solar heater

[Poconos]

Been a long time since I had Thermo (40 years?) but what you have to know is that the temperature differential is what governs heat transfer. You are trying to get maximum heat transfer from the surface that is heated by the sun to the flowing water. In the case of a solar heater the differential would be the temp of the heater surface with 'normal' water flow compared to the temp without water flow. Pretty close, not exact. Without flow probably pretty hot...120F+. In tphaggertys post, a couple above, you can think of a term like 'GPM x degree rise'. What it means is you have some constant heat input to a panel therefore higher flow yields less temp difference input to output of the heater and visa versa. 'GPM x degree rise' is a constant...almost. If you are trying to squeak every last BTU (British Thermal Unit...a measure of heat) from the system then you want max flow. Do what is best for the equipment, circulating, and sanitizing and enjoy the warm water flow. Whatever that implies.
Al