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Ground Water
(Open-Loop)
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Wells

Ground water is usually supplied to the heat pump by a drilled well with a submersible pump system. If a recharge well is to be used, it should be drilled at the same time as the primary well. The ground water should be tested for acidity, dissolved solids, and mineral content.

In some large commercial installations, a separate water-to-water heat exchanger is used to keep only pure water in the heat pump.

Since the system design is dependent on the performance of the well, some understanding of water well terminology may be useful.

Static water level is the water level in the well that exists under static non-pumping conditions. Usually this level is closer to the surface than when water is encountered during drilling.

Pump level is the water level that exists under specific pumping rates. The higher the pumping rates, the lower the pumping level.

Drawdown is the difference between the static and pumping levels.

Specific capacity is a measure of well performance, usually in gpm per foot. For example, a well with a static level of 50 feet produces 500 gpm with a pumping level of 200 feet. The drawdown is 200 minus 50 or 150 feet. The specific capacity is 500 divided by 150 or 3.3 gpm per foot.

After a well is drilled, a drawdown test pump is used to draw water from near the bottom of the well. This test is conducted for 24 hours at twice the operating flow of the heat pump to be installed. The test results are then interpreted to determine the expected drawdown for use in sizing the well pump. This test also serves to clear the well bore of drilling debris and residue.

In operation, the water is used on a once-through basis and then must be disposed of in some acceptable manner. The reinjection and disposal issues are discussed under the Environment - Water Issues section of this program.

The advantages of using well water include:

• The well drilling and plumbing trades understand the system.
  
• An open-loop system can be installed economically if there are existing wells or a high water table allowing the use of a shallow well.
  
• An open-loop system is easier to start since there are no concerns about anti-freeze, purging the piping of air and debris, or hidden water leaks,
  
• and in areas with cold well water, intermediate "free cooling" is available by passing the cold well water through a coil in the air handling unit.

• The uncertainty of the well output after drilling.
  
• There is a disposal problem, which is addressed under Water Issues.
  
• Scaling in the heat pump's heat exchanger can occur if there is a high rate of dissolved solids, minerals, calcium, or iron content in the water.
  
• Water quality can deteriorate over time. Good water this year can become poor water next year.
  
• Pumping costs are usually higher. For example, a 3/4 horsepower pump might be needed to deliver 10 gpm from a 200 foot well. The same 10 gpm can typically be maintained with a 1/6 hp circulating pump in a closed-loop system.
  
• Many well water systems use a higher cost cupro-nickel heat exchanger in the heat pump.
  
• An open-loop system will require more long term maintenance to de-scale the heat exchanger, and redevelop or clean out the return well.
  
• Consumers are reluctant to pump large quantities of water and discharge the cooled or heated water to waste.
  
• Local or other restrictions may limit or prohibit use.

And furthermore . . .

Water Issues