# Heat retention of greenhouse pond



## ghmerrill (Feb 22, 2011)

I'm putting a pond in the greenhouse- the greenhouse is 12 x 40, the pond is going to be 4' x 20' x 18" deep... Broad, long, and shallow. The purpose of the pond is to raise some fish, and act as thermal mass in the greenhouse. I'm digging the hole for it now, and got to thinking, why is it that all the photos and videos I have seen show the ponds above ground? Now I'm getting worried that the soil won't help hold the heat, it will suck it out of the pond.....

Ideally, I would like to raise tilapia, but I have to keep the water warm for that, and this is a completely off grid, low budget project.

So, is a below ground pond a bad idea? 

Can I use a heat reflective insulator like Mylar to help? 

Any thoughts?


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## Ky-Jeeper (Sep 5, 2010)

http://www.pbase.com/ericjeeper/image/66320513 good example here.


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## TnAndy (Sep 15, 2005)

ghmerrill said:


> ... and got to thinking, why is it that all the photos and videos I have seen show the ponds above ground? Now I'm getting worried that the soil won't help hold the heat, it will suck it out of the pond.....


Probably the reason you see most of these type deals above ground is simply cost and ease.....same reason folks will put in an above ground swimming pool versus the cost of an inground one. Knock some wood side walls together, drop a plastic liner in and fill it up, versus all that digging.

As for the earth "sucking" the heat out faster, having it up in the air will cause the heat to "suck" faster than in the ground....air being a far better heat exchange medium than earth.

Either way, I'd probably insulate the sides/bottom with some foam board, and plan on some kind of removable cover for nights depending on your night temps.....like the covers on hot tubs.


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## Micheal (Jan 28, 2009)

I could see how an "above ground" pond could be easier to maintain/clean or even drain if needed....


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## ghmerrill (Feb 22, 2011)

I didn't even think of a cover for night! 

Thanks!


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## pancho (Oct 23, 2006)

Since you live in Oregon it would probably be a good idea to raise some other kind of fish.


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## SolarGary (Sep 8, 2005)

Hi,
I think that you would want to insulate the pond with rigid foam insulation (as in Jeepers photo), and then line the insulation with a single folded piece of EPDM or PVC. Not sure if either of these linings effect fish or not?

Reflective insulation is only effective if it faces an air gap, and that would be hard to do with a pond. The reflective bubble wrap style insulation they sometimes use under concrete slabs has been shown by test to be ineffective.

If you want the pond to be thermal mass for the GH, then it has to be able to exchange heat with the GH air. Maybe you would get enough of that with just the surface of the pond, but not sure.

Gary


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## Cyngbaeld (May 20, 2004)

The ground is acting as a heat sink but not as "thermal mass" because it is not insulated away from the surrounding earth which is sucking the heat away. You might try a simple method of insulating the ground around the outside of the greenhouse to see if that helps. Basically lots of straw covered by a tarp or plastic. Do it on the warmest, driest day you can. Go out at least 6 ft from the greenhouse.

If that doesn't work, you'll need to isolate some thermal mass away from the earth and insulate the floor. Some people use barrels of water and put their plants on trays over the barrels.


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