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Old 06-27-14, 06:17 PM   #5
jeff5may
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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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