Which heats quicker High or Low speed pumping

[mas985]

14000 * 8.34 * (85 - 69) / (400000 * .8) = 5.8 hours

I have a little different view on the flow rates and efficiency. A pool heater operates at much higher temperatures than a solar heater. Therefore, the reduction in efficiency is much less for lower flow rates. Natural gas burns well over 3000 degrees F and heat transfer is proportional to the temperature difference between the water and the flame. So at half the flow rates, the temperature increase of the water in the heater will be twice as much but heat transfer will be nearly the same. So the relative heat transfer for each flow rate is

Te = Temperature of of heat exchanger ~1000 degrees F (conservative estimate)
Tw = Temperature of water = 70 degrees F
dT1 = Temp rise for flow rate 1 = 400000 * .8 / (500 lbs / GPM * 80) = 8 degrees for 80 GPM
dT2 = 16 degrees for 40 GPM

(Te - (Tw + dT1) ) / (Te - (Tw - dT2)) ~ 1

So it is nearly the same heat transfer rate. In fact, this is a conservative estimate since the heat exchanger will have a slightly higher temp for the lower flow rates making the transfer rates even closer. The reason for the flow rate limit is that they don't want the heat exchanger to get to hot.

Also, your low speed flow rates should be half of your high speed so you should be at about the 40 GPM limit assuming your high speed is 80 GPM.