The ground water heat pump is an open loop system. It requires a reliable supply of good quality water, and a suitable means of disposing of the thermally altered water after the process.

The water source must be adequate to provide 1.5 to three gallons of water per minute per ton of heat pump capacity. Adequate disposal must also be available.

The quality of water is a major concern. Bad water can corrode the heat exchanger, and reduce the service life of the equipment.

In deciding whether or not to use available good ground water, the pumping cost must be considered. Should analysis indicate that pumping costs will be 25 percent of more of the space heating and cooling energy costs, an alternative closed loop system should also be investigated.

Water issues, such as reinjection and disposal and legal restrictions on use, are a major concern, and should be carefully examined.

Groundwater temperatures

The temperature of groundwater remains relatively constant throughout the year. The ground temperature for an area is approximately equal to an area's annual average air temperature.

The earth and groundwater temperatures are much more stable than the highly variable seasonal air temperature.

This stable and relatively constant source temperature makes the sizing procedure somewhat different than with earth-coupled piping loops.

Wells

Groundwater 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 groundwater 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.

Advantages and disadvantages

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 antifreeze, purging the piping of air and debris, or hidden water leaks.

• 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.

Some of the disadvantages include:

• The uncertainty of the well output after drilling.

• The problem of reinjection and disposal.

• Scaling in the 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 ¾-hp 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 will require more long-term maintenance to descale 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.

When a well is being considered, consult with a reputable well driller who has knowledge of both local conditions and of all the required permits, codes and restrictions. You can also talk with local officials.

Lakes and ponds

When a lake, pond or other body of water is conveniently located, it offers the potential of a source of heat, as well as a heat rejection sink.

In-ground - or more precisely in this case, water-coupling - pipe coils are submerged several feet below the low water level. This is typically at a level where the water does not freeze in winter, usually a depth of eight feet or more.

Special considerations

• Never place a loop in moving body of water subject to flooding. Flood stages can destroy the loop.

• Use at least a 20 percent by volume of an antifreeze fluid.

• The body of water should be close to the structure. If the distance to the water and back would accommodate a horizontal field, the submerged loop would offer no advantage.

Installations normally use a parallel configuration. The supply header is on one side, loops in the middle and the return header on the other side.

A word of caution: The performance of these systems is hard to predict due to water stratification, algae growth and other such factors.