Below ground the temperature remains constant

Ground source heat pumps are designed to transfer heat from the ground into a building to warm water used in radiators or underfloor heating systems.  

A ground source heat pump is comprised of lengths of pipe buried in the ground, either in a borehole or a horizontal trench.  The pipe is usually a closed circuit and is filled with a mixture of water and antifreeze which is pumped around the system absorbing heat from the ground.

The temperature of the ground beneath the surface remains constant so a ground source heat pump can be used all year round, and because the pipes are buried underground the loops are virtually maintenance free.  The system can also run in reverse to function as an air conditioning unit. 

Ground source heat pumps can be more efficient than air source heat pumps but due to the need to excavate the site to install the pipes the initial installation costs can be higher.