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Old 06-26-14, 10:21 PM   #3
AC_Hacker
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Quote:
Originally Posted by thowell2695 View Post
I have a 2 1/2 ton AC Heat pump that I want to modify. I have a Cistern that is 10Ft deep by 10 ft wide by 20 ft long full of water.
FordGuy is right, if your cistern is full, and you also have 125,100 pounds of water.

Depending on the heat loss of your house (you really need to do a heat loss analysis before you go any farther) ... and just for just for arithmetic purposes:

1 BTU will raise one pound of water 1 degree F.

So, if your house needed 60,000 BTU/hour (* pulling a number out of thin air *) to keep it from getting colder, and your night was 12 hours long, your house heat load could drop the cistern temp by maybe six degrees-ish in one night.

So you can see that it might not take so many nights before you would have a 125,000 pound chunk of ice buried in your yard.

The mitigating factor is that the surrounding soil will slowly give up it's heat to the cistern... and I'm afraid I don't have the math to figure out how fast the rate of heat migration from the surrounding soil would be.

BUT, for a little perspective...

A rule of thumb (a very fallible rule) is that a 250 foot bore hole with a water loop circulating through it could yield 12,000 BTU per hour. Now that is considering that the bore hole is acting like a cylinder 250 feet long, and has a diameter of maybe > 16 feet. So you see, that is a big bunch of dirt, and to keep your house warm, you would need => 5 of these bore holes.

So that's 5 times "a very big bunch of dirt".

About 251328 cubic feet, or 30,159,360 pounds.

Does this give you any perspective?

I did a fairly detailed study of this kind of thing in the Homemade Heat Pump Manifesto thread.

I think you would find it very interesting.

Best,

-AC_Hacker
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Last edited by AC_Hacker; 06-27-14 at 06:42 PM..
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