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Old 01-04-21, 12:46 AM   #3
MN Renovator
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The coil inside of the water heater would be exchanging the heat from the coil into the hot water in the tank, so it's not going to "run out after 3-7 gallons" as you said it, the capacity of the hot water tank remains the same because the water being fed to the house is in the tank, not the coil. The coil is connected to the solar heater. You've got the configuration backwards.

..but with that being said, it's not so great to have a small differential in temperature for the sake of efficiency. Ideally you would want to store the solar heated water in a tank not exposed to electric or fuel heating, you ideally want it to be exposed to water incoming at ground water temperature and have the solar heating heat that up as much as possible. From there, I think it would be ideal on a cost basis to use a tankless natural gas water heater capable of handling a warmer than expected(not ground water temp) water source. ..or I suppose you could use a second standard tank type of cold water heater. This whole thing gets expensive when hot water tanks generally only last a decade.

I would almost want my solar heat collection tank to be one of the 275 gallon(1 kiloliter) cube tanks if they can handle the heat without getting soft. ..but that would be a problem in the summer because you'd be heating your house, but it wouldn't be too difficult to build an insulated enclosure out of XPS or polyiso. Even a heavy layer of rockwool insulation would work if the whole thing is air sealed and thermal bridges are minimized.
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