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Old 12-28-20, 06:34 AM   #12
Helper EcoRenovator
Join Date: Mar 2012
Location: Warwick, Rhode Island
Posts: 85
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Originally Posted by jeff5may View Post
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).
Jeff how are you doing?

I've had my drain back system in place for over 7 years now and no issues at all even with stag temps going off the wall..
Just wanted to chime in on your thoughts

2 - 20 tube evacuated tube collectors
1 - 6 gallon used electric hot water heater for drain back tank with site glass (electric disconnected
1 - 40 gallon used Superstor storage tank with heat coil in bottom connected to collector piping
1 - Grundfo's circulator pump
Resol solar controller... allow me to slow pump / water speed down which works beautiful. // uses less watts at lower speed and water picks heat quickly running thru evacuated tube collector manifolds.

Couldn't be happier..
Hardly no maintainence at all other then flushing system with vinagar now and then to remove rusty water from solar loop and now and then I have to add water to the loop because it evaporates over time.
Pat from Warwick, RI

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Comments and/or suggestions I make here at the forums on 'your' projects as well as my own have all been carefully and scientifically calculated by 'the seat of my pants'
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