Heat pumps are worth considering if you do not have access to mains gas and you have a well-insulated home. A heat pump is like a fridge or air conditioner (using a refrigerant and a compressor), but works in the opposite direction. Heat is extracted from outside the house and released inside it. For an air source heat pump, the heat is extracted from the outside air. A ground source heat pump typically has coils of pipe buried in a trench in the garden, or occasionally a deep vertical borehole. In this case heat is extracted from the ground. The compressor in the heat pump is run by electricity. For each unit of electricity consumed by the compressor, about 3 to 4 units of heat are produced. In winter, a ground source heat pump is likely to operate more efficiently than an air source heat pump. The system operates more efficiently if the output heat is not at too high a temperature. This makes a heat pump more suitable for underfloor heating which is running most of the day than for higher temperature radiators operating for short periods.