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Lakes/Ponds/Others (Closed-Loop)

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. The direct use of this water is considered open-loop and is addressed under Ground Water Systems.

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 8 feet or more.

Submerged water loops have some special considerations:

• Never place a loop in a moving body of water subject to flooding. Flood stages may destroy the loop.
  
• Use at least a 20 percent by volume of an anti-freeze 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.

Since plastic pipe is buoyant even when filled, the loop, headers and service lines must be anchored in place. The loops, anchors and service lines should be assembled, leak checked and pressure tested on land. The assembly should be filled with air, not water, and floated into place with the weights attached. When in the proper location, the system is filled with water and sunk into position. Service lines must be buried from the structure, across the shore and out into the body of water. They may be secured to docks or piers at or near the bottom.

There have been a number of successful installations where copper, polyethylene or polybutylene pipe coils are set in lakes or ponds. If copper coils are used with plastic pipe headers, NEVER thread a plastic fitting into a threaded copper or brass fitting if it is to be buried. This type connection will tend to leak over time due to wide temperature swings.

Experience indicates copper loops should be at least 150 feet, and plastic pipe should be 300 feet, per cooling ton of installed heat pump capacity. The coils should be installed either vertically or horizontally with provision to keep them above the bottom.

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