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Thread: Pond water winter temperature

  1. #1
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    Pond water winter temperature

    Hi, I posted in the AOP res section but after sifting through the forum topics I think the question is best asked here. Without getting into detail I’m contemplating a closed loop through a pond to be built on my property. The pond volume will be 1.3 million gallons, 16 ft deep with +/-18,000 sf area. Once the surface freezes,(it will, zone 6) the water will be 39deg f. Is there a commercially heat pump to efficiently extract heat at that temperature?

  2. #2
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    I don't have any specifications in front of me at the moment.
    But that's right around our well water temperature here so almost any system would likely work with that temperature
    You don't squat with your spurs on.
    And you NEVER put the torches away before pressure testing.

  3. #3
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    Good idea to post here.

    The standard design conditions for GSHPs use a minimum entering water temperature of 30 deg F. So, 39 degree pond water will not be a problem for a standard GSHP.

    My 4T GSHP with a ground loop is currently working with an entering water temperature of 38.5 deg. My GSHP is spec'd to operate down to 20 deg, but the COP is pretty poor with water that cold.

    Pond loops are particularly good for heating. As you know, the water at the bottom stays at 39 deg. The water cooled by the pond loop is lighter and will rise, creating currents across the coils for good heat exchange.

    Despite having a sizable pond at my house with about 7 million gallons of water, I chose not to implement a pond loop. I have beavers, muskrats, and otters, with a tendency to chew on things, and I did not care to have them chew through the loop. There is also a need to periodically scrub the coils to remove algae, etc. and I did not want that maintenance headache.

    I don't know offhand whether your pond size is an issue. You will need enough space to install several thousand feet of pipe on the bottom and you'll need enough water to supply the heat your house requires without lowering the overall pond water temperature.

    If your pond does not have sufficient capacity for the size of your house, you also have the risk of water freezing onto the outside of the loops. This will reduce the heat transfer effectiveness and can cause the loop to rise to the surface. In extreme cases the pond could freeze solid.

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  5. #4
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    Thread Starter
    Thanks, that’s encouraging information. The house is designed with 4 big bedroom suites on the lower of the two levels and those are designated B&B rooms and occupancy in late winter will be minimum so Baseboard electric resistance supplemental heaters are part of the plan. The house will be R40 Walls and R60 roof, all SIP, triple glazing and very tight. I’ve tested the design against a heat loss calculator as I added or changed features to find what works best and think it’s workable with 5 tons heating, cooling isn’t an issue at all, but HRV is a code requirement as is wastewater Heat recovery. The pond as a heat source was an after thought as a friend looked at the model and said ‘ you should have a pond’. It helps when one of the toys is a CAT 955L with a 2 yd bucket.

  6. #5
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    Quote Originally Posted by sshaw View Post
    Good idea to post here.

    The standard design conditions for GSHPs use a minimum entering water temperature of 30 deg F. So, 39 degree pond water will not be a problem for a standard GSHP.

    My 4T GSHP with a ground loop is currently working with an entering water temperature of 38.5 deg. My GSHP is spec'd to operate down to 20 deg, but the COP is pretty poor with water that cold.

    Pond loops are particularly good for heating. As you know, the water at the bottom stays at 39 deg. The water cooled by the pond loop is lighter and will rise, creating currents across the coils for good heat exchange.

    Despite having a sizable pond at my house with about 7 million gallons of water, I chose not to implement a pond loop. I have beavers, muskrats, and otters, with a tendency to chew on things, and I did not care to have them chew through the loop. There is also a need to periodically scrub the coils to remove algae, etc. and I did not want that maintenance headache.

    I don't know offhand whether your pond size is an issue. You will need enough space to install several thousand feet of pipe on the bottom and you'll need enough water to supply the heat your house requires without lowering the overall pond water temperature.

    If your pond does not have sufficient capacity for the size of your house, you also have the risk of water freezing onto the outside of the loops. This will reduce the heat transfer effectiveness and can cause the loop to rise to the surface. In extreme cases the pond could freeze solid.
    I hope you meant refrigerant saturated temps instead of 30 & 20*f entering water temperature.

  7. #6
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    Quote Originally Posted by ehsx View Post
    I hope you meant refrigerant saturated temps instead of 30 & 20*f entering water temperature.
    I meant EWT. Standard practice is to design loops such that the minimum EWT for the season is 30 deg F. Obviously, this is for closed loop systems using antifreeze. Typical design assumption is 15% methanol, providing freeze protection down to 20 deg.

    20 deg EWT is the lower limit of operation for a Waterfurnace Series 7, as an example. Operating there would warrant a higher antifreeze concentration, and would require higher water flow, to ensure the LWT doesn't go below the freeze protection point. It wouldn't be recommended to design a system to operate at that point.

    These systems typically have freeze protection sensors that measure the refrigerant temperature that are set to trip at 15 deg for closed loop operation. For open loop it's typically 30 deg.

    You can review the attached screenshot for the performance of a Waterfurnace Series 7 at various EWTs.

    A properly designed pond loop will be surrounded by a constant flow of 39 deg water in the winter, and the loop will be large enough that the EWT will be near 39.
    Attached Images Attached Images  

  8. #7
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    How much space ought to be allowed between loops to have effective exchange and is it temperature dependant? By that I mean is the exchange the same in 39f water as 60f water.

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    Quote Originally Posted by sshaw View Post
    I meant EWT. Standard practice is to design loops such that the minimum EWT for the season is 30 deg F. Obviously, this is for closed loop systems using antifreeze. Typical design assumption is 15% methanol, providing freeze protection down to 20 deg.

    20 deg EWT is the lower limit of operation for a Waterfurnace Series 7, as an example. Operating there would warrant a higher antifreeze concentration, and would require higher water flow, to ensure the LWT doesn't go below the freeze protection point. It wouldn't be recommended to design a system to operate at that point.

    These systems typically have freeze protection sensors that measure the refrigerant temperature that are set to trip at 15 deg for closed loop operation. For open loop it's typically 30 deg.

    You can review the attached screenshot for the performance of a Waterfurnace Series 7 at various EWTs.

    A properly designed pond loop will be surrounded by a constant flow of 39 deg water in the winter, and the loop will be large enough that the EWT will be near 39.
    Your correct. But vendor should have been listed as “eft” and “ fpd” since words have meanings and not good to make assumptions. Firm believe in cya.

  10. #9
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    Quote Originally Posted by JimV8 View Post
    How much space ought to be allowed between loops to have effective exchange and is it temperature dependant? By that I mean is the exchange the same in 39f water as 60f water.
    If possible, you should consult with a GSHP installer experienced with pond loops in your area, to specify a design that will work for your scenario. It could be difficult to find someone though.

    The design tools I've used specify a 10' minimum spacing between each coil or slinky loop.

    There are other forums dedicated to geo, but they are not as active as this one, because there are relatively few GSHP installed compared to ASHP. Search for "geoexchange"

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  12. #10
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    Here is my advice. Get an engineer involved and maybe get a couple different engineers to size it for you.

    Almost every single geo system I have seen that has problems, the loop is too small.

    Everyone wants to try and save a buck and will try and do the minimum. It never ends well with geothermal and you spend years and/or piles of cash trying to fix it.

    Sent from my SM-G965W using Tapatalk

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  14. #11
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    Good advice. In addition to the Manual J/S/D that should be done for an ASHP, but often gets skipped, the GSHP needs design work to size the loop for the building load and design work to optimize the loop to minimize pumping costs. Not all installers will know how to do this, nor will take the time to do it.

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  16. #12
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    There are two "range" class of units. One for the colder North(Extended range) and one for the warmer South(Standard range). It has a lot to do w/ the EWT.

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