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Old 09-21-20, 01:33 PM   #6
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Drain back is not generally recommended with evacuated tube collectors. Mainly two reasons:
First, stagnation temperature can literally melt down gasket and sealing materials. When the sun shines brightly and there's no fluid to absorb the heat, temperatures in the collector can skyrocket to over 150 degC. If there's any liquid pooled in the collector, it expands to steam, which causes more things to cook and melt.

The other big problem is cold shocking. When the collector is under full sun, and fluid pumping stops, you get the previous paragraph. Stagnation and steam generation and pressure and yadda yadda yadda. When the fluid starts pumping again, all of the hot collector plumbing gets a sudden blast of cold fluid. At first, your manifold acts as a large coffee maker boiler, boiling and spewing shots of hot water with steam downstream. If the pump is strong, it pushes the hot froth down the pipe, overcomes the boiling action, then the boiling froth is replaced by lower temperature fluid. This heat shocks the whole plumbing all the way to a mixing vessel (if there is one).
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