First, I think we are talking about a solar hot water collector. Secondly, I believe you are talking about a thermo siphon system where the storage tank is located above the collector.
I think you are asking about the slope of the collector with a vertical collector allowing the best thermo siphon operation. You are correct that a flat collector, with a passive hot water collector plate does exhibit water flow stagnation.
Can you offer some additional details so we can understand better?
Steve
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consulting on geothermal heating/cooling & rational energy use since 1990
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