I'm a new pool owner with a fairly advanced question. I got into a discussion today about flow rates and heat transfer coefficients. I was discussing my disappointment with the fact that when running my natural gas heater, the water coming out the returns isn't very hot(perhaps about 10-15* hotter at full heat). I know that this will heat up the pool over an extended period of time. However, I blamed it on the fact that I currently have a 3.5 HP spa/pool pump running my recirculation system. I claimed that the water was running so quick through the heater, it didn't have time to heat up much. Someone else claimed that as long as I was having turbulent flow (vs. laminar), the transfer of heat was about the same, regardless of flow rate. Does anyone have any input on this? If it matters, my pump is plumbed for 2" lines, but everythingon my pool is 1.5", so everything is reduced to that at the pump. Thanks in advance for any input.