Ground source heat pumps
Ground heating systems absorb heat from the ground at low temperatures into a fluid inside a loop of pipe (a ground loop) buried underground. The fluid then passes through a compressor that raises it to a higher temperature, which can then heat water for the heating and hot water circuits in the house. The cooled ground-loop fluid passes back into the ground where it absorbs further energy from the ground in a continuous process as long as heating is required.
Normally the loop is laid flat or coiled in trenches about 2m deep, but if there is not enough space in your garden you can install a vertical loop down into the ground to a depth of up to 100m for a typical domestic home. Heat pumps have some impact of the environment as they need electricity to run, but the heat they extract from the ground, the air or water in constantly being renewed naturally.