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Burying Groundsource heat pump beneath pond?
#536097 06/04/21 11:06 PM
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Falison Offline OP
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Hi, buying my first home and it comes with a project of finishing 40x100ft pond that's 30ft deep in the middle.
Before finishing it I'm wondering about installing some geothermal hosing before laying the clay on one side of the pond. All the pond based systems I've seen professionals will sink the system but that must be some sort of swimming hazard? Also wouldn't being under the clay make it more efficient and like a horizontal GSHP than laying it in a body of water?

Thanks for any explanations and advice you have.

Re: Burying Groundsource heat pump beneath pond?
Falison #536098 06/04/21 11:24 PM
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Welcome to the forum. All we see around here are open loop systems. I would think burying it would act sort of an insulator for the loop, being in direct contact with the water might minimize that, plus if you have to work on the line for any reason it is a lot easier to access if it isn't buried.

As for a swimming hazard, how many swimmers do you know that go down 30 feet?


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Re: Burying Groundsource heat pump beneath pond?
Falison #536107 06/05/21 08:07 AM
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I strongly suspect (disclaimer: it's been forty years since I took Heat Transfer class) that the fact that water would circulate through the coils of an open loop in a pond (which would occur due to natural convection from the resulting temperature difference) means that temperature transfer loops in water would be more efficient than ones in clay that was pretty much the same starting temperature.


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Re: Burying Groundsource heat pump beneath pond?
Falison #536113 06/05/21 09:40 AM
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Sorry, I should have specified close loop system!

With horizontal systems the minimum depth required is ~6ft.

Definitely not a swimming hazard concern with 30ft, but the demo videos I've seen the ponds aren't 30' deep.


Isn't "working on the system" harder if it's burried in dirt than burried in a pond?

Just found this table of conductivity, clay saturated with water looks the highest

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Last edited by Falison; 06/05/21 09:51 AM.
Re: Burying Groundsource heat pump beneath pond?
Falison #536125 06/05/21 11:45 AM
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Falison, I am a geologist, NOT a heat pump expert. However, the conductivity of water is several orders of magnitude greater than soil materials. (Primarily, due to movement of the water, which is impossible for soil materials.)

Your question quickly becomes very complicated. Using water as the heat sink or the heat source in your closed-loop system is more efficient based on conductivity. However, it may become less efficient due to temperature differentials over the course of a full year cycle. The soil temperature will be close to the average annual surface temperature. The water temperature will slowly increase above the soil temperature during the summer. The water temperature will be highest when you are trying to dump heat during the summer. Likewise, the water temperature will be lower than the soil temperature during winter when you want a heat source.

Finally, I believe "efficiency" may NOT be the most important variable. I think(?) the reason people install closed-loop systems in existing water bodies is purely due to the installation and maintenance savings. Digging the trenches or excavations for the ground loops is a large percentage of the costs.

My advice would be to search the net for water-based heat pumps in general, and in your corner of the state.

You probably already know this info, but I included a link to some general heat pump discussion that also includes a section on surface water heat pumps.

https://www.wbdg.org/resources/geothermal-heat-pumps

Good luck on your project!

Re: Burying Groundsource heat pump beneath pond?
Falison #536129 06/05/21 12:35 PM
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Thanks everyone! I definitely have some gaps in my understanding. The water being greater effected by the ambient temperature is definitely one of those concerns, I guess I'm trying to avoid the additional cost of digging a horizontal GSHP line by using the existing hole that will be a pond. Trying to understand if I can combine the ideas.

I'll get on the phone with some people next week. Thanks for listening to my ideas and giving me feedback, it's been very valuable at sharpening my understanding.

I'm completely reconsidering incorporating a pond. Im going to buy an excavator to finish the pond and do yard work anyways. Maybe I put the hosing under what will be pathways and follow the horizontal GSHP idea without modification.

Last edited by Falison; 06/05/21 12:45 PM.
Re: Burying Groundsource heat pump beneath pond?
Falison #536130 06/05/21 02:23 PM
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About 15 years ago when I was researching geothermal I was told that during a prolonged winter the efficiency of a closed loop system would drop if it were installed in soil due to drawing heat from the soil and cooling the area right around the coils. It's a toss-up here between open loop dumping 5-6 gpm into a pond and closed loop systems that have the loops in the pond.

If the ponds ice over, the bottom water in a pond is typically 39-42 degrees..... Here the open loop systems need to be "flushed" once a year due to mineral and iron build-up.


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Re: Burying Groundsource heat pump beneath pond?
Falison #536132 06/05/21 03:45 PM
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esshup gives a good example of the trade-offs between two variables.

His example is where decreased efficiency is due to lower conductivity in the soil surrounding the ground loop. The lower the conductivity of the soil, the higher the thermal gradient from unaffected soil to the colder soil immediately adjacent to your ground loops. (You want a lower thermal gradient for maximum efficiency.)

Re: Burying Groundsource heat pump beneath pond?
Falison #536133 06/05/21 04:13 PM
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When we built our house in 1995 we went with an open loop system, if I remember correctly it ran off our well at under 3.0 GPM. A few years ago we converted to a closed loop system, in my situation I prefer the closed loop. We would flush our open loop system every 2-3 years, this isn’t necessary with our closed loop. There really shouldn’t ever be a need to maintain the coils underground so I don’t think it matters from that standpoint if they are under water or dirt. I did most of the dirt work myself, I was impressed with the directional boring guys. They set up 75’ east of my house, went under the footing and came up in a 16” hole in the concrete floor next to the heat pump. I took them less than 30 minutes. Running the heat pump hard today with the air conditioning as it’s about 90F outside, I like my house around 67F. Good luck.

Bryan


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