Geothermal heat exchangers almost always use condenser water or glycol as the heat transfer medium — and, sometimes, refrigerant. Wastewater heat recovery systems typically preheat domestic cold water directly or use a refrigerant as a heat extraction medium. Solar thermal systems preheat domestic hot water by means of a drain back system or glycol medium. Heat recovery air-handling systems use heat wheels, plate heat exchangers, heat pipes, or runaround coils to exchange heat or cooling from the exhaust air to the supply air.
Heat recovery chillers can simultaneously generate 130°F heating hot water and 42° chilled water, but water-cooled chillers have to have these loads balanced, which sometimes but rarely occurs in most buildings. Six-pipe water-cooled chillers use a geothermal heat exchanger in the ground as a source or sink to balance the loads. Four-pipe air-cooled heat recovery chillers use the condenser coil as a source or sink with the ambient air.