Geothermal systems use the earth as a heat source and heat sink. A series of pipes, commonly called a "loop," connect the geothermal system's heat pump to the earth. There are two basic types of loops: closed and open.

Open Loop Systems

The simplest of geothermal systems. Used successfully for decades, 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 distance from the first. 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.

Closed Loop Systems

These have become the most common. When properly installed, they're economical, efficient, and reliable. Water (or a water and antifreeze solution) is circulated through a continuous buried pipe. The length of loop piping varies depending on ground temperature, thermal conductivity of the ground, soil moisture, and system design. (Some heat pumps work well with larger inlet temperature variations, which allows marginally smaller loops).

Horizontal Closed Loop

Generally most cost-effective for small installations, particularly for new construction where sufficient land area is available. These installations involve burying pipe in trenches dug with back-hoes or chain trenchers. Up to six pipes, usually in parallel connections, are buried in each trench, with minimum separations of a foot between pipes and ten to fifteen feet between trenches.

Vertical Closed Loops

are preferred in many situations. Vertical loops are also used where the soil is too shallow for trenching and minimize the disturbance to existing landscaping. For vertical closed loop systems, a U-tube (more rarely, two U-tubes) is installed in a well drilled 100 to 400 feet deep. Because conditions in the ground may vary greatly, loop lengths can range from 130 to 300 feet per ton of heat exchange. Multiple drill holes are required for most installations, where the pipes are generally joined in parallel or series-parallelconfigurations.

Slinky Loops

Increasingly, coils -- overlapping coils of polyethylene pipe -- are used to increase the heat exchange per foot of trench, but require more pipe per ton of capacity. The trench length decreases as thenumber of pipes in the trench increases -- or as Slinky coil overlap increases.

Pond Closed Loops

A special kind of closed loop system. Where there is a pond or stream that is deep enough and with enough flow, closed loop coils can be placed on the pond bottom. Fluid is pumped just as for a conventional closed loop ground system where conditions are suitable, the economics are very attractive, and no aquatic system impacts have been shown.

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