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I have a question about how to provide air relief in a drain back solar system that a client wants. The problem as I see it is that the run is more horizontal than vertical with a couple of steps in the piping. Slope will be pretty good about 1/2"+ per foot for a majority of the run. Length is my concern we are running about 125' horizontally under ground. Most of this run is on a slope and we can get below frost line. Total elevation from tank under house to collector high point is about 22' I am concerned that the pipes will not allow air into the collectors quickly enough to prevent freezing and that I will have to install a relief at the collectors to allow air in to drain the piping but then that eliminates the siphon for the flow of the water meaning I have to keep the larger pump running constantly.
I am tinking about using a spring check on the high point in the collector bank and set it so that it opens to allow air in. I know that there are vacuum breaker for pool collectors but this system is going to hotter than the breakers can handle. I tried years ago to get large drain back array working but the airvents seemed to create problems and the collectors constantly stamed through the air vents.