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Old 06-26-14, 11:21 AM   #1
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Default Modify heat pump to use cistern water

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. I would like to use that water to raise the efficency of my heat pump.

My idea is to replace the outside coils with a flat plate heat exchanger (Refrigerant to water) and pump the cistern water through it. I also wanted to add a AQUEFIERŽ Heat Recovery Unit to provide hot water from the waste heat in the system.

Does this sound realistic? Given the depth of the cistern and the volume of water my assumption was that it should maintain a fairly consistant temperature, even with the heat pump attached. Does anyone know of any studdy on below ground water and thermal conductivity. I am sure their are volumes out there. I have read that a 10 foot cube of water at a depth of over 10 feet underground is capable of cooling a house but I am skeptical.

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Old 06-26-14, 12:23 PM   #2
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I'm not a pro at any of this by any means but you have roughly 15000 is gallons of water..

If you still use the cistern for you domestic water the only problem I see is that in the winter time you will have very very cold water to use and that will bring the efficiency of your water heater down a lot..
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Old 06-26-14, 11:21 PM   #3
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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.


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Last edited by AC_Hacker; 06-27-14 at 07:42 PM..
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Old 06-27-14, 12:54 AM   #4
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If there is not enough of a heat source to make it a geothermal system, add some solar thermal panels.
To my surprise, shortly after Naomi Wu gave me a bit of fame for making good use of solar power, Allie Moore got really jealous of her...
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Old 06-27-14, 07:17 PM   #5
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This is akin to the question "Can I heat and/or cool my home with my swimming pool?". The answer is: it depends.

If you live in a tropical climate, and don't mind swimming in 130 degree F water, then yes, you can cool your home. In your case, if you don't mind 130 degree cold water on tap, then yes you can. That is, if the cistern is also your water source.

As the temperature of the vessel increases, the efficiency of the heat pump will decrease. At a certain point, you will be better off using an air coil, especially at night. With a swimming pool, evaporation does wonders as far as massive heat transfer. With your sealed cistern, you will have to rely on the surrounding dirt. Refer to the manifesto and AC's words of wisdom...

During heating season, you have a lower limit as to how far you can drain the heat out of your vessel, and this limit is around 40 degrees F. Below that temperature, you risk freezing your heat exchanger and releasing all manner of mayhem. Not recommended.

The main question here lies in whether you have enough surface area to transfer all the heat your heat pump can move. My guess is probably not. I imagine the setup would run for a few days or even a week, then top or bottom out, depending on what you're doing to the cistern water. However, it would work well in spring and fall, when the temperature is rising and falling every couple of days. Or, you could use it as a reserve to use during sub-zero or above 90 degree temperatures, when your regular system is getting a workout.

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flat plate heat exchanger, geothermal, midofy heat pump

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