Which heats quicker High or Low speed pumping

[dp_vegas]

Hi guys.
I have a question for the Smarter ones out there. I will be turning on my heater soon. Two speed pump with flows of @ 80 GPM and 30 GPM . Given a 400K BTU heater. Which speed would heat the pool faster ?

Any thoughts would be much appreciated.

[chem geek]

The faster flow rate will heat more quickly, but not by the factor of the flow rate. It's simply more efficient to transfer heat by a small amount that keeps the temperature differences small. A slow flow will be less efficient because the water gets heated hotter and therefore accepts getting more heat a little less (due to a smaller temperature difference between the flame/pipe and the water) and also loses more heat down the pipe (due to the greater temperature difference between the water and the pipe/ground).

However, I don't know how big a difference this is and you are welcome to experiment with both flow rates and let us know what you find out!

Richard

[CarlD]

The faster flow rate will heat more quickly, but not by the factor of the flow rate. It's simply more efficient to transfer heat by a small amount that keeps the temperature differences small. A slow flow will be less efficient because the water gets heated hotter and therefore accepts getting more heat a little less (due to a smaller temperature difference between the flame/pipe and the water) and also loses more heat down the pipe (due to the greater temperature difference between the water and the pipe/ground).

However, I don't know how big a difference this is and you are welcome to experiment with both flow rates and let us know what you find out!

Richard

HOORAY! I couldn't have said it better!
In fact, I couldn't have said it nearly as well!

[dp_vegas]

So High flow will heat faster but increase electrical cost since the pump costs more. I have to decide the if the trade-off is worth it.

Will it heat 10% faster 50% .. 80% Any guesses ?

[JohnT]

So High flow will heat faster but increase electrical cost since the pump costs more. I have to decide the if the trade-off is worth it.

Will it heat 10% faster 50% .. 80% Any guesses ?

The only advantage of the low flow would be less per hour pump cost. Assuming the pump's electrical use is in direct proportion to its flow, the high flow would win hands down because it is more efficient and the total amount of heat needing to be added is the same.

[chem geek]

The specifics of whether a high speed or low speed are more or less efficient is dependent on how your system curve (feet of head vs. flow rate) intersects your two pump curves. Let's just assume that the system curve intersects your pump curves at their most efficient points. At very slow flow rates the "fixed" inefficiencies of electrical and other losses make the overall efficiency lower. For example, the Intelliflow variable-speed pump has an output power (product of head and GPM scaled to appropriate units) to electrical power efficiency ratio and other parameters as follows:

 RPM   GPM   Head   Watts(Input)   Efficiency
3450   100    77       2780          52.1%
2070    60    28        316          50.2%
 690    20     3         95          11.9%

However, note that from a heating perspective, the output power is not relevant since it doesn't matter what the pressure is, only the flow rate. Essentially, half the flow rate is achieved at one-fourth the electrical power (the formula is more complex than that, but this is close enough). So lower pump speed is more electrically efficient from a flow rate (GPM) perspective, but such efficiency improvement is far less at very slow speeds (flows). I don't know the specs on your specific pump, but it would certainly be true that the slower pump speed would be more electrically efficient in terms of GPM (even if efficiency relative to output power were the same).

As for how much of an efficiency difference there is vs. flow through a gas heater, I do not know, but I can refer you to what happens in solar panels at this link where the efficiency through a panel at 1 GPM was 60%, at 2 GPM was 70%, and at 4 GPM was 80%. Obviously at some point, doubling the flow rate won't continue to add 10% since you can't get to 100% efficiency. Typical gas heater efficiencies are around 80% unless you have a super-efficient heater at 90%. These are probably rated at typical flow rates so my best guess is that at your lower flow rate which is 37.5% of your higher flow rate, that the efficiency difference might be around 15% less. So if your high flow rate was 80% efficient, then perhaps your low flow rate might be 65% efficient.

Furthermore, be aware that gas heaters not only specify a maximum flow rate (typically around 120 GPM), but also a minimum flow rate that varies depending on heater size. The minimum flow rate appears to be approximately 1/10,000th the output BTU so your 400,000 BTU heater (assuming that's output BTU and not input BTU) probably has a minimum flow rate requirement of 40 GPM. This is to make sure that the heat transfer pipe and other internals of the heater do not get too hot. If the hot is not transferred as efficiently, that means that something else (i.e. the heater itself) gets hotter and that can cause damage.

So this is a hard one to determine because the slower pump speed, if it isn't TOO slow, will cost you a lot less overall though you will heat your pool perhaps 15% more slowly due to the lower thermal efficiency. However, if the slow pump speed is very electrically inefficient, then you may not realize a net cost savings (though my hunch is you probably still will). Also, if the pump speed with the 30 GPM is lower than your gas heater can handle, then obviously that is not an option for you.

Richard

[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.