Air Source heat pumps utilise the outside air as their energy source. Air source heat pumps can even extract heating energy from the outside air at temperatures as low as -25˚C. The UK has a relatively moderate winter climate compared with central Europe with average winter temperatures of around 5°C and air source heat pumps can achieve seasonal co-efficient of performance comparable with ground source heat pumps without the additional cost of expensive ground loop collectors ground work. An air source heat pump system is normally designed to deliver 100% of the heating and hot water demand. Taking into account investment costs and efficiency the heat pump itself is typically sized to provide at least 95% of the heating requirement and the remaining energy demand is normally from a supplementary heat source, most commonly an electric immersion heater, however in retro-fit applications it is also possible to combine the heat pump with an existing boiler.

The earth stores an enormous amount of solar energy from both solar radiation and rainfall. At just 1m below the surface, the earth provides a stable source of heat throughout the year. At depths of 15m or more, the earth provides a constant 10°C temperature. Ground source heat pumps extract heat from the earth all year-round via ground heat collectors which consist of flexible poly ethylene pipes buried in the earth, either horizontally or vertically. A mixture of water and anti-freeze is then circulated through the pipe loops, attracting the heat energy and transferring it to the heat pump.

Water-to-water heat pumps extract enormous amount of thermal energy with high levels of energy efficiency by circulation the ground water directly through the heat pump evaporator "open loop collector". Year round high underground water temperatures 7-12ºC allow water to water heat pumps achieve very high levels of efficiency with COP's of 5 or over. Water to water heat pumps requires environment agency consent to extract and discharge water from/to the water course.