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How big a pond for heating/cooling average size house?

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Steamback
Steamback Member Posts: 37
in Geothermal
A friend of mine asked me if their pond is big enough to support year-round heating and cooling using a heat pump for their average size home. I have experience sizing ground loops, air-source equipment and direct exchange ground loops, but I do not have experience suggesting minimum pond size for heat exchange applications. If you have done such an install and it worked OK, can you tell me where it was, how many gallons the pond held and how many feet of piping you used in the pond exchanger coil at minimum? Basically, I'm looking for a safe recommendation of pond size (total gallons) which makes switching to a pond loop more practical than a ground loop. Any feedback from someone who has done this before is appreciated greatly. Thank you!

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  • Zman
    Zman Member Posts: 7,569
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    I think there are quite a few factors. Can you see the pond on google earth? Do you know how deep the pond is? What climate zone?
    A very small deep pond would likely work very well. Shallow ponds are heavily influenced by outdoor temps
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Jamie Hall
    Jamie Hall Member Posts: 23,283
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    In some ways this really isn't all that hard. First, as @Zman said, you do have to consider how deep the pond is, and what your climate is like. Around where I am, for instance, it is reasonable to assume that the top foot of a pond will freeze solid by mid-January, if not sooner. So that water is unavailable.

    But leaving that aside, it's pretty basic: you need to figure out how many BTU per year you need to extract from the pond. Then you need to know how much water and at what temperature will get into30 the pond (probably very little in the winter, but that varies). Then you need to figure out just how much you can afford to lower the water temperature. Then you have a delta T and a BTU figure and a flow figure at a certain temperature, and you can figure out how many pounds of water you need.

    Just for the sake of giggles, an example. Suppose I know that I have a situation which uses 300,000,000 BTU per year. I have -- for simplicity -- negligible inflow into my pond. I can start in October with a mean water temperature of 50, and I can afford to drop to 35 by May. So... I need 20,000,000 pounds of water. About a million cubic feet. My pond is on average 10 feet deep. So I need about 100,000 square feet of pond, or about 3 acres.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • hot_rod
    hot_rod Member Posts: 22,137
    edited December 2019
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    I have a 3 ton system in about 1 acre of pond 10’ deep in the center. I’m lucky to have a small spring feeding in, that makes a big difference. I should sink a temperature sensor in the bottom mud to see how much it changes. Surface can go into high 80s in cooling mode.

    Temperature does of course change HP efficiency.

    IGSHPA the heat pump association in OK has good sizing info at their website
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • HVACNUT
    HVACNUT Member Posts: 5,835
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    I have absolutely no experience on this subject but just wondering if with deep ponds, spring/ fall turnover effects anything?
  • hot_rod
    hot_rod Member Posts: 22,137
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    I think it’s all about temperature deltas
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • psb75
    psb75 Member Posts: 833
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    Lakes and ponds: Winter-- cold on top, warmer below. Summer- warm on top, cooler below. That's what 'turnover does.'
  • SteamingatMohawk
    SteamingatMohawk Member Posts: 1,005
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    @Jamie Hall I tried that calculation several years ago for my 30,000 gallon (250,000#) swimming pool. At the time I had no clue as to how to estimate the recovery by drawing "heat" from the underground surface of the pool. So, I just laughed and put it on the back burner for some other time.

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