An air source heat pump extracts heat from the air outside and transfers it to the building's interior for space heating or water heating. It works like other types of heat pump, using the same principles as a refrigerator. In this case, it is the air that acts as a store of renewable energy, rather than the ground, as with ground source heat pumps.
Systems based on an air source heat pump can be installed in virtually any building - the only requirements are an outside wall for the collector and a supply of electricity to run the exterior fan (which blows air over the collector) and the compressor (which drives the heat extraction circuit).
The compressor unit can be located indoors or outdoors.
There are two basic types of air source heat pumps: air-to-air systems and air-to-water systems.
Air source heat pumps can be installed in smaller properties with limited exterior space. This air-to-water unit supplies heating and hot water (courtesy of the Heat Pump Association and HeatKing)
These take heat from the air and transfer it to water in a storage tank for distribution via underfloor heating or radiators. The heat can also be used to provide hot water, although a supplementary electric immersion heater is required to boost the temperature to 60 celsius or more, depending on thermostat settings. In a system using radiators, these are oversized compared with a conventional system, to compensate for the lower operating temperatures. Typical domestic models consist of the heat pump and a distribution unit; the latter feeds warm water to the heating system and also helps to heat a hot water tank.
Thermodynamic solar panels are heat collectors that work without a fan and can be sited on roofs, exterior walls or the ground. Hence, there is no exterior noise and, it is claimed, little or no risk of icing up in winter. The heat is transferred to a hot water tank or to a heating system via a heat exchanger, with the aid of a compressor. The simplicity of the system should mean that relatively little maintenance is required.
In these the heat from outside is distributed directly as warm air by fans inside the building for space heating. They can also be used in reverse to help cool the interior during hot weather. More than one unit may be needed to heat several rooms, and these systems are most suitable for apartments or small houses.
Typical air-to-air system has an interior unit (top) and an exterior unit (bottom) (courtesy of IVT Heat Pumps, Sweden)
How efficient are they?
The efficiency of any air source system depends on the outside air temperature and the temperature required indoors. This means that the coefficient of performance (COP) - the standard measure of efficiency - varies according to the difference between air temperature and the required output temperature indoors. The closer these two values are, the better the efficiency. A COP of 2.5, for instance, means that the systems delivers 2.5 kW of heat energy for every kilowatt of electricity supplied to drive the system.
A typical 6 kW system for a domestic house might have a COP of 3.5 for output at a temperature of 35 celsius, compared with only 2.5 for an output at 45 celsius., with the same external air temperature.
Useful heat can still be extracted from air at temperatures down to about minus 15 celsius, and the system will work effectively, albeit less efficiently than a gas boiler in cold conditions.
How much will it cost?
Typical domestic air-to-water systems are rated at around 6-12 kW, and will cost in the region of £7000 to £12,000 installed. However, the cost can be reduced dramatically if you are prepared to shop around and install the unit yourself, as described in the case study Happy with hot air.
The air-to-air systems are much cheaper to buy and install, with a typical 6 kW model costing around £1000-2000 installed. Note, however, that two or more such units may be required for heating multiple rooms or storeys.
Can I get a grant?
Air-to-water heat pumps qualify for payments under the domestic Renewable Heat Incentive (RHI) scheme. Annual RHI payments for an installation suitable for a typical four-bedroom house are likely to be in the range £900 to 1445, according to the Energy Saving Trust. Air-to-air heat pumps are not eligible for the RHI scheme. See Renewable Heat Incentive for homes. You can find out how much money you are likely to receive by using the government's own Renewable Heat Incentive Calculator.
What are the benefits?
Air source heat pumps are an attractive option where connection to the gas grid is not possible, and land area is restricted. They generally give savings on heating bills compared with systems fuelled by electricity, oil, liquefied petroleum gas (LPG) or solid fuel. Payback is most favourable when comparing with fully electric heating when, according to the Energy Saving Trust, the annual saving could be over £800. This represents a saving in carbon dioxide emissions of 6 tonnes per year. Financial and carbon savings are more modest when compared with oil and solid fuel, and the benefit over a gas-fired system is marginal at best.
Points to consider
- No fuel store or handling of fuel is necessary
- Units can be noisy so think about their location carefully - avoid disturbance to yourself or neighbours
- Ensure the external unit has an auto defrost function
- The system should be matched carefully with the size and layout of the building, the desired indoor temperature and rate of heat loss from the property.
- The building must be well insulated to maximize efficiency
- The heating distribution system will run at a lower temperature than one heated by a conventional boiler, and underfloor heating is most effective
- You must check with your planning authority whether or not you will need planning permission for external units
- Heat pumps require regular maintenance, so consider taking out a warranty extension or similar maintenance contract.