When used with geothermal heat pump systems, the slinky coil is a flattened, overlapped plastic pipe circular coiled ground loop heat exchangers. It concentrates the heat transfer surface into a small volume, requiring less land area and shorter trenching.

"Slinky" is the term commonly used rather than the imposing scientific name - curtate cycloid.

Fabricating the coils

The coils are fabricated flat for transportation by tying the return pipe to the base of the coil. Before the coil is placed in the trench, the return pipe is cut free from the bottom and is installed on top of the coils. In this case, the spacing is about 20 inches between each loop, and the loops intersect only at the bottom.

In another method, the coils are 30 to 32 inches in diameter, making it easier to shape while the worker is standing up and the coils are tied together. Slinky assemblies can be fabricated on the jobsite, or pre-assembled in the shop on rainy days or slack periods using a simple fixture and later trucked to the jobsite.

Slinky configurations

The compact slinky at 10-inch pitch is equivalent to 12 feet of pipe per foot of trench, and will reduce trench length by about two-thirds compared to two pipes at four- and six-foot depths.

An extended slinky at 56-inch pitch is equivalent to four feet of pipe per foot of trench, and will reduce trench length by about one-third. Specific design lengths will vary with climate, soil and the heat pump's run fraction.

Slinkies can be installed horizontally at the bottom of a three-foot wide trench or vertically in a narrower trench.

Contractors have developed other slinky configurations specifically to meet their installation needs. For example, a Georgia contractor innovated a way to install the coils vertically in shallower - 20 to 30 feet deep - but larger diameter boreholes. A series of plastic strips keep the coils properly spaced when the assembly goes into the ground.

Evaluation tests

Evaluation tests performed by a Pennsylvania utility indicated:

• No significant difference was found between vertical or horizontal slinky installation.

• Proper backfilling is critical for good performance.

• Horizontal installations seemed easier to properly backfill.

• Loops for one circulator should use no more than 750 to 800 feet of pipe to minimize pressure losses and maintain turbulent flow.

The test results recommended for each ton of capacity a 100-foot trench, three feet wide and six feet deep in the slinky area, four feet deep in the pipe header area, using 750 to 800 feet of 0.75-inch pipe on a 17-inch pitch.