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The #1 rule for drainback solar collection loops has always been that both the supply and return must be sloped continuously back to the tank with a minimum slope of 1/4" per foot. That means no traps.
Well...... sometimes the job just can't accommodate rule #1, and we've found ways to break it. You can run a 1/4" air line from the top of the tank (assuming closed loop drainback) to some point on the collector return line above all the water traps. This provides reliable drainback and no possibility of freezing if none of the traps are outside the heated envelope.
With an atmospheric tank, it's a bit simpler, the air relief line can just be a stand pipe about 24" long somewhere on the return line inside the heated envelope.
My question is has anyone you seen systems with traps that work fine, or have you experimented with this problem? Perhaps some design criteria can be developed.
Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experiments