Abstract Heat pumps extract heat energy from a low-temperature source and transfer it to a higher temperature sink, usually via a closed loop of volatile ‘refrigerant’ fluid in a compression/expansion cycle. They can be efficiently used for space heating (and cooling), extracting heat from seawater, rivers, lakes, groundwater, rocks, sewage, or mine water. Electrical energy powers the heat pump’s compressor. The ratio of total heat output to electrical energy input, called the coefficient of performance, typically ranges from 3.0 to 6.0. The use of mine water for space heating or cooling purposes has been demonstrated to be feasible and economic in applications in Scotland, Canada, Norway, and the USA. Mine water is an attractive energy resource due to: (1) the high water storage and water flux in mine workings, representing a huge renewable enthalpy reservoir; (2) the possibility of re-branding a potentially polluting environmental liability as a ‘green’ energy resource; and (3) the development of many mine sites as commercial/industrial parks with large space heating/cooling requirements. The exothermic nature of the pyrite oxidation reaction (> 1000 kJ/mol) implies added benefits if closed-loop systems can harness the chemical energy released in mine-waste tips. An appreciation of geochemistry also assists in identifying and solving possible problems with precipitation reactions occurring in heat pump systems.