Slinky Loops and the Laws of Physics
Filed under ask a geothermal expert
Don’t do this unless a P.Eng signs off on it OR it’s a swamp with advective flow
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.
Hello,
I just read your comments on slinky loops. I am building a home in Comox, British Columbia and I do not have room for the tried and true horizontal loop. There is no machinery in the area which can poke the holes necessary for vertical loop. Can you tell me, if any, under what conditions the slinky loop would best preform. We have very a temperatate climate here. The lot is on a large golf resort with plenty of surface water through out the course which would suggest a high water table. The house will have roughly 3000-3500 sq ft of living space.
Thank you,
Wanda
What pitch are you figuring to say that slinky loops will not work? Say that 40″ diameter loops are at a pitch of 10″ the each loop will be crossed by other loops about 6 times. If the same 40″ loops are installed at a pitch of say 35″ each loop will be crossed only twice.
Hi Wanda,
The information from this company is absolutely correct. I am so happy to see another company that is promoting the correct earth loop heat exchanger sizing and designing information. As they said an earth loop designer never gets bored since each earth loop should be designed and sized for each geothermal heat installation.
Your home must have the heat loss and heat gain calculated for it so you can get the proper size geothermal heat pump. Guessing the size, or basing the size on the size of the equipment you now have, or adding to the size just to make sure it is big enough to do the job will only rob you of more than half of the money you could be saving on your heating and cooling bills. You must size a geothermal heat pump correctly or it will not operate correctly. Just as important the earth loop must be sized, designed, and installed correctly, or it will not supply to your geothermal heat pump the heat transfer that it needs so it can be the most efficient and thereby save you the most money on your heating and cooling bills. geojerry geojerry.com
Thanks for the compliment Jerry. I’m sure your clients are glad to have a knowledgeable and responsible installer like you around!
A longer pitch is better, yes. In practice though, we’ve seen several slinky systems fail (done by others I should add), resulting in huge operating costs to the owner. It bites to spend the money on geo, and then pay for conventional heating anyways! Part of the problem is that the industry is unregulated/buyer beware, and there are enough low bid contractors putting in poorly designed systems that don’t perform.
Word to consumers: do your due diligence on contractors and of all the trades in your project, Geo is not the one to take the low bid on!
Hi,
I like the idea of a conventional horizontal bed, and I have the space for it (5 acres), but one concern I have is how to make sure the ends of the pipe doesn’t pull loose from the header pipes or u bends. I know thermal expansion of plastic pipe is substatial and the seasonal output temperature swing of the water from the heat pump is significant. What design features can prevent this from happening?
I am also curious why it is not acceptable to pick a geothermal heat pump size similar to the one it is replacing (assuming the old 2 ton unit kept the house comfortable)? I understand soils vary, so the ground loop design must also vary.
SO are you saying you should not use a slinky loop under any condition? I will be installing a system soon and was going to use slinky loop.
Hi Randy,
The only approved pipe for geo is HDPE and the only approved method for joining pipes is fusion welding. Fusion joints are stronger than the pipe itself.
There should be no pipe separation issues when this is done properly.
Regarding sizing of heat pumps in retrofits, sizing should always be done based on the heat loss of the building. Using the previous size sounds reasonable if the house was comfortable, though bear in mind that new heat pumps are often more efficient so you’ve got more capacity with the same nominal size, upgrades to the building envelope may have reduced the heat loss, or additions may increase the heat loss – common sense I know. And yes, ground loops should match the house load and allow for the geology of the site.
You’ve got to be very careful with slinky loops. We’ve seen so many of these done wrong we usually caution people to stay away. In the right soil conditions, the right pitch, the right offset between coils, you can get away with it, but linear loops are more efficient and a much better practice in general. Whatever you do DON’T use close coupled loops in a pit! In our design software this configuration will not get a positive result, and this is the loop that we’ve seen fail repeatedly in the field.
I recommend finding a designer and having the math back up whatever you end up doing.
How about slinky loops in a lake ?
I totally agree that slinky loops are often not the way to go, however I am now combining solar thermal with my systems which are oversized most of the year. This transfers that extra heat back into my ground. This allows for a smaller storage tank because my ground is the heat storage. I am obviously in a 95% heating area of the country. Even on a cold verry sunny day in Feb I get extra solar thermal heat. Why not use it. Do any of you think that a slinky loop is the way to go with this?
When you say “don’t use close coupled loops in a pit” – What do you mean. Are you referring to the spacing between the loops? The photo at the top of this post shows a series of loops next to each other in one big pit. I understnad why that configuration may not exchange heat well. What about a series of 4 foot wide trenches with a slinky loop in the trench?
Yes, we mean the slinky pits. We’ve seen these running at 23F, which give absolutely abysmal COP’s and can lock out heat pumps. Slinky loops in trenches offset from each other is an acceptable design. Make sure that you’ve had someone model the system before installation to ensure that your ground loop is right sized for the load. Horizontal loops with straight pipes are by far the best.
Yes, in a pond, lake, or ocean slinky’s work great. They are more efficient per foot of pipe than the “loose bundle” style of lake loop.
So where is a good geothermal design program to calculate lake BTU’s to match the home BTU’s
I am thinking about putting in a geothermal system and have come across some DIY kits. I have no problem with using the heavy equipment necessary to dig the trenches.I noticed you were warning against the slinky coil system and just need a little clarification since the DIY systems utilize the slinkys in a trench. What should I look for in these DIY system companies? How should they be designed assuming the trench method is good? How deep should the trench be?
I am going install a geothermal system in my house. i have talked to a couple of people about it and one is sold on slinky tubes and one says six pipes in a trench…3 on the bottom and 3 above about 2 feet, so a total of 6 HDPE tubes in each trench. My soil is clay/silt and a fairly high water table. I want to make sure I do this correctly but there are some shhady characters out there. I have about 4000 sq ft heated in a ranch style house(2400 sq ft main floor, 1600 sq ft in an exposed basement. What are your thoughts? I have a 6 acre lot so I have room. Any input would be appreciated.
I am to understand that the modelling done at Oklahoma State are not to believed or considered on slinky loop design over linear design?