An air source heat pump (ASHP) absorbs heat from outside air at low temperature into a fluid, then boosts it to a higher temperature by passing the fluid through a compressor, and transfers its higher temperature heat to the heating and hot water circuits of the house. The pump needs some electricity to run, but it should use less electrical energy than the heat it produces. Alternatively you could consider using solar photovoltaic panels or a wind turbine (if you are in a suitable area) for a greener source of electricity.
There are two main types of ASHP systems.
An air-to-water system distributes heat via your wet central heating system. Heat pumps work much more efficiently at a lower temperature than a standard boiler system would. This makes them more suitable for underfloor heating systems or larger radiators, which give out heat at lower temperatures over longer periods of time.
An air-to-air system produces warm air which is circulated by fans to heat your home. They are unlikely to provide you with hot water as well.
Depending on the type of system you install, this heat can then be used:
The benefits of air source heat pumps: