Geothermal Slinky Loop Construction

Slinky loops are used in horizontal closed loop system. The name “slinky loops” comes from the shape that is created when the tubing is tied into concentric coils. The purpose of this procedure is to make the long lengths of pipe more manageable during installation and to be able to construct a consistent heat exchanger in a given amount of space. Not all horizontal closed loop systems are constructed in the slinky method, some just lay out the pipe in a down and back layout.

A slinky is most easily constructed by using a jig made out of plywood with edges to contain the tubing. Measured marks can be drawn onto the jig to give you a reference point for sizing the overlap (pitch) of the slinky as you build it. It will take three people to most effectively build a slink. One will do the tying, one will feed the coils to the one doing the tying, and one will pull the slinky along out the back of the jig as it is built. It will take three people approx. 20 min to tie an 800′ slinky once they are up and rolling.

The first step is to determine how long of a slinky you need to build based on the loop field design. A chart that shows the lengths of slinky coils based on tube length, pitch of coils, and diameter of coils can be found here. A length of tubing will be pulled off the roll that is the needed length to be the return from the very end of the coil back to where the manifold will be; this portion will be straight. Once this is pulled off the coil you can begin tying the tubing into loops.

The first loop will be tied into whatever diameter you made the jig (36″ is good). The second loop will be laid over the first loop by whatever pitch is needed to make your total length work. For example if you have a 36″ coil with a 18″ pitch you would lay each coil approximately half over the one in front of it. It your length called for 36″ loops with 36″ pitch you would not overlap them at all but simply place each loop end to end as you tie the coil. Plastic zip ties are the preferred product for tying the loops.

When you get near the end be sure to leave enough left untied to make it back to the manifold location. The first part of the coil that was left uncoiled can now be tied along the coil as it makes it’s way back to the “front” of the coil. Both ends should now be close to equal distance from the loops and both at the front of the coil where you will build the manifold. Tape the ends of the pipe to keep dirt out if they don’t already have caps.

The final step is rolling the slinky into a donut roll which will actually resemble more of a giant birds nest but will in fact be fairly manageable and transportable. Simply start at the “end” (not the “front” where the lines are) and roll the thing up. Try to keep it as in line as possible when rolling it up.

The slinky is now ready for transport. It will take two people to carry it due to both weight and shape. It can be transported in a truck of trailer to the job site if you built it elsewhere. Once onsite carry it to the trench or pit where it will spend its life and unroll it. If it is going in a trench I would suggest unrolling it besides the trench at the top and then with several people throw it into the trench. Be sure that the coil lays flat in the bottom of the trench but be careful if entering trenches that could collapse. The safer way would be to use a long pole to situate the slinky as needed.

When bedding the loop field be sure the fill material is sufficient to provide good contact with the pipe without leaving air gaps and be sure that big rocks are not dropped on the pipe. In rocky soil screening the soil and bedding the pipe in two feet of fines is recommended. After proper bedding the tubing the rest of the back filling can be done and the area compacted.