New Pond Geothermal System Design

Discussion in 'Surface Water Loops' started by nd96, Jun 21, 2012.

  1. nd96

    nd96 New Member

    I made my final decision on the pond loops. I'm going with two 1300ft loops of 2" HDPE (basically one loop for each 5-ton geothermal unit). That should be very low resistance and a large volume of coolant in the lines. I know that won't meet the minimum speed to prevent laminar flow, but each HVAC unit should be able to run for about 15 minutes for the coolant to make one full loop. I think that amount of time will allow for enough heat transfer even with some laminar flow.

    What I haven't decided yet is what mix of water:antifreeze I'll need in the loops. Given that I'm in Texas, I think I'm at a lower risk of freezing, so although the line volume will be about 200 gallons, hopefully I won't need too much propylene glycol.

    I put in the order for the HDPE line today, and plan on sinking the 2600ft of line next weekend if all goes well.

    Thank you to all those that gave some input to the thread.

  2. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader


    You will need A LOT more weight for sinking it and you do not want to sink it in the sediment - that is a ground loop. You lose the best step - convection. And 2" pipe will be hell to work with in this situation.

    Just speaking from experience - not numbers.

    Edit realized I'm repeating myself somewhat. Apologies. Losing track.
    Last edited: Aug 25, 2012
  3. nd96

    nd96 New Member


    I appreciate your concern. I know this is not going by the textbook for a geothermal loop. I could be wrong, but I'm willing to bet any professional out there that this will work as well (if not slightly better) than your standard slinky ground loop or pond coil, AND create the no-maintenance (and fish hook resistant) pond loop that I wanted.

    I've got 15 sticks of 40ft rebar that I will cut into 10ft lengths and space about 10ft apart down the length of the loop. (See the attachment - it is not exactly to scale, but it should give you the idea). If my math is correct, that should be plenty of weight to keep the loop on the bottom until the sediment fills in. I already have the rebar left over from a storm shelter project I did last year.

    Storm Shelter


    Attached Files:

    Last edited: Aug 25, 2012
  4. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    Have at 'er

    Give 'er. You just have to report back here with the amount of swearing involved in the process:)
  5. Calladrilling

    Calladrilling Member Industry Professional Forum Leader

    Where are you getting 1300' loops in 2" is my first question?
    Isn't 2" a bit extreme for a 9-10 Ton system?
    This just seems like a over engineered thought plan here.
  6. nd96

    nd96 New Member

    I'm going with 6 - 500 ft rolls from a local supplier, It is actually 1500 ft per loop (about 1300ft will be in the pond). I will butt fuse the ends to make the required length as I feed it into the pond. 1 1/2" might work (3 loops), but the resistance is higher. I decided to go with 2 loops for lower resistance, easier installation and larger volume of circulating fluid.

    You are probably right about it being over engineered. My primary concern is longevity and a no-maintenance loop. I might be able to save a few hundred dollars with the loop by doing a different design, but I like the extra piece of mind. I plan on living here for the next 50+ years. (If you want to see over-engineered, see my storm shelter link above). :)

    Last edited: Aug 26, 2012
  7. nd96

    nd96 New Member

    The weekend work went well. I got all the pipe in the pond and made it about 1/3 of the way through sinking it with the rebar. One more day of attaching rebar and I think it will be done. The tractor and floating work barge (boat dock) worked out well. The fusion equipment wasn't too hard to figure out after doing a little on-line research.

    Working with the 2" SDR 11 pipe was a bear. No cussing involved urthbouy, but plenty of sore muscles. I did pick up 2 laborers to help me out, so I can't take all the credit.

    Attached Files:

  8. Palace GeoThermal

    Palace GeoThermal Well-Known Member Industry Professional Forum Leader

    looking good
  9. nd96

    nd96 New Member

    It took just 4 more hours of work, but we sank the remaining lines without any problems. The rebar worked great. One important step is to make sure you fill the pipe with water as you advance it into the pond. Otherwise you won't be able to sink it with air in the lines (or you need a really good flush to get all the air out).

    Overall, it was a good bit of work, but a great experience. I'm pretty sure I have a 75-100+ year geothermal loop that will require no maintenance. Total cost (materials and labor) for the 3000' of 2" HDPE loop was about $5000. Definitely worth it.

    When I get the house built and the HVAC units installed, I'll give an update in a couple of years.

    Thanks again for everyone's input.


    Attached Files:

  10. Calladrilling

    Calladrilling Member Industry Professional Forum Leader

    Looks like you had fun!
    Nice way to keep them spaced apart too.
  11. Pond loop pipe length

    I have good luck in south Texas with 300' coils per ton. On a really cold winter loops can run a little cool, so 400' coils is better. If yo ran straight pipe probably 300'/ton would be fine. It does need to be in water contact, not in the silt, to work best. Typically in Dallas we need 250-300' vertical bores, that is 500-600' pipe, per ton in a ground contact loop. For horizontal loops here I find you need about 880'/ton in a 5' deep trench, by comparison.
  12. The best way in Texas is to used coiled loops 300'/ton. have to be at least 8' deep, and have enough surface area or circulation to keep pond cool enough in summer. Slinky loops not necessary, since icing not much of a problem. Just have to make sure pond does not dry up or overheat. Make up water and pond fountain usually overcomes this.
  13. I find the best way to do pond loops is 300' coils/ton about 6' apart, left on 8" concrete blocks off the bottom. as long as the pond is deep enough not to over heat in summer, it works fine. That is here is South Texas.
  14. ryoung9

    ryoung9 New Member

    Jumping in on an old post, weighing options for a new install, considering closed loop horizontal trench, horizontal bore, and pond loop. I believe I have enough pond to support a 10t project, and I understand that the initial install cost would be lower with pond loops, but as is suggested in this post, won't the siltation degrade the performance of the system over time? If it's designed for thermal transfer to water and over time essentially becomes thermal transfer to heavily saturated soil (silt), doesn't that sort of invalidate the original design parameters? I'm starting to think now for long-term consistency (through droughts and extreme heat/cold) and high(er) efficiency performance that horizontal boring the lines under my pond might be the best approach. Thoughts?

  15. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    Silt is just thick water.

  16. ryoung9

    ryoung9 New Member

    So this thick water does not meaningfully impact performance? Do loops really need to be refloated to shake off the muck from time to time, and if so how often? Don't they just sink back into the silt when they go back down? Is this all much ado about nothing?

  17. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    I spent a good bit of time in my youth boy scouting. The group I was engaged with spent lots of time in water.

    Silt is thick water.

    Go find a pond and try to walk to water being at your waist. Then walk back out. This will teach you what silt is. Your feet where never dry.

    My point is that if there is water above the silted pond loop, one still gets the benefit of the water above it.

    The easiest way to move heat is with water, I think. So pond loops work very well.

    Allowing pond loops to "silt in" is a good thing. Less loss of fishing gear.

    I do not trust your information source.

    Note the picture to the left. That is me installing pond loops.

  18. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    I'm curious to learn exactly why an 8k SF 2 story ICF house needs 10 tons of HVAC in the Dallas area. I know everything is bigger in Texas, but it is not uncommon for larger ICF houses to come in at 1200 - 2000 SF per ton in my (Florida) area when a PROPER load calculation is performed. I'm not real jazzed by the load calculation company cited earlier in this thread.

    I have an 8200 SF two story ICF house project coming out of the ground needing right at 4 tons (we are installing 2 x 3 tons) and another at 10,100 SF needing right at 7 tons (we are installing 3 x 3 tons). Another 4400 SF ICF project has the potential to come in at just two tons, but I think we'll go with a 3 ton S7.

    Getting back to the project at hand, I smell three more rats:

    1) It is uncommon for the first floor of a two story house to have anywhere near as much cooling load (54 vs 58 kBtuh) as the 2nd floor since the first floor has the benefit of conditioned space above; no roof or attic heat gain

    2) the recommendation to meet a 54 kBtuh load with a 4.5 ton system (if one could be found) suggests they simply divided load by 12 to get tons. That ignores the sensible / latent split of the equipment relative to that of the house. The splits rarely, if ever match and proper design needs to take that into account.

    3) Did the load calc specify whether the ductwork be located in conditioned space compared with where yours will be (Hint - you'd be a complete fool to have any ductwork outside the conditioned (thermal and pressure envelopes) yet I find most load calculations assume all ductwork is in a hot attic and use that selection to justify radically oversized systems even when ductwork is within conditioned space.

    GIGO - Garbage In; Garbage Out.
  19. nd96

    nd96 New Member

    It has been a while since I checked this thread, and figured it was about time to give an update for those interested. I finished the house and moved in about 8 months ago. I am very happy with both the house and the geothermal system. I did stick with my plan for the two 5 ton units (Climatemaster Tranquility 30 series). Average heating/cooling bills for the 8,800sqft conditioned space has been about $90/month (high about $150, low about $20). As far as I can tell, the unit size is about right for the house. I prefer the units running in the first stage, so if the units are a little oversized for my house, that is fine with me. On the hottest days in July/August, the units seemed to be running about 50-60% of the time.

    I have not noticed any humidity problems (yet). Humidity inside the house usually stay between 50-60% (spring and summer). We haven't gone through a winter yet.

    I attached a screen shot of the system info while running this summer (I think in second stage). Maybe some of the experts can give some feedback on the numbers. Pump speed usually runs between 40-60% (never seen it above the 60's). The GPM only displays some of the time, but is usually between 10 and 14. Entering and leaving water temps are pretty much about 5 degree difference running first stage, and about 9 degree in second stage. As far as I can tell, the pond loop seems to be working okay, but I'd like to hear what others think.

    As far as having the same size units for both floors, I don't think that is much of an issue. ICF forms (Insuldeck) were used between the floors (including 2nd floor and attic). So there is 2.5-11.5" of foam insulation separating the attic and 2nd floor, plus a drop ceiling space. I don't know that there is much more heat gain/loss through the attic than through the foundation perimeter on the first floor (less well insulated).

    All the duct work runs in conditioned space and the units are both in a utility room on the first floor (conditioned). Nothing runs through the attic.

    System data.JPG Front-small.jpg Back-small.jpg
  20. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    "On the hottest days in July/August, the units seemed to be running about 50-60% of the time."

    Suggests oversized by a factor of 2-ish as I surmised. Low stage is helping with dehu as is ICF and other thermal mass. I'm glad to read you are happy

    My off-the-cuff guess is that a 4T would have been a good solution upstairs and 3T downstairs, maybe 4T down if heating load required or reserve for summer parties. 45% RH can result from proper sizing

    No big deal, just a bit of extra cost up front, more blower noise, minor loss of EER, minor loss of operating efficiency.

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