The average energy savings quoted above imply a simple payback time of between 3 and 7 years with respect to direct electric systems. Domestic systems in Scandinavia providing heating only, have similar payback periods.

In Switzerland, although capital costs are much higher elsewhere, they are only about 25% higher than the alternative oil fired system and running costs are about 25% lower. Payback periods are longer at between 10 and 12 years, but still well within the lifetime of

the system.

Other benefits include low noise, good aesthetics and good security of supply.

Typical energy savings are based on the heat output per energy input ratio required to drive the compressors and pumps. A nominal ratio of 4:1 is often assumed, i.e. for every 4 kW of heat energy delivered 1 kW of electrical energy is required.

Heat pump systems consume energy in moving heat from one place to another. The principal use of energy in a heat pump is to generate the motive power to drive the compressor. The associated emissions depend on the power source driving the compressor. In the UK the heat pump will typically create around 55% of the greenhouse gas emissions from the most energy efficient domestic gas heating system (where the heat pump is providing space and domestic hot water heating).

So, for a typical household, savings in CO₂ emissions are likely to be in the range of 35-50%, or 1.5-2 tonnes per annum, when compared to gas boilers and would be greater for other fuels.

Further carbon savings can be made if the electricity used to power the pump comes from a renewable energy source such as photovoltaics or a renewable electricity tariff.

Refrigerants such as hydrochloroflurocarbons (HCFCs) are present in many GSHP systems. These are potentially toxic, flammable and can lead to increased global warming. It is recommended that systems are installed by a qualified engineer to avoid leakages.