Heat can be extracted from the ambient atmosphere using a device called an air source heat pump. This is rather like an air conditioning system running in reverse, and indeed many air source heat pumps are able to operate either way, in heating or cooling mode. Heat pumps generally deliver their output at lower temperature than fuel burning boilers but, unlike the latter, operate continuously. Water source heat pumps, rather than taking the heat from the air, use a body of water, for example, a neighbouring pond, river or lake. Ground source heat pumps use a similar principle to extract heat from the earth. A heat collection fluid of water with antifreeze is pumped through a ground loop and absorbs heat from the surrounding soil or rock. The fuel is burnt and the heat is delivered to the space heating and hot water system of the premises, usually through a hot water loop.