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A: In order for a geothermal heat pump system to operate at its specified heating and cooling capacity and efficiency, the proper ground water flow rate through the water-to-refrigerant heat exchanger must be maintained. The ground water aquifer, source well, and pumping system must be able to supply the required flow rate.

The water flow requirement of a geothermal heat pump is dependent on its sizing (which varies over a wide range for individual home, office, or other commercial applications), its design, water flow per Btu (British thermal unit)/hour of heating (which varies over a wide range for different manufacturers), and the temperature of available ground water (which varies from 44°F in the north central states to 80°F in the extreme southern states).

The requirements of geothermal heat pumps sized to provide 50,000 Btu/hr space heating output (typical sizing for an average modern home) can range from 5 to 15 gpm (gallons per minute), approximately 7,200 to 21,600 gallons per day, depending on the design of the specific equipment. Such geothermal heat pumps can require even higher water flows during the summer if equipped with ground water heat exchangers for space cooling, and an additional water flow of 1 to 3 gpm (1,440 to 4,320 gallons per day) can be required if a separate GeoExchange unit is used for water heating in the home. The required water flows for geothermal heat pumps are therefore much larger than the 300 to 400 gallons per day required for most domestic water supply systems.

The consumption of geothermal heat pumps supplying 75, 20, and 15 million Btu/year of space heating, hot water heating and space cooling, respectively, can range from 500,000 to 2,000,000 gallons per year. This depends on the characteristics of the specific equipment and installation.

The water flow requirement per ton varies with the water temperature and the manufacturer’s choice of design. As a general rule of thumb, a minimum flow of about 2.5 to 3 gpm for every 12,000 Btu per hour (ton) of heating and cooling will be needed (though some units specify flows as low as 1.5 gpm/ton for open-loop systems and 3 gpm/ton for closed-loop systems). For instance, one manufacturer says that if the ground water temperature is 55°F, the company’s 3-ton (36,000 Btu) unit requires a flow rate of 2.5 to 3 gpm/ton. If the water temperature is 50°F, the required flow rate for this equipment in the heating mode grows to 5 gpm, while at forty-five degrees water must flow at the rate of 10 gpm. Water temperature has the opposite effect in cooling mode. The system should be designed to handle peak water consumption (including any ground water used for other domestic needs), whether peak consumption occurs during cooling or heating.