These types of pumps use special pipes that are buried under the ground (in the garden). They work by extracting heat from the ground; This is then used to warm underfloor heating systems and water for radiators. It has some other uses as well like the ability to pre-heat water that is to be transferred to a more conventional boiler.
These pumps can be used all year around even in the middles of winter; This is because the temperature underground places the same at a constant temperature. So how do they work? A ground loop is buried in the garden; the pump is used to circulate a mix of antifreeze and water around the ground loop pipe. Heat is absorbed by the liquid traveling through the pipe; This is what is used to heat water and radiators etc.
The size of the loop can vary depending on a number of things, the amount of heat you require and the size of your home; the longer the pipes underground the more heat can be absorbed. There are a few different way in which the piping can be laid; it is usually laid flat (where possible), sometimes trenches are made about 2 meters deep and the pipe is coiled inside. If you do not have enough space in your garden for this then the pipes can be installed vertically in loops, they can go down as far as 100 meters deep.
If you are looking into ground source heat pumps have a look online to find out more information about them.