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Old 06-21-15, 11:16 AM   #5
SDMCF
Apprentice EcoRenovator
 
Join Date: Jul 2014
Location: Finland
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My solar hot water system is not a drain-back system, but maybe some of my experience may apply anyway.

I have had the system stagnate & overheat when the power failed - my system relies on a pump for circulation - and this would be the same overheating panel scenario you are concerned about. The system did indeed overheat, to the point where it was producing hammer in the pipework and I was genuinely concerned about an explosion or at the least a burst. Thinking about it later I don't understand why the pressure relief valve did not activate. Either it is faulty or there I have made a design error in the layout.. Either way, it was not a nice experience. The end result was no major damage, just one evacuated tube lost all its thermal covering on the pipe inside the glass. It could have been a lot worse.

It could be a good idea to design in a method of losing any excess heat produced when your tank temperature gets up to your required maximum. If the power is on I have 2 possibilities. One is to lose the heat into my central heating system and the other is to send the heat to my front steps which I keep clear of snow using solar heat in the winter. Not sure how much heat the steps can take but the central heating can certainly take anything the panels can produce. Neither of these options is effective if the power is off though. Think about that in your system design.
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