New Pond Geothermal System Design

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

  1. nd96

    nd96 New Member

    I will be building a new house in 6-12 months and will be going with a closed loop geothermal system for heating and cooling. I live in the Dallas area and have a nice 3 acre pond on the property that is 10-20ft deep in the deeper areas.

    I will get the exact numbers from a professional, but my current estimate is that I will need a 8-10 ton system. So probably two 4 ton or 5 ton units.

    My preference would be to lay straight runs of the PE pipe on the bottom of the pond and eventually let sediment cover the lines so that I don't have to worry about getting fishing hooks caught or other potential damage to the lines. It is a private pond and I could do straight runs of pipe up to about 500ft each way (1000ft loop) without any problem.

    To simplify the system I would like to run one larger size PE line for each geothermal unit (say 800-1000 ft of 1 1/4 inch line). It may be a little more expensive for the 1 1/4 line vs using 3/4, but I think it would eliminate the manifold system, simplify the plumbing, and increase the reliability of the loop. Doing a long straight line would also dissipate heat better than having overlapping slinky or coiled pipe.

    Has anyone done a system similar to this? Any recommendations on pipe length for 4-5 ton loops? The middle depth pond water in the summer tops off about 80-82 degrees and drops to about 40 in the winter. On the bottom of the pond it is probably a few degrees cooler in the summer and a few degrees warmer in the winter.

    I've attached a picture of my property and proposed loop lines. Any thoughts?
     

    Attached Files:

  2. waterpirate

    waterpirate Well-Known Member Industry Professional Forum Leader

    Hi and welcome,

    I think at first glance your pipe lengths are short for your tonnage. You will be better off I think useing the slinky method to get more pipe in the same footprint. Others here have pond loop experiance that they can share. Good luck.
    Eric
     
  3. Palace GeoThermal

    Palace GeoThermal Well-Known Member Industry Professional Forum Leader

    I agree with Eric, I think you need more pipe in your pond.
     
  4. nd96

    nd96 New Member

    Maybe I'm thinking about the issue wrong, but it seems like there would be some benefit to creating a larger field area/footprint by laying the pipe out straight, and not overlapping loops. I know the coils and slinky are done primarily to minimize field size. I don't really need to minimize the field size, so I'm trying to see if I can use a larger field area to my advantage.

    I just don't know if that is a 1% advantage or a 25% advantage (for reducing pipe length). My guess would be somewhere around 250ft/ton. But that is a completely uneducated and random guess, and most professionals don't endorse that. :)

    Any good geothermal guys in the Dallas (McKinney) area?

    Thanks for all the input and replies.

    Thomas
     
  5. waterpirate

    waterpirate Well-Known Member Industry Professional Forum Leader

    Hi Thomas,

    All good thoughts. The gross rule of thumb for vertical loops where the ground temp is stable at -30 feet from the ground is 150 to 250 depending on soil types and design temps that were planned for.

    The gross rule of thumb for pond loops is generally at least twice the amount of pipe as vertical. 500 feet per ton is not uncommon.

    In regard to the slinky and field size the answer is: yes and no. When we are moveing btu's and or doing a thermal exchange it is all about the " feet of pipe in the ground ".

    It is about surface area available to do the exchange, particularly in a pond loop. Think the more surface area I have on my radiator, the more freely it will do the exchange without altering the ponds base temprature.

    That is the principle behind the new hyper loops, and the tried and true plate exchanger like the slim jim and simmilar products.
     
  6. Palace GeoThermal

    Palace GeoThermal Well-Known Member Industry Professional Forum Leader

    Hey Thomas,

    The proper way to approach this is to determine how many feet of pipe you need in the water to dissipate all of the heat that you need to reject into the water.

    Once you know how many thousand BTU/hr you need to reject, then the length of pipe can be calculated, then you can figure out how to place that length of pipe in your pond.
     
  7. Palace GeoThermal

    Palace GeoThermal Well-Known Member Industry Professional Forum Leader

  8. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    Pond Loop

    If you're planning on burying it in the pond, it is a ground loop - not pond loop. And design needs to factor that in accordingly.

    You lose the biggest heat transfer - convection - by burying it.
     
  9. nd96

    nd96 New Member

    "If you're planning on burying it in the pond, it is a ground loop - not pond loop"

    That is an important factor. I would lay the pipe on the bottom of the pond, but over 10-20 years, sediment will eventually bury it. But I'm wondering if as the sediment builds and the system drifts away from convective heat dispersal, if it would more closely match the transfer of a vertical loop, than a horizontal loop. It would essentially be 20 feet below ground and in completely saturated soil. Don't vertical loops on average require less ft/ton pipe than horizontal (or pond)?

    I'd probably need to find someone who has actually done this or who is really good with math and can appreciate all the variables involved.

    Thanks for the link to the installer directory. I haven't quite decided if I want to use a local installer to do the whole system or if I will go with one of the DIY kits they sell on line. When I put the pond in about 7 years ago, I put several 4" PVC conduits that go from down in the pond up to where the new house pad will be. I plan on running the PE lines through those pipes, and should be able to do the loop installation pretty easily (especially if they are just straight line loops that lay on the bottom of the pond).
     
  10. Calladrilling

    Calladrilling Member Industry Professional Forum Leader

    Disregard this poSt if I'm reading you incorrectly.
    You want to run your pond loop through 4"PVC piping you had previously installed in the pond? If that's correct then forget about that idea, it's not going to accept or reject the heat when encased inside PVC.
    You can run the supply and return lines to the loop field through the PVC but not the whole loppfield itself.
     
  11. nd96

    nd96 New Member

    No, the 4" pvc are just short (50ft) conduits to thread incoming/outgoing PE pipe through. The PVC conduits run under a couple levels of a stone retaining wall and open up/end in the pond. I put them in 5 years ago when when I built the walls. That way I wouldn't have to dig up the stone walls or their concrete foundations to put in the geothermal loops when it came to building the new house.

    They just make it easy for me to get the PE loops from the house pad into the pond.
     
  12. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    Couple of thoughts.
    Straight pipe can pick up or dissapate more btus than a slinky in horizontal earth loops, but not so much in pond loops, where water should speed transfer regardless of pipe style.
    Straight runs and 11/4 pipe should be fine and may offer impact and fishing lure protection. Size/length will come when you get your loss/gain load of new home prints.
     
  13. nd96

    nd96 New Member

    Thanks for the thoughts AMI and everyone who has provided input. I think I'm going to stick with my current plans for the general layout. Once the HVAC system size is determined, then work on the loop size/length.

    I'm planning on a rather large ICF house and will have a professional ( www.energywisestructures.com ) size the system. Hopefully they can provide some numbers next week.
     
  14. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Kepp in mind that 1.25" pipe only has marginal, about 10%, better heat exchange than 0.75" pipe. But it requires about 3 times the same flow than 0.75" pipe for the same Reynolds number to get a marginal turbulent flow. Make sure a pressure drop calculation is done if you deviate from the design standards...the whole system has to work together! Flow requirements, temperatures, pressure drop and overall capacities all matter.
     
  15. nd96

    nd96 New Member

    I received the recommendations from HVAC engineer. The first floor has a load of 63K Btuh heating and 54K Btuh cooling, recommending 4.5 ton. The second floor it is 45K Btuh heating and 58K Btuh cooling and 5 ton unit.

    So it looks like I will round up and get a 5 ton unit for each floor.

    The house is 3963 sqft 1st floor and 3932 sqft 2nd floor (conditioned space).

    After a little more amateur research, I'm thinking a single 1200-1250 ft loop of either 1.25" or 1.5" PE in the pond for each unit. I think the pressure drop on 1.25" may be too much for a run that long. I'm going to see if I can get some feedback from the engineer or the HVAC company I purchase the system from, but would love to have some additional opinions from anyone else who might know.

    Thanks!
     
    Last edited: Jul 12, 2012
  16. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    You need to know Flo

    A manifolded system will require less pump. Bigger is not always better.
     
  17. Jim S

    Jim S New Member

    nd96,
    You might look into a plate type heat exchanger like the one pictured. It is 10T capacity, consisting of two 5T plates in a parallel connection. The rated capacity is for the plates themselves, the piping from the house to the plates added approx another 2T although I did not include that in my heat rejection calculations.
    I had to go through a local contractor to purchase the plates, but built the frame and manifold myself. I was able to pressure test it on-shore and then float into position and submerge. Mine is at 8 ft depth - your greater depth would be a significant advantage. Slim Jim is one brand but there is at least one other.
    Jim
     

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  18. nd96

    nd96 New Member

    Jim,

    Thanks for the input. I did look into the plate options. The main drawback for me is that the plates require maintenance. You need to re-float the system every few years to keep sediment from becoming a problem (just like with coils). If sediment builds up on or around your plates, they loose their thermal exchange.

    I'm trying to go with an option that will require no maintenance for the next 30-50 years. Long runs of HDPE sitting on the bottom is probably going to be the best option for me.
     
  19. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    There is a hidden cost

    That you may be missing. What are you planning to use to keep the pipe on the bottom? HDPE floats.

    Mark
     

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  20. nd96

    nd96 New Member

    I do plan on weighing it down. I'm thinking a 10ft stick of rebar every 20ft or so, or PVC pipe filled with some concrete. Zip tie the HDPE lines spaced 2-3ft apart under the stick of rebar (or pvc). That should weight it down and keep the lines evenly spaced at the same time. The sediment will eventually bury it.

    I ran some quick numbers, (and if my math is correct) for 3000 ft of 1.5" HDPE, it would take less than 100 pounds of weight to counter the buoyancy of the HDPE (assuming no air in the lines). So 10-15 pound weights spaced out properly should do the trick.

    Thomas
     
    Last edited: Aug 22, 2012

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