double layered slinky

Discussion in 'Geothermal Loops' started by bjl, Apr 21, 2015.

  1. bjl

    bjl Member

    I am nearing the end of a large diy new construction home. I am about to put in the driveway and my loopfield. I have a particular design in mind that i came up with from reading posts on here.

    My units are miami heat pump 2.5 ton and a 3.5 ton. I live in southern louisiana were the water table is very high (about 5'). I have some clay but mostly very well compacted dirt, easy to dig though (I have my own excavator). The 2.5 ton requires 7.5 gpm and the 3.5 ton requires 10.5 gpm.

    From my research on this it will require about 2550 ln.ft. + 30% for the slinky. Total would be 3315 ln.ft. which i would round up to 3600' of 3/4" hdpp. I only have about 90' (at most) of possible trenchs. If i do 18" pitch with 3' diameter, i would have about 20' of pipe per 3' of trench. So each trench would have around 600'.

    The design i would like to do is dig down to 9' or 10' (no one would be entering the trench) and lay the first layer of pipe. Backfill to 6' and lay the second layer. The two layers would be one continous flow at 1200' long. 3 trenches = 3600'. I was going to run the six ends into the attic to the flowcenter into a manifold. Each unit has its own pump supplying the manifold.

    Is this a good way to set up the manifold or should i separate to 2 loop fields?
    Is this enough pipe to adequately exchange heat for 6 tons?
    If i do the double layer slinky is it best to supply to the upper layer with the heated fluid and return from the lower or vice versa?
    Should i set up the slinky with the return along the whole 1200' run? In other words enter the top layer slinky run the 90', make the bend to the lower, make the 90', then return the lower 90' on a straight run, make the bend back along the top 90 and back to the unit. This way seems that the heated fluid would effect the upper cooled return, but i don't know.

    this system was designed by a igshpa accredited designer. It called for 5 tons, but he advised stepping up both units 1/2 ton because the size of the house and amount of windows, so i did. It was also designed with verticle loop field, but the installer disappeared and now i dont have the money to hire someone else. I more than have the capabilities of doing a horizontal field with a little guidance of course. There are very few people down here that knows anything about these systems so i am turning to you guys. Thanks in advance for the help.
  2. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    I have trouble with your last paragraph first. Why up a right sized system?

    Feet of pipe for ground loop are based on load, depth, foot per foot of trench, and moisture content of soil. So the more feet of pipe per foot of trench the more pipe you need. The deeper you are the less pipe you need and the more moisture in the soil the less pipe you need. Seperation between slinkies matters as well.
    So when you say you need 3600 feet of slinky but wish to pack it into a relatively small area, we don't have enough info to agree/disagree but I am suspicious you have left something out of the equation.
  3. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    1200' of 3/4" is twice the recommended length for minimizing headloss.
    heatoldhome likes this.
  4. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Hmmmm....pressure drop, pressure drop, pressure drop. I always like to keep the loop field together, so water is mixed for better efficiency.
    You need about 15-18 gpm total flow. 3 x 1200' circuits are a bad idea, since the pressure drop is too high. 600' of slinky per ton is a bit short in my area, but I do not know anything about your ground. We have done a fair amount of stacked loop fields in limited spaces.
    If you have about 90', you get about 750' of 18" pitched pipe in it, don't forget the tail going back. With the pipes going in the attic you are talking about 800ft loops. How much width do you have for your loop field?
  5. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    We did some research and testing found it slightly more efficient to offset the slinkies by a foot of vertical separation. Attached is a slinky field for a 54 kBTU manual J heating load and a 17 KBTU cooling load as an example. Obviously we focus on heating only, you obviously will be cooling dominated.

    Attached Files:

  6. bjl

    bjl Member

    I am not sure why he would recommend upsizing the system. I did get a second and third opinion from a couple hvac installers and they agreed. They do not do gshp though. I have also notice down here, a lot of the pros likes to use rule of thumb. I didn't design a rule of thumb house. It's very well insulated and sealed but still allows the house to breathe, if that makes any sense. The house is 3800 sq ft including the 2x6 exterior walls. 10' ceilings throughout and 12' in the living room (550 sq ft) with lots of low e insulated windows (approx. 500 sq ft) and 6 french style exterior doors 2/3 glass. I know the larger unit is a 2 stage and the smaller is a single if that matters here any. I am sure i am leaving something out i just don't know what.

    I am assuming what your saying is the same as doc with pressure drop.

    So with the pressure drop i am not going to have the reynolds number i need, if i'm saying this right. What do you mean when you say you like to keep the loop field together for efficiency? Should i put each unit on its own pump or one pump for both units? The ground down here is very wet. My yard is @ 5' above sea level. I dig down 5' and water starts seaping in. At 10' it'll almost flow in. I have about 50' of width to play with. I am working off my phone, but as soon as i get my computer ill check out the site plan you attached.

    I also saw you guys recomended some software for diyers. Ill check that out also.
  7. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Usually you need the pipes shorter, around 600ft, to have less flow resistance, so you need lesser pumping power and save energy. To shorten the circuits and instead increase the number of circuits for the needed footage in the ground also splits up the needed flow (15-18 gpm total) needed for your heat pumps between more circuits, and the reduced flow per circuit also reduces the flow resistance (pressure drop).

    We always try to put multiple heat pumps on one common loop field, with a variable speed heatpump, but you can do individual pumps for each unit, too. Your Reynolds number will be low when only one pump is running, but you then more than compensate with the larger loop field.

    So if you have 50 feet width, I would put in 8 circuits, 600 ft each, 3' slinkies, with 3 feet between them (6' o.c.), 75 feet long trench. That would be in my area for 6 tons, and I have no idea if you could get by with less. Your soil sounds nicely saturated, which is good.
  8. bjl

    bjl Member

    oh wow. that is a lot more digging than what i was planning on doing, but doable. this whole build turned out to be more than i was planning on doing. i really like the avalon site plan. that may come in handy when the time comes to install. what about stepping the pipe up to 1"? i know it raises the pipe cost, but with the larger surface area of the pipe, and less resistance, can there be less circuits and less piping? last big question, i will eventually have a swimming pool and i would like to heat it in the summer with the units. i know i will need some sort of heat exchange from the pool equipment to the a/c equipment. what do you recommend?
  9. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    With 8 circuits, your flow resistance is pretty low already. I did some 1" 4' wide slinkies when I needed 1200' loops, not much benefit but a lot more pipe wrestling. For the swimming pool I use Aquacal air source heat pumps, hands down. By the time of the year you supplement your pool it is 60-70F outside, much warmer than the ground. Plus your ground must recover to a certain degree. Plus I don't like the idea of pumping the pool water through the basement. Imagine a leak....
  10. bjl

    bjl Member

    I'm sorry I have one more question. To try and minimize digging would it lower efficiency of the loop field to put the slinkies verticle. In other words i would dig a 3' wide trench and hang a slinky on each side and backfill. The lowest part would be at 10' or so and the highest at a little over 7'.

    On another note.
    What i was thinking with the swimming pool was placing a heat exchange by the pool pump equipment outside (we dont have basements here. High water table. Lol) i was going to use 2" pipe and run the heated water to the exchange then to the loop field and have a valve system to shut it off in winter. Im not looking to get the water hot, just a little warmer for swimming comfort.
  11. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    You can put them vertical, but you usually have to fixate them on the wall for backfilling, meaning you have to get in the 10ft trench, meaning you have to bank it 6 ft down....By that time you usually have a 2n trench dug.
    With the pool, you are trying to reinvent the wheel. If you live in southern Louisiana, I don't know if you heat at all, but at the beginning of the cooling season you might be at 75F LWT temp? So your pool is 70 F, not much heat transfer going on. Then at the end of the cooling season, your ground might be 85-90.

    The point is that when you want to supplement your pool, you usually do not A/C you house very much. Thus you do not have much heat rejection. Especially in your climate, air sourced pool heat is the way to go.

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