The earth, water or air absorbs and stores heat from the sun and from the earth’s core. In the Uk the average constant temperature of the earth at about 1.5m deep, is 11 to 12°C. A similar situation exists with underground water sources or large surface water sources. It is also possible to extract considerable heat from the ambient air at temperatures as low as -15°C.
Heat Pumps are a very cost effective menas of providing space heating, because of the high efficiency of the units, the output heat energy can be up to 4 times the electrical input power - something not possible with other methods of heating.