Which heats quicker High or Low speed pumping

[dp_vegas]

Thanks for all the info guys.

I guess it didn't matter, Turned on the heater this morning and it wouldn't work on low. Turned to high and fired up no problem.

So any bets on how long it takes to raise 14K gallons from 69 to 85 degrees, 14K gallons. Current outside temp 67 projected high today 91

[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.

[chem geek]

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

[mas985]

I think your second point is probably why they set the lower limit on flow rate. I suspect that the heat exchanger could handle much lower flow rates but you run the risk of scalding water comming out of the returns. Although not quite the same, tankless water heaters work at very low flow rates, < 6 GPM. If the flow rate dropped to 10 GPM on a 400k BTU heater, the delta T would be close to 64 degrees! On top of 80 degree water would present a significant hazard.

[salinda]

I have a variable speed pump (Ikeric) and a 400,000 BTU heater. When the new pump was initially installed, it was only programmed to go to high speed when water is flowing to solar. When I ran the system in spa mode the pump was on low. (Note that I have a separate pump for the spa jets. The spa shares the pool's filter pump.) The water coming into the spa was way too hot. I suspect that something in my almost new heater should have prevented this as a safety mechanism but I had the contractor that installed the pump set the pump to go into high speed whenever the water is flowing into the heater as well, when I close the new automated heater bypass valve. The apparently scalding water mixing into the spa concerned me. Be careful.