A few questions about a Horizontal Slinky Design

Discussion in 'Vertical and Horizontal Loops' started by nlocal, Oct 9, 2012.

  1. nlocal

    nlocal New Member


    I have been reading and taking notes here for a while and think it is great that there is a community out here to support and direct others when considering a Geothermal system, so thanks in advance.

    I was hoping that some of you with some experience designing and/or installing Geothermal Systems with a closed ground slinky loop would look over this basic design and let me know if we are on the correct track. The house is 1200 sq ft located in Charlotte, NC. Ground temp at 5’-6' depth is 62 degrees year round. I need about 2.2 -2.5 tons to efficiently cool the home.

    An approximate numbers I have observed is: 100 ft of trench (3 ft wide, 6 feet deep) = 600-800 linear ft of pipe = 1 ton. I have approximately 275 linear feet of trench space available in soft damp clay beside a small stream.

    This is what someone noted:
    “Each loop of the slinky is 3’ diameter. The circumference of one loop = ∏ x D = 3.1415927 x 3’ = 9.42 ft is the length of pipe for one complete circle (or loop) of the pipe. At 18” pitch, it means you have need two loops of the slinky for every 3’ feet of ditch (18” x 2 = 36” = 3’). Two loops = 2 x 9.42 ft. per loop = 18.85 ft. of pipe for every 3 ft. of ditch. So 18.85 / 3 = 6.28 ft. of pipe used for every 1 linear foot of ditch. So you have 275’ of ditch x 6.28 ft. of pipe used in the slinky = 1,729 ft. of pipe used in the total slinky design to fill 275 of ditch with 18” pitch loops at 3’ diameter per loop”

    The way I see it at a pitch of 18, every 3 ft is a loop and a half or 14.5’ of pipe, or 1 linear foot is 4.5 ft of pipe. My first question is, Which of these is correct? And if incorrect, what pitch or formula would I use to fit 1800 linear ft in 275 ft of trenches?
    Next question does 1800 pipe feet of loops (not including supply and return footage) sound like enough to work with a 2.5 ton system.
    Another design question I have is serial loops VS parallel loops. Example:

    Three 600 ft loops parallel to each other with a header using ¾” HDPE
    One continuous 1800’ loop without a header using 1” or even 1.25” HDPE.

    Which seems a better route to go? I have heard arguments for both designs based on pressure drop/flow rate/ equal H20 distribution/ energy exchange/etc. One of the “kits” sold for a 2.5 ton system suggests 3- 600 ft runs of ¾” HDPE with header and a ½ hp flow center pump. I’ve been told that a larger diameter pipe in a continuous 1800’ loop would give the H20 more time to exchange with the ground, because the flow rate would be slower and more evenly distributed over the continuous loop. Another factor to consider is that the three parallel runs cannot be the exact same length. Any advice or opinions here would be much appreciated.

    Thanks for your time.

    PS I attached a picture of the area I have to work with.

    Attached Files:

  2. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    Not enough time to do the math today.

    I would suggest using using a manifold and .75" pipe for the loops. When using a manifold only one of the loops counts for your head loss math, meaning a smaller pump will do the job.

    One wants to keep the loops as close to the same lenghth as possible, the reason is water is lazy and will take the shortest route. So the longer loops will not make a good heat transfer.

  3. nlocal

    nlocal New Member

    Thanks Mark

    Hi Mark,

    Thanks for the reply. I certainly would love to hear more advice on the "math" or other general thoughts you may have had when you read my post, especially regarding the 0.75 vs 1.25. I look forward to when you have additional time to help. Anyone else have any thoughts?

    Thanks Again,

  4. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    Can of worms

    This is one of those "a few questions" becoming an entire book:).

    You have a lot of questions. And it isn't even the "no, don't do that. Do this." It is the questions that will come from those answers.

    I'm going to step back and say that first the loads need to be sorted out. And by loads, you need an idea as to the expected equipment run times on top of your 2-2.5 tons cooling approximation.

    edit- I'm going to add to this as it comes up a lot. Peak loads is what everybody seems to focus on. But one quickly finds out that a loop designed for peak cooling loads 1 day a week (church) is not the same as the loop for 5 day a week cooling. Even if both of them are a peak cooling load of 4 tons. Now residentially, you will find the experienced installer in your area likely has come up with a rule of thumb for tonnage. But that will be based on his experience in that climate with that type of home construction in those soils (etc.). Any approximations for loop sizing that are gathered on reading the internet are either nonsense or specific to that particular load.

    So, sort your loads out. Then you can work on the mechanics of your loop.

    Plenty of people will design you a system that you can install yourself. If that is where this is going.
    Last edited: Oct 11, 2012
  5. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    Ditto Chris.
    I will add that as Mark said paralell circuits take a lot less pumping energy.
    Did you say 1/2HP pump?!:eek:
  6. mtrentw

    mtrentw Active Member Industry Professional Forum Leader

    The maths

    Just a quick look at the numbers. See attached diagram. In my diagram, I have three overlapping loops. Let's assume 3' diameter with 18" pitch. Between point A and B (shaded yellow) is 3 linear foot of trench. In that length, I have right half of the first loop, full second loop and left half of third loop, so I have 2 full loops inside of a three foot section of trench. Additionally, those loops are connected to each other and the loop will have a return, so assume along the whole length of the bottom, i have continuous line from loop to loop and along the top, I have a continuous run of return pipe. .
    I would say for three feet of trench you would have 2*(pi)*D + 2*L and since L=D, this becomes 2 * (pi +1) * D or 8.28*D. I'd go with 8.28 feet of pipe per foot of trench in your notional layout.

    Attached Files:

  7. mtrentw

    mtrentw Active Member Industry Professional Forum Leader


    I am in Southern Maryland, so probably a not too unsimilar climate. My loops are near the creek with very moist and clay/sand like soil. We have 5 tons of Heat Pump (3 ton and 2 ton) connected to the loop. Mark Custis designed it with me. We had originally planned on 5 loops at 300' of pipe each. I got a bit nervous at the last minute and had him throw a 6th loop on. My loops all run straight out and back 150' on either side of a 2 foot trench. 6 Trenches are probably spaced ten feet apart ..... My numbers now work out to about 360' of 3/4" pipe per connected ton............... Through a pretty warm MD summer, my loop temperature never got above 77 degrees. Waiting to see how the first winter goes......

  8. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    What kind of heat pump are planning to use? What are the flow requirements? You likely need a 3 ton heatpump, which needs between 7.5 and 9 gpm of flow.

    What load do you put on it. Slinkies require slightly more pipe about (25% more) than straight pipe, so 800ft/ton is a good number if you have moist soil. You can bring them closer together (less than the 5 ft or even stack them at the same trench, one at 8 ft and one at 5 ft. If you run with a header pipe outside, it should be 1.25", otherwise your pressure drop (friction loss) is too high and you need more pumping power than you want to use.
    Do not go with one continuos pipe! You can have a 5' wide slinky, you can run 2 circuits (1.00") pipe out with a 30" pitch, 1200' each.

    So you go out 60 ft at 8" depth and come back at 5" depth. You do that twice. So you have two trenches, 60ft each with 2400' of pipe in it, the edge of the slinky maybe 3 ft apart, for 8ft OC.

    Counting in some elbows, 20 ft of header pipe outside and 20 ft inside to the heatpump (each way), you have about 31ft PD at 9 gpm and 20 ft PD at 7 gpm. A flowcenter with a single 26-99 pump will do it all.
  9. nlocal

    nlocal New Member


    Thanks for the various replies Mark, Chris, Trent, urthbuoy, and docjenser...

    I did have some load calculations calculated myself using some software I found, however, probably need a professional. My current system is a 20 year old 2 ton. Anyway, I have a few more questions.

    What are the major disadvantages against a continuous loop, as that is what my "engineer" is suggesting?

    It sounds like it is best to run 1.25" supply/return to a header in the field, then go 0.75" for 3 loops. Should the supply and return be in separate trenches about 4 feet apart?

    docjenser, you seem to have some alternative ideas as far as designing the field (th 5 ' loops, 5' and 8' deep stacked ect). Did you look at my diagram for the space I have? I see what you are saying, with the 60" runs x2. I take it you would suggest 2400 feet of pipe in the ground?

    Again thanks,
  10. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Yes, 2400ft, 2 trenches, 2 circuits, 60ft long, slinky going out at 8ft, coming back at 5, so you only need 2x60ft in your constrained space. 1" pipe for the slinkies, 1.25" for the header pipe.

    I went with 25ft of 1.25" inside the building (each way), 6 elbows, 50ft of 1.25" outside each way, and 2 circuits of 1200' of 1", at 8 gpm it gives you 29ft/head, which is what a single 26-99 can handle.

    I don't know your soil and your water conditions, but this supports a 3 ton load in my climate.

    You don't seem to know your load, which is obviously crucial.
  11. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    ....and there's the problem others pointed out. Loops are designed to extract a known quantitiy of btus. Its just plain handy to know how many you need before you design the loops.
  12. nlocal

    nlocal New Member


    So loads are in at BTUH Gain : 26,082 + 20% = 31,298/ 12,000 = 2.6 TON SYSTEM
    BTUH Loss = 10,743.
    What other info can I provide to you folks?

    It looks to me like I am right on with most the numbers discussed above. So it's q 2.5 vs 3.0 ton system with about 2400 ft of loops fit in the the area of earth I have as seen by the attached diagrams. I don't think I can do the 8' and 5' in the same trench as I will probably hit the water table at about 6'. I appreciate and look forward to any additional suggestions in design at this time.

  13. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    If you have water at 6', what is then wrong with your design you have sketched out? You will have a high conductivity, and if you are running out of room with one of the trenches, you can put the pitch a bit closer together. As long as you keep the loops the same lengths, a closer pitch or spacing the trenches a bit closer together will not matter.
  14. nlocal

    nlocal New Member


    Ok, that makes me feel good about it.. Yeah the probability of H20 at 6’ was a definite positive when I started looking at this 2 years back. I am getting close, thanks for the support. So…

    The 2400’ of HDPE in the field under the three circuits, can that include the returns to the header from the field or should it all be in loops? Example, a slinky 75’ out, then a straight pipe 75 back, is that ok to include in the numbers as conductivity?

    That said, I will have three 800’ circuits with a pitch greater than 18 at the end of the day. You somehow guessed the fact that I will need 6 elbows total was right on. The 75’ each way from the house top the field is close but is about 90’. So I will have 2 runs of 90’ of 1.25” out to a header and three 1” 800’ runs. Does that still make sense or is there any reason to go down to ¾” on the circuits?

    Thanks again,

  15. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader


    3/4" circuits should be kept under 500' for head loss. People still run longer though.
  16. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Sure, it is OK to include the 75' straight pipe back for conductivity. Both 1" and 0.75" are OK with your design.<br />

    With 0.75" and a one pump flowcenter with a 26-99 pump gives you 8 gpm flow at 28 ft/head total system pressure drop with a 3 ton heatpump, which usually needs between 7-9 gpm flow, so you would be right within specs.1" pipe reduces the pressure drop, getting your flow up to 10 gpm while still having a reynolds around 2500 in the loopfield, so both a 1" pipe or 0.75" pipe would work fine with (3) 800 ft slinkies.
    Last edited: Oct 29, 2012

Share This Page