Air source heat pump


Air-Source Heat Pumps (ASHPs) take solar-derived heat energy out of the outside air and ‘upgrade’ it, by means of a compressor, to a higher temperature to heat a building and its hot water. 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. It chills a liquid refrigerant and pumps 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 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 key elements to an ASHP system:

  • Evaporator (resembling a car radiator) – in a cabinet out in the open air, the heat-exchanger that absorbs the heat from the air causing the refrigerant to warm and evaporate.
  • Compressor – compresses the gaseous refrigerant up to the high temperature needed in the house and moves the refrigerant on around the heat pump circuit.
  • Condenser – the heat exchanger which absorbs the heat from the compressed refrigerant, usually to a buffer tank which then feeds the heating and hot water systems.
  • Heat distribution system – consists of radiators and/or underfloor heating for space heating and often a buffer tank for the central heating circuit and a storage tank for a domestic hot water supply. Underfloor heating is more efficient and ideally-suited to heat pumps which most efficiently produce 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 fluctuations in 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 may be sized to provide less, so needing some form of supplementary heating for the very coldest days of the year, either immersion heaters in the heat pump system, or some separate heat source such as a wood-burning stove. Heat pumps will usually heat domestic hot water to 50-60OC so an immersion heater must be programmed to raise this periodically for a while to kill any bacteria.

Is my house suitable?

Heat pumps operate most effectively in very energy-efficient buildings. Whilst they will provide heat in any situation, their efficiency is 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. These will reduce your peak 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 more or replace existing radiators with either larger or fan-assisted ones.
  • 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? The running cost will be lower than electricity, oil or LPG, but currently similar to 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.
  • How much heat do you need, and do you have a single, dual or 3-phase power supply? A single phase supply can operate heat pumps up to about 25kW of thermal output.

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 typical mains electricity) causes 40-75% 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 are supported under the Government’s Renewable Heat Incentive (RHI) IF registered before end March 2021, which would pay off a proportion of your capital cost. Interest-free loan may be available via the Energy Saving Trust.

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 comparative running costs per kWh as at January 2019 were as follows:


Per kWh

Heat pump (at 300% efficiency)


Mains Gas




Night Storage Electricity




(Source: Sutherland Tables January ‘19)

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

For information about the Renewable Heat Incentive scheme (closing end March 2021) see

or information on possible loan funding contact Home Energy Scotland on 0808 808 2282 or visit