Mark,

I see what you mean about the gas heater efficiency and that makes sense given the very high source (flame) temperature. Also, the losses down the pipe due to higher water temperature in that pipe are also probably low even though the increase in water temperature would be more than doubled. So it sounds like the lower speed is generally the way to go to save on electrical costs with little loss in efficiency, assuming the flow is faster than the minimum specified for the heater. Too bad the heater wouldn't turn on with the low speed in this case.

I suppose another consideration would be safety since the gas-heated water coming out of the returns in my own pool is already quite warm and at a slow speed (when I get my IntelliFlo) it will be nearly doubled in temperature. I believe the current increase in temperature at my current 65 GPM flow rate is 6 degrees so when I'm close to my 88ºF target the output from the returns is at 94ºF. At half the flow rate, the output would be at 100ºF. Well, I suppose that's not so bad and is less than the 104ºF typically found in spas.

Richard