Discussion in 'Geothermal Loops' started by Superchief, Dec 11, 2022.

  1. Superchief

    Superchief Member

    Considering geothermal for a 12 ton project in zone 4a but only if horizontal loops work out as vertical makes it cost prohibitive versus ASHP, but with the tax credit it could work if I can do horizontal loops professionally designed but installed with my crew which I can't do for horizontal.

    There is an unusual condition that makes this particularly attractive - I will already have a large excavator on site and I am already trenching and/or filling about 12000ft2 of area to depths up to 10' which will significantly reduce the excavation cost.

    So, ... my question is in a horizontal loop design at least 6' below grade and in clay soil, what is reasonable ft2 needed per ton? I could use straight or slinky and I'll get it engineered if I decide this is worth pursuing.

    Follow on question - if the building has a new foundation with 2" rigid insulation below the slab and a footer and frost wall insulated with 4" (2" form on both sides) to 36" would you ever consider installing part of the field below the slab but not as deep (maybe more then 36" but not necessarily 72")?
  2. gsmith22

    gsmith22 Active Member Forum Leader

    can't answer the ft^2 /ton estimating question, but don't put the loop below your building/foundation and make it as deep as practical. The shallower the loop, the greater the variabliity in ground temp and worse the loop performs so the longer the loop you need. Also, in zone 4a, you will probably have loop temps below 32degF in the middle of winter. The loop liquid won't freeze (because it has ethanol/methanol/antifreeze), it will freeze any liquid in the adjacent soil surrounding the loop pipe which can cause ground to heave. With clay soil, it will always have some water in it. You don't want to heave your building foundation for obvious reasons.
  3. Superchief

    Superchief Member

    heaving is great point I had not considered, thank you! i will not install anything under the building.

    In our area most systems are smaller lots and vertical bore closed loops so I'm having trouble finding someone experienced in horizontal, advice is greatly appreciated from those of you who design and install it.

    I have 4 areas I can install horizontal and have to decide what makes the most sense - I need 12 tons in the field and my frost line is 3'.

    70x40 2800 ft2 area being backfilled with lawn above, easy to install as deep as 12'.

    60x35 2100 ft2 area being significantly regraded with lawn above, easy to install as deep as 8'.

    6800 ft2 L shaped area being disturbed with lawn above, directly adjacent to the mechanical room and can install to 6' with trenches

    There is also a 4th even larger area that I could do trenches 150' long perpendicular to the grade but it would be 60' further away and 24' in elevation lower than the building, so I'm not considering that area unless someone advises me differently.

    Since I have clay soil that should be well saturated, I am thinking of using doing a stacked installation with 2' horizontal and 3' vertical separation so that for example in the 70x40 area if I did 3 layers at 12', 9', and 6' I believe based upon my readings that I could get 7-8 tons and I could get the rest in the 60x35 area. One advantage of using the two smaller fields is I can design the header manifold with extra ports for future expansion because the 3rd area (6800 ft2) is by far the easiest to add extra more capacity if I determined the system was not performing adequately and as that area won't be finish graded for 18 months I can actually go through a full year and measure performance.

    Alternately I could do slinky at a couple of levels but I think in this case straight runs make more sense based upon my reading.

  4. gsmith22

    gsmith22 Active Member Forum Leader

    "looplink RLC" is your friend (google it). its free online (or at least used to be free) and you can run through all types of horizontal system design scenarios for sizing - I don't think anyone is going to be able to advise you better than that program given the multiple variables and huge system (12 tons must be at least 3 heat pumps, no?). All things being equal, you want seperation between pipes, pipes to be as deep as possible, and you want to put as many heat pumps on one loop as you can (especially if they are variable speed). If the heat pumps are distributed around the house, you may have to distribute the fields too or else pumping/friction losses become too great and you waste a bunch or energy moving water around. Don't know how you got the 12 tons sizing but "coolcalc" (google it) can do all of the Manual J sizing. I recall it being free to use but you have to pay a nominal fee for printout of results.

    If you search this site, you are bound to find other examples of horizontal fields. I personally have vertical wells so I'm not as well versed.

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