Can you use slinky loops under a building?

Discussion in 'Vertical and Horizontal Loops' started by tc_courtco, Oct 21, 2014.

  1. tc_courtco

    tc_courtco New Member

    I just took over maintenance on a school and they have slinky loops installed under two buildings about 12 inches below slab, the slabs are 18 feet below grade. I have trouble with both systems. One system has a heat pump and the other just a heat exchanger. My question is could the slinky be installed this way? And if not where can I find documentation on this so I can prove my scenario doesn't work so I can start working on a new solution. Every thing in have seen and read points at a open field loop install but nothing I have looked at says it can't be under a building, or if okay to be under how deep it should be.
    Thanks for any advice.
  2. SeekingAdvice

    SeekingAdvice Member

    I am a novice, so take my information with a grain of salt....
    From a scientific stand point, there should be no reason you couldn't put them under a building. From an engineering standpoint, the reason you might not want to is the cost of repair (if the loop starts to leak, you would be better off abandoning the loop and building a new loop somewhere else).
    In regards to depth, I need more clarification regarding depth.
    You say the loops are 12 inches below the slab and the slap is 18 feet below grade. I am going to assume here that the slab is a basement floor? or is the slab 18ft thick?
    One of the professionals would need to respond, but I believe you want/need 3-5 feet minimum below "surface" (top of the slab) for the surface temperature to not affect the loop temperature too directly. And deeper is better.

    Also, are the loops in dirt below the slab? or are the encased in the slab?

    I cannot answer all of your questions, but if you can answer mine, hopefully one of the experts will have enough information to guide you.
  3. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    Yes, you can install loops underneath buildings. Lots of commercial sites have boreholes drilled in their foundations for example.

    But something is really odd about only having a slinky loop 18" beneath slab. You really are losing 1/2 of the "ground coupling" an normal slinky loop would be exposed to.

    And 12" beneath a slab likely consists of some nice dry porous gravel for the slab to sit on. So the material itself would be a poor thermoconductor.

    So, yes, I could see issues being associated with this particular design.
  4. tc_courtco

    tc_courtco New Member

    Both buildings have a gym dug into the ground about 18 feet below grade, the slab is only about 6-7" thick (gym floor).
    The slinky loops were covered with sand according to the drawings and 2" of styrofoam on the building with the heat pump, nothing but the sand and slab on the one with the straight heat exchanger.
    With the slinky loops so close to the slab I'm starting to see that this system will never work as intended.
    My next problem is backing this up with documentation so I can show the powers at be what scale of problem they have, any help linking me to this would be great.
  5. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    This is not something I could easily show the effects the building has on a loop. One could show what a properly designed/placed loopfield would be generating in entering water temperatures. You could use those theoretical results vs. measurements done on your current setup to help detail the underperformance.

    The fact it is in sand is horrible in itself.
  6. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Dry sand is perhaps the worst possible soil for a slinky loop. It greatly reduces available heat transfer (by 3x or worse) regardless of whether it is under a building. If the slinky depresses (or elevates, depending on heating / cooling mode) soil temp immediately beneath a conditioned space with just 2" foam insulation there will be a coupling / short circuiting penalty, though I can't begin to guess at quantity.
  7. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    Yes dry sand is a horrible conductor, but is there moisture in this gravel below the building? I know one PE who suggested buring loops under buildings so that they would benefit from the basement heat loss.......momentum engine thingy I guess.

    The fact is TC, we don't know if the loops are a problem. We don't know how many are servicing how many tons. We don't know the entering water temperatures. We don't know if the soil is damp or dry. We don't know the design load. 19 feet below grade would generally call for shorter than average slinkies which are generally 6-8' deep, so we need a lot more info if you want help.

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