Slinky Loops and the Laws of Physics

Q: I have limited yard space for a horizontal loop. I’ve seen “slinky” loops in photos. How do they work?

A: The premise that slinky loops will let you get more heat capacity from a smaller loop footprint is inherently flawed. When applying the laws of thermodynamics to ground loop design, it’s clear that there is a limited amount of energy available from a given amount of earth, and a limited amount of energy that the earth can absorb. Cramming in more pipe by using a slinky pit will have a number of ramifications:

1. In heating mode, the loops compete with each other for the same thermal energy, interfering with each others ability to do the intended work. This results in very low temperatures in the loops in the centre of the pit, and lower overall loop temperatures. This results in what we call the toilet effect. The loop temperatures start out OK, but spiral downwards until they are “flushed” and can’t recover. We’ve seen these loops running at 20F, abysmal performance. The earth is frozen and the moisture is crystallized, air pockets appear and the thermal conductivity nose dives. The system is forced to run on back-up heat, your operating costs go through the roof, and the high efficiency system you invested in becomes a black mark on our industry.
2. In cooling mode, the loop can’t dissipate the heat, so the loop heats up far above design temperatures. The slinky loop has hundreds of potential pinch points. When the pipe is softened by the high temps, and with 5 or 6 feet of earth on top of it, it tends to flatten at each pinch point, causing flow restrictions that decrease system performance even further. We use slinky loops in surface water loops (pond, lake, ocean), and in wet, swampy earth with high ground water flow. We don’t recommend this layout unless the conditions are ideally suited.

Q: So if I don’t use a slinky loop, what do I do now?

A: The tried and true horizontal loop system uses single straight pipe in trenches and requires way more space. Other established horizontal designs can use two, four, or six pipes in a trench. Each time you add a pair of pipes, your trench length per nominal ton of capacity goes down, but your pipe length and antifreeze volume goes up.
Another method is the “parking lot” loop – a pit loop using straight runs of pipe on 2’ centres. This will require a space typically more than twice the square footage of the building depending on the load and the ground conditions. If the loop field is large, temperatures in the centre may be low and impact the overall average temperature. This will be considered when GeoExchange experts are evaluating design options.
The ideal design for your property depends on the space available, the ground conditions, and the cost of the system making the system site specific – one of the things that makes GeoExchange so much fun and the reason we don’t get bored doing this over and over again!
If your lot doesn’t allow a properly designed horizontal loop, vertical boreholes are the order of the day: more upfront cost but well worth the investment.