A ground source heat pump uses a ground loop or borehole to exchange heat with the ground: the advantage is that a relatively constant temperature is available throughout the winter if the ground loop is large enough. There are alternatives which can also be attractive for larger buildings including open loop systems which heat exchange with open water or aquifer systems: water source heat pumps or marine source heat pumps. It is also possible to heat exchange with ambient air, but an air source heat pump is significantly less efficient when the external air temperature is cold – and this corresponds with the time when space heating is most needed.