Open loop systems are the simplest and usually lease expensive to install. Used successfully for many decades, the ground water is drawn from an aquifer through one well, passes through the heat pump's heat exchanger, and is discharged to the same aquifer through a second well at a safe distance from the first well. Generally, two to three gallons per minute per ton of capacity are necessary for effective heat exchange. Since the temperature of ground water is nearly constant throughout the year, open loops are a popular option in areas where they are permitted. Open loop systems do have some associated challenges:
1. Some local ground water chemical conditions can lead to fouling the heat pump's heat exchanger. Such situations may require precautions to keep carbon dioxide and other gases in solution in the water. Other options include the use of cupronickel heat exchangers and heat exchangers that can be cleaned without introducing chemicals into the groundwater.
2. Increasing environmental concerns mean that local officials must be consulted to assure compliance with regulations concerning water use and acceptable water discharge methods. For example, discharge to a sanitary sewer system is rarely acceptable.
How much groundwater does an open-loop system need?
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 is usually expressed in gallons per minute (g.p.m.) Your heating contractor should be able to provide this information. Generally, the average system will use 6-10 g.p.m. An extremely hot or cold day might result in usage of 6,000-10,000 gallons of water. Your well pump combination should be large enough to supply the water needed by the geothermal unit and your domestic water requirements.
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