Air source heat pumps have the advantage of not requiring boreholes or trench excavations on adjoining land. They work in a similar way to ground source heat pumps, but by extracting natural heat from the air via a surface absorber (part of the heat pump) fitted outside the property.
In air to water heat pumps it is possible to extract heat energy from air at temperatures as low as minus 15 degrees Celsius. They work in the same way a fridge uses refrigerant to extract heat from the inside of your home, keeping your food cool. An air /water heat pump extracts heat from the outside air and converts this to useful energy to heat your home and hot water requirements.
The efficiency of a air / water heat pump is measured by the coefficient of performance (COP). This is the ratio of units of heat output for each unit of electricity used to drive the compressor and pump. Typically Cop's range form 3 - 4 although high efficiency heat pumps can often achieve Cop's of in excess of 4.5 (e.g. 450% efficiency)
Air / water source heat pumps normally should cover the requirements of a well insulated home in all but extreme winter temperatures. When used in combination with hot water requirements then an immersion heater or buffer vessel in conjunction with electric/oil /gas fired boiler would be used to ensure hot water and heating requirements are met.
The air / water option is an ideal system for retro fit or new installations.
A typical 8 Kw system budgets around £7500 - £10000 but you must remember to add to this the cost of the radiators or underfloor heating.