Geothermal heat pump: soil or pond?

coffeehausSeptember 29, 2007

Soon to be building and planning geothermal. Our geothermal guy is interested in using our nearby pond for the heat-exchange medium, rather than laying loops in the soil. Deepest spot in the pond is about 12 ft., size around 0.75 acres. Conditioned house size around 2300 sf. We're located in central VA and are more interested in AC than heating as we will have a woodstove for the latter. Pond has been in since 2001 and has never gone dry, but this year has certainly taxed it! Anyone out there with experience using water as the heat exchanger?

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marknmt

"It's generally thought to be a good thing".

I would expect (I'm no engineer, so be prepared to hear better views) that the pond would carry off a lot of heat from your AC needs- but whether it was enough for all of it, I just don't know. It depends on how well the house is insulated, its exposure, the windows, the habits of the owners, how humid the summer is -just everything.

But certainly it's going to be much cheaper to lay coils in the bottom of the pond than to drill a well or dig deep trenches. Climatemaster endorses the procedure on their website. In my mind it would most likely be worth trying it at your location, with the understanding that you might end up digging trenches to increase the capacity later. Perhaps it would be worth digging the wide trench to and from the pond, and then laying the coils out, into the pond, and back. It might be enough to lay coils out through the pond and then bring the return back in the same trench. I imagine your contractor has ideas about this.

Any way to increase the volume or retention of the pond? Is it spring fed or does it depend on rain?

I take it your pond, if a square, would be about 180 feet on a side. If you "slinkied" coils four feet wide back and forth through that entire area you'd have the equivalent of 45 trenches each 180 feet long. I don't even want to think about how much pipe that would take! But seat of my pants thinking tells me it's a lot of capacity.

Should observe, although you probably already know it, that water and wet soil are many times better at carrying off heat than dry soil, even clay. That makes me imagine that you would not need as much pipe in the pond as you would in the soil.

Overall, I think you should give it careful consideration.

Good luck,

M

    Bookmark   September 29, 2007 at 4:30PM
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minnt

I think there are websites that give specifics about putting the tubes in water. I did the research for it last fall and figured that we would need too deep of a pond to make it work for us. We just finished our geothermal trenching and laying of the pipes. All I can say is that the trench was BIG, DEEP and LONG! You can read about it below (go forward several days to see the final stages of installation.

Here is a link that might be useful: Geothermal trench

    Bookmark   October 1, 2007 at 12:22AM
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bob_brown

To do geothermal for coolimg only in a cold climate does not make much sense. Utilize the heat aspect of the system and you will not need the woodstove.

    Bookmark   October 1, 2007 at 1:42PM
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fsq4cw

Could work, although your pond is not very deep. Make sure your contractor knows what heÂs doing. Compare costs of lake loops to other configurations, then make the decision.

What guarantees is your contractor offering? You pond hasnÂt been around for very long.

Keep in mind that lake loops can be more problematic than other configurations. Have you discussed ÂSlim JimsÂ?

Any idea what temperature that pond is July/August?

Ideally, a deeper pond/lake would be better as there would be a Âlayering effect and the water on the bottom is a rather constant temperature. Your pond is too shallow for that and will be more affected by surface temperatures.

BTW: I know of people who have had their pond loops eaten through by beavers; bet you never thought of that one!

SR

Here is a link that might be useful: Slim Jim

    Bookmark   October 6, 2007 at 2:03AM
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coffeehaus

Whoa! No beavers here, yet...just muskrats and snapping turtles...but they are in the area. Frankly, with the current drought that we are having, we are less inclined to pursue the water option. Pond is down a couple of feet, fish are looking worried, and no rain in the forecast. I expect we will probably play it safe and go with the trench. Thanks to all for the input.

    Bookmark   October 7, 2007 at 5:08PM
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