Horizontal Slinky vs. Straight Pipe

Discussion in 'Geothermal Loops' started by bengee, Nov 12, 2010.

  1. bengee

    bengee New Member

    Hello all! My first post here so all the usual newbie conditions apply.

    First, the question(s): Is there a noticable performance difference between a horizontal slinky and a horizontal straight pipe loop configuration? What are the loop design "gotchas" with a slinky?

    I have receieved quotes from 4 different contractors for my geothermal system, 3 with straight pipe and one who does only slinky. The straight pipe guy I'm considering says to stay away from the slinky, or to make sure that there's 800' of pipe per ton (he and the other guy both quoted 3 ton).

    I trust the two guys on the tonnage spec with our upgraded insulation, but I'm not sure about the slinky. It makes sense to me that there is only so much energy in the ground. There has to be a point of diminishing returns. The slinky guy also said there's less energy in the ground in the winter so his systems may not perform as well. But could this mean that he's just not going down deep enough, or that the slinky is pulling too much heat from the ground in too small an area?

    Both contractors have good references and a successful track record, but the slinky guy is local, and about $4500 more for the same "features" (dual zone, variable speed, etc). He installs W.F. and the straight pipe guy does HydroTemp.

    I'm going to be laying down a huge sum and want to make the best decision. Thanks in advance for any info!

  2. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader


    Both slinky and straight can be designed properly. But, one can argue that a straight pipe layout will be more "forgiving" if design conditions are not met in the real world.

    We only consider slinky when space is limited. But I know many installers use them far more than we do.

    An installer that only does one or the other is a bit suspect.
  3. bengee

    bengee New Member

    Slinky Considerations

    Thank you urthbuoy. Are there things I can ask my contractor about his slinky installs that could make me feel more comfortable about his expertise?
  4. Palace GeoThermal

    Palace GeoThermal Well-Known Member Industry Professional Forum Leader

    We do mostly slinkies for horizontal loops. The main reason is we can build the slinky and drop it in the trench, we don't have to walk up and down the trench. Which is a good thing if the trench is very deep. Trying to lay four pipes in a trench and keep them spaced out is not the easiest thing to do.

    I have attached a design report for a recent job.

    There are 3 designs:

    #1 is a four pipe trench 8' deep, 10' spacing, total pipe length- 5808, total trench length 1452

    #2 is a four pipe trench 5' deep, 3' spacing ( which makes it a pit) total pipe length 12,968, pit size 3240 square feet.

    #3 is a slinky trench 8' deep, 10 foot spacing, total pipe length 6952, total trench length 1386.

    We are doing loop #3 on this job for the above reasons of not wanting to work in a trench 8' deep.

    Attached Files:

  5. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    Deep trenches

    As per Dewayne's comments: working in trenches is not supposed to happen without all the safety measues. We've adapted by using a long pole and hook to space piping and sandbags tossed in to hold in place. Or we just open everything up like a swimming pool type excavation

    For a slinky, the biggest thing to note would be the assumed soil types vs. actual.
  6. Palace GeoThermal

    Palace GeoThermal Well-Known Member Industry Professional Forum Leader

    thanks for the tip. We use long poles, now we can use sand bags.

    These forums are a great place to learn:D:D
  7. bengee

    bengee New Member

    Slinky Overlap

    Thanks! palacegeo, I noticed in your attachment that the slinky configuration was non-overlapping. This seems to make better sense. The slinkies I've seen pictures of had overlapping circles, which seemed like a waste. I will ask the contractor what he does.

    Also, I saw that you were going down 8'. My contractor only goes down typically 4 to 5, and sometimes 6'. Of course, soil conditions vary greatly, but I have read that even slinkies should be buried at least 6'.

    Any further thoughts on the matter? Thanks again, I LOVE this forum!
  8. Palace GeoThermal

    Palace GeoThermal Well-Known Member Industry Professional Forum Leader

    If I change the design to overlapping slinkies, the total pipe goes up to 10,475' and trench length goes down to 1316', compared to 6952' and 1386' with non overlapping.

    Going deep means less pipe and trench, but more excavating cost, so you have to weigh the differences. Lot size is big factor.
  9. jvolkmar

    jvolkmar Guest

    Bengee, My company has installed over 150 horizontal systems and we learn more with each one but what I do not see in the responces is pipe length to pump ratio. You need a flow center to pump the water and antifreeze mix throught he loop. We do not install slinky loop systems unless in water but we dig down 6-8 feet using one 500 to 600 feet of loop in a straight configuration. We have developed a non in the trench design and we use only one coil per trench. The idea is to get as much seperation per coil as possible. If you use to much coil you will have to use a bigger flow center. You can use a QT non pressurized single pump 26-99 flow center. You will use less energy to circulate you fluid while getting system performance.So my two cents is to dig three trenches minimum 10 feet apart, wider is better, using 500 or 600 foot pipe with a buried 3 way manifold with 1-1/4" diameter supply/return lines , short as possible.
  10. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader



    Careful about suggesting a design without knowing the spec's involved.
  11. Palace GeoThermal

    Palace GeoThermal Well-Known Member Industry Professional Forum Leader

    what does this mean?
  12. jvolkmar

    jvolkmar Guest


    Urthbuoy good point. I should have stated in our area we do the calculations for the specific area in NY we are installing. Just seems to me that that length was excessisive.
    palacegeo- Should have stated that no person goes in a trench that deep. Thanks
  13. bengee

    bengee New Member


    Thank you everyone for your comments!
  14. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    the main reason people select a certain loop style is due in large part (i think) to circumstances that have little to do with performance.
    There is much talk here about 8' deep trenches for instance, but that is seldom the practice in mid MI (due to nature of soil). 8 vs 6 feet would literally double the excavation time.
    The up shot for us is that compaction is rarely an issue.

    What is obvious in the slinky vs multipipe discussion is that both systems have X number of feet/ft of pipe in a trench. Therefore a 4 pipe system performs similarly to a 4'/foot slinky while a 6 pipe system compares to our 6'/foot slinkies etc.

    Of the contractors I have employed to do our loops, I've noticed the younger guys (with little fear and limited storage facilities like the 6 pipe system, as there is no off site prep and they are unafraid to work in a trench (ahh the marvel of youth). The older guys have big barns and prepare slinkies off site to limit time in trench or on site. This also requires larger equipment to transport loops but almost no "in trench" time.

    As long as design is correct the difference between loop size is much like the difference between 2 fifties or a $100 bill....who cares?

    Good Luck,
  15. bengee

    bengee New Member

    Thanks Joe. From everyone's comments it seems that the slinky is a perfectly viable solution as long as it is done correctly. Now I just need to figure out why the slinky contractor is $5K more than the straight pipe contractor for the exact same equipment, especially if the slinky loop is a more cost effective configuration in my climate/soil conditions. Thanks again.
  16. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    5k more could be very different features if you really dissect the bids or experience/overhead etc.

    The biggest contractor in my area charges that much more than me to pay for his building, new trucks, receptionist and other overhead.
    As a country contractor with the shop unmanned during the day and trucks I drive the wheels off of I might make more per unit than them.

    His customers are served well as are mine. Some folks prefer the fleet of new trucks and office staff, some disinterested in paying extra for it.

  17. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    De Specs....

    Are bifocals. What is a design?

    Plan ahead? Guess?

    How is it up north Bouy?
  18. mwinn

    mwinn New Member

    Loop Pipe Diameter, Material of construction

    Regarding metal vs plastic loops, I understand that the dirt is the limiting factor in overall heat transmission. But, (if cost is not an issue) suppose metal pipe with say, a 2 inch diameter, is used. Would the extra pipe surface area (per foot) contacting the dirt, plus the better heat transmission through the metal, reduce the length of pipe necessary? (I assume of course that the metal pipe is suitably protected from corrosion by cathodic or other means). Also, anybody have experience with horizontal loops within the water table? Would the slight movement of groundwater across the array assist in heat transfer?
  19. Looby

    Looby Member Forum Leader

    One problem with large diameter pipe is that it requires much higher GPM
    to maintain turbulent flow (i.e., a high Reynolds number). Non-turbulent
    water has surprisingly poor thermal conductivity.

    BTW, "if cost is not an issue" why would you be concerned with reducing
    the length of pipe necessary? Anyhoo, whatever the budget, I'd bet a
    nickel that spending the available $$$ on a greater length of HDPE would
    prove more efficient and (trouble-free) than a shorter length of metal pipe.

    BTW2, cathodic protection can be very effective, but it's far from a simple
    install-and-forget solution. It requires careful monitoring and maintenance.

  20. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    Ahh but if you have enough feet of pipe, turbulent flow is a non issue.
    Wait, were we trying for a shorter field or which? I've forgotten.

    seriously, for those who have been putting lots of pipe in the ground over the years we would love to shorten the digs, but we have settled on a way that suits us best.
    It may not even be the same way from one company to the next.....i.e. slinky, 6 pipe, 4 pipe etc.
    Many designs and materials will work; copper, metal and pvc have all been tried. Design must be on and all else is preference (albeit educated preference where material is concerned).
    Presence of a high water table is the quickest way to get more btus through less ground loop.
    Last edited: Jan 3, 2011

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