Find out what an air source heat pump is likely to cost to buy and to run

 Image credit: BakerBrown | Grand Designs Magazine

BakerBrown developed the plans for this gabled extension. A 4kW photovoltaic array of solar panels powers a 3.5kW air- source heat pump.

With the possibility of financial support from the government’s Green Homes Grant (only available in England) and the Renewable Heat Incentive (RHI), the prospect of switching a gas- or oil-fired boiler for an air-source heat pump (ASHP) seems more enticing than ever before. Powered by electricity, ASHPs extract warmth from the air, and increase the temperature to supply heating and hot water. But how much does air source heat pump cost?

Calculating the cost

The price of ASHPs varies depending on the size of the unit, its manufacturer, heat output and its level of performance. Efficiency is measured by its coefficient of performance (COP), which shows how much usable energy is extracted from the air compared to how much electricity is used to power the pump, so a COP of four means that for every 1kW of electrical energy used, 4kW of heat is generated. The higher the ratio, the more efficient the unit and the more efficient its heat delivery. You should also look for the seasonally adjusted indicator of performance (SCOP), to compare like with like. This shows how effective heat pumps are on an annual basis; it indicates the ratio of the heat delivered to the total electrical energy supplied over the year.The price of ASHPs varies depending on the size of the unit, its manufacturer, heat output and its level of performance. Efficiency is measured by its coefficient of performance (COP), which shows how much usable energy is extracted from the air compared to how much electricity is used to power the pump, so a COP of four means that for every 1kW of electrical energy used, 4kW of heat is generated. The higher the ratio, the more efficient the unit and the more efficient its heat delivery.

You should also look for the seasonally adjusted indicator of performance (SCOP), to compare like with like. This shows how effective heat pumps are on an annual basis; it indicates the ratio of the heat delivered to the total electrical energy supplied over the year.

Some models provide both heating and cooling, and these tend to be more expensive. ‘The average cost of an ASHP for a three-bedroom detached house is between £8,000 and £10,000,’ says Max Halliwell, heat pump specialist at Mitsubishi Electric . It is possible this outlay can be offset against financial incentives provided by the RHI and, if you live in England, the Green Homes Grant – as long as you meet the necessary criteria. ‘The latter scheme, for instance, provides homeowners with vouchers covering two thirds of the cost of eligible improvements, up to a maximum of £5,000,’ Max adds.

Barn conversion designed by Pad Studio. Image credit: Nigel Rigden | Grand Designs Magazine

This barn conversion was designed by Pad Studio. The owners incorporated photovoltaic and solar thermal panels, along with an Electric Ecodan ASHP from Mitsubishi

Running cost comparision

Estimates from quote-comparison website Greenmatch show that for an average four-bedroom house with an ASHP, annual heating costs start at £759. ‘Compared to a gas boiler, an air-source heat pump requires a lot less input energy. But gas can be cheaper than electricity, at 4p per kWh compared to 14p per kWh,’ says John Taylor, managing director of Enhabit. ‘The lower energy use of the heat pump is offset by the cheaper cost of gas. By this reckoning, both systems have similar running costs.’

But bear in mind that improving your home’s thermal envelope by ensuring the building is airtight and well-insulated will reduce demand on the heat pump and keep the running costs in check.

Arco2 Sustainable Architects ASHP1 | Grand Designs Magazine

Architect Ian Armstrong, director of Arco2, chose underfloor heating and an ASHP, designed and installed by Abode Heat abodeheat.com, for his new-build home. Ian was able to get cashback from the government’s RHI scheme.

Further cost implications

Retrofit tips from David Hilton from Heat & Energy

  • ASHPs are not a universal replacement for boilers. Yes, they replace the primary heat source, but there are differences that need to be accommodated if you want to avoid high electricity bills.

  • You may need to change your radiators to those with an increased surface area, compatible with the low flow temperatures of an ASHP.

  • Underfloor heating is particularly effective when used alongside a heat pump. Be sure to design it to have close pipe centres, no further than 150mm apart. You’ll also need to install good insulation under the pipes to avoid conductive heat loss.

  • An existing hot water cylinder will need to be changed for one that has a bigger, heat pump-compatible coil, plus a larger storage capacity. Hot water is stored at a slightly lower temperature, so a larger percentage of the water that comes out of the taps is from the hot water cylinder, and a small amount of blended cold water.

  • Ensure your installation meets the conditions required to qualify for any potential incentives or grants that you are applying for. For example, minimum Energy Performance Certificate (EPC) standards need to be met to be eligible.

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