heat pump

A pump for extracting heat from subsoil (ground source heat pump), air or water. The heat energy is renewable provided the substrate is not over-taxed, while the electricity to power the pump must come from another source which may or may not be renewable. Air source heat pumps are the cheapest to install but the least efficient on cold days, while the temperature of the subsoil or sea is much more constant.

This definition is abridged from A – Z of tree terms: A companion to British arboriculture.
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