Geothermal heat pumps used in open-loop systems need differing amounts of water depending on the size of the unit and the manufacturer’s specifications. The water requirement of a specific model is usually expressed in gallons per minute (g.p.m.) and is listed in the specifications for that unit. Your water well and heat pump combination should be large enough to supply the water needed by the heat pump in addition to your domestic water requirements. ECONAR heat pumps have the smallest water requirement in the industry, ranging between 1.5 – 2 gallons per minute at 50° EWT.