water heat pumps

How an air-to-water heat pump works

An air-to-water heat pump sits outside and draws heat from the outdoor air, even in cold weather. A refrigerant circuit and compressor raise that heat to a useful temperature, and it is passed into the water that runs through your radiators or underfloor heating and into a hot water cylinder. It does the job your boiler does today, using electricity to move heat rather than burning gas, oil or LPG.

The main difference you will notice is the water temperature. A heat pump is most efficient when it runs at a lower flow temperature for longer periods, so rooms are kept steadily warm rather than heated in short bursts. That is why the design matters so much, and why we start with a proper survey.

The heat loss survey

As an MCS certified installer, we design every system from a room-by-room heat loss calculation. On the survey we measure each room, look at the walls, windows, floors and roof, ask about the insulation and draughts and talk through how you use the heating and hot water. From that we work out how much heat each room needs on a cold day, which sets the size of the heat pump and shows whether each radiator is big enough at heat pump temperatures.

We also look at the existing pipework, where a hot water cylinder can go if you don’t have one, the electrical supply and where the outdoor unit can sit. You get a written design and a fixed-price quote that shows the heat pump, the cylinder, any radiator or pipework changes and the controls.

The £7,500 Boiler Upgrade Scheme grant

In England and Wales, the Boiler Upgrade Scheme gives £7,500 towards an air source (air-to-water) or ground source heat pump (GOV.UK: what you can get). Until March 2027, an extra £1,500 is available for air source or ground source heat pumps in homes heated by oil or LPG with no mains gas. The system must be installed by an MCS certified installer, there is one grant per property, hybrid systems that keep a boiler are excluded and you can’t get a grant to replace an existing low-carbon system.

The installer applies for the grant, so we check your eligibility on the survey and make the application before the work goes ahead. The grant is taken off what you pay.

Cost and VAT

Air-to-water heat pump installations are typically £8,000–£16,000, before the £7,500 grant. Installing a heat pump in a home is zero-rated for VAT from 1 May 2023 to 31 March 2027, then 5% (HMRC Notice 708/6). Business premises pay the standard rate. This is general information, not tax advice.

Where your job sits in the range depends on the size of the heat pump, whether a new cylinder is needed, how many radiators need upgrading, pipework changes and access for the outdoor unit. Our heat pump cost guide explains each of these. We don’t quote running costs or payback figures without knowing your home, your tariff and how you use the heating.

Planning permission and noise

On a house in England, an air source heat pump can be permitted development if the outdoor unit, including any housing, is no more than 1.5 cubic metres and complies with MCS 020(a), which includes a noise check. It must not be on a pitched roof, must be at least 1 metre from the edge of a flat roof, and in conservation areas and World Heritage Sites must not be on a wall or roof facing a road. Detached houses may have two, other houses one. Listed buildings, flats and areas with an Article 4 direction have different rules (Planning Portal). We check the position and the noise calculation on the survey. This is general information, not planning advice.

Brands we install

We install air-to-water heat pumps from leading manufacturers, including the Daikin Altherma and Mitsubishi Electric Ecodan ranges. We recommend a model to suit the heat loss, the space for the outdoor unit and cylinder and the controls you want. Warranty is the manufacturer’s, on its terms. For example, Daikin’s 5–7 year warranty extensions apply to its Altherma air-to-water units, subject to its conditions (Daikin UK warranty). We explain the terms for the model we quote.

Air-to-water or air-to-air?

Both are air source heat pumps, but they do different jobs. An air-to-water heat pump runs your radiators or underfloor heating and heats a hot water cylinder, so it is a straight replacement for a boiler, with the £7,500 grant where you are eligible. An air-to-air heat pump delivers warm air through indoor units in each room, can also cool in summer and doesn’t heat water, and the grant towards it is £2,500 with stricter conditions. We talk through both on the survey.

Living with a heat pump

Heat pumps work best left to run steadily, with the controls set up for your home. At handover we show you the controller, the hot water settings and what the system does in very cold weather, and you get the MCS certificate and commissioning record. Keep the outdoor unit clear of leaves, snow and clutter, and book an annual service to keep it running efficiently and to meet the manufacturer’s warranty conditions.