[stevehull]

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