Air source heat pump

Air-Source Heat Pumps

Air-Source Heat Pumps (ASHPs) take solar-derived heat energy out of the outside air and ‘upgrade’ it to a higher temperature to heat a building and its hot water, using a compressor. Heat pumps are considered to generate renewable heat because each unit of electricity consumed by the compressor can deliver several units of solar-derived heat ‘out of thin air’.

How do ASHPs work?

ASHPs work quite like an air conditioner operating in reverse, by chilling a refrigerant liquid and pumping it through an outdoor heat exchanger resembling a car radiator where, being colder than the air being drawn through it by the fan, it absorbs the relative warmth and evaporates. The warmed vapour then passes through a compressor, greatly increasing its temperature.

This heat is absorbed, via a second heat exchanger, into your home’s heating and hot water systems causing the refrigerant to cool and condense back into a liquid. Its temperature drops much further again as it passes through an expansion valve, ready to start the cycle again.

There are four important elements to a ASHP system:

  • Evaporator (resembling a car radiator) – In a cabinet out in the open air, it absorbs the heat from the air, causing the refrigerant to warm and evaporate.
  • Compressor – moves the refrigerant round the heat pump and compresses the gaseous refrigerant up to the high temperature needed in the house.
  • Condenser – the heat exchanger which transfers the heat, usually to a hot water tank which then feeds the heating and hot water systems.
  • Heat distribution system – consists of radiators or underfloor heating for space heating and often a buffer tank for the central heating circuit and a storage tank for domestic hot water supply. Underfloor heating is more efficient and ideally suited to heat pumps which produce water efficiently at lower temperatures than gas, oil or biomass-fired boilers.


Correct sizing of the heat pump and its radiator or underfloor system is crucial to the effective and efficient operation and will depend on your building’s heat requirements. At the outset it is important to implement all possible energy efficiency measures such as roof, wall and floor insulation and draught-proofing to minimise your heat demand.

When sizing a system it is important to consult a professional installer for expert advice. A heat pump can be designed to meet 100% of space heating requirements but often they are sized to provide less, needing some form of supplementary heating for the few coldest days of the year, either immersion heaters in the heat pump system, or some other form of heating such as a wood-burning stove. Heat pumps will usually only partially heat domestic hot water so secondary heating (ie. an immersion heater) must be programmed to top it up.

Is my house suitable?

Heat pumps operate most effectively in very energy-efficient buildings, ideally new builds. Whilst they will provide heat in any situation, their efficiency is very sensitive to the flow temperature demanded of them, which is a function of the size of emitter (radiator or underfloor pipework) and the rate of heat loss.

You should consider the following issues if you are considering a heat pump. An accredited installer will be able to provide more detailed advice regarding suitability.

  • Can you incorporate insulation measures? These measures include wall, floor and loft insulation, as these will reduce your heat demand, making a heat pump more efficient.
  • The type of heat distribution system. Underfloor heating is better as it works at a lower temperature. ASHPs can be combined with radiators but existing ones will give out less heat than before, so unless you can greatly improve your insulation and draught-proofing, you will need to install some more.
  • Is there space available to site the outdoor unit away from bedroom windows and where the chilled air can disperse easily?
  • What fuel is being replaced? If it is electricity, oil or LPG the payback will be more favourable, but currently less so if it is mains gas. This makes heat pumps a particularly good option for off-gas-grid areas.
  • Do you require a backup heating system? If so it will be less cost-effective.
  • Is there also a cooling requirement? Heat pumps can cool as well as heat.
  • Do you want to be 100 per cent renewable? If so, purchase green electricity or install solar PV or some other form of renewable electricity generating system, if it would be feasible in its own right, to power the compressor, fan and pump.

How much does it cost?

The cost of a professional ASHP installation is dependent on property and location and ranges from about £800 to £1,200 per kW of peak heat output, excluding the cost of the heat distribution system (eg. underfloor heating).

The installed cost of a typical 12kW system, for example, would be in the region of £10,000 plus the cost of the distribution system. The price per kW gets lower as systems get larger.

What are the advantages? 

By harnessing an inexhaustible resource, a heat pump uses less energy than a typical heating system to provide the same benefit. In a well-designed system, up to 3.5 units of heat are delivered for each unit of electricity consumed. Because of this, it also has lower CO2 emissions. A good heat pump system (running on mains electricity) causes 40% fewer CO2 emissions than an equivalent oil boiler.

Air-source heat pumps are low-maintenance, low noise solutions. The fan motor or compressor may need replacing after 10 years or so, while the other parts of the system have a reasonable life expectancy. The actual lifetime will depend on the quality of the unit and its installation. If it is over-sized and therefore starts up much more frequently than it should, the compressor’s lifetime may be significantly shortened.

Air-source heat pump systems may be supported under the Government’s Renewable Heat Incentive (RHI), the full domestic phase of which is expected in 2014 as part of the ‘Green Deal’ scheme, which would eventually pay off your capital cost or even provide some of the cost in the form of a loan recouped through your electricity bills.

A well-designed ASHP system in an energy-efficient dwelling will cost less to run than an oil, LPG or direct electric (eg. storage heater) system, and potentially mains gas. Approximate running costs per kWh, and to heat a 3 bed house in Scotland for one year, might be as follows:

  Per kWh Per year
ASHP 5.5p - 7.9p £1135 - £1493
Mains Gas 6.1p £1128
Oil 9.0p £1538
Night Storage Electricity 8.1p £1748
LPG 10.3p £2181

(Source: Sutherland Tables April ’13)

*Price for radiator use only, not peak tariff backup or hot water heating

In 2013-14 the RHI pays non-domestic users 4.8p/kWh for each unit of ‘green heat’ generated by accredited non-domestic GSHP systems, and is similar to the Feed-In Tariffs already in place for renewable electricity generators. ASHPs may be eligible under the domestic RHI when it starts, but probably not for those with access to mains gas.

For information about the existing non-domestic RHI, the Renewable Heat Premium Payment and proposed domestic RHI schemes, see .