Pump Speed and Heating Efficiency

[Rancho Cost-a-Lotta]

Looks like spring will finally be arriving this week after unseasonably cold and rainy weather. Temps will be rising about 25 degrees from the 60s to lower 90s in just a couple days. I want to heat up the pool to so we can kick off our Covidcation.

Does pump speed have any effect on the heating speed and efficiency?

I thought I saw a recent thread indicating a higher speed was more efficient. With a balance of solar generation credit, I'm not worried about electrical costs. With a VS, my lower pump speed is 1600 1800 rpm and higher speed is 2500 rpm.

I found a Pool Heater Time Calculator online that indicates I can increase water temp of 18,500 gallons from 65 to 85 degrees in about 6 hours with a 400K BTU heater and solar cover. Does this seem accurate?

 

[ajw22]

I think that calculator is optimistic. By my calculations your heater heats your pool at 2 degrees an hour. 20 degrees will take 10 hours. Although you may get a boost from a strong sun.

Let me know who wins.

 

[Rancho Cost-a-Lotta]

Let me know who wins.

Thanks Allen, I'll be heating overnight, but I'll try to keep an eye on it.

Any opinion on whether flow rate/speed has any influence on heating efficiency?

 

[ajw22]

Thanks Allen, I'll be heating overnight, but I'll try to keep an eye on it.

Any opinion on whether flow rate/speed has any influence on heating efficiency?

Theoretically higher flow has less in transit heat loss. I am not sure you will see a material difference in practice. Hard to do tests under the same environmental conditions.

 

[Newdude]

This is a great question and one I’ve always wondered myself.

My brain figured that even if so much water was going through the heater that it didn’t fully heat to the set temperature, All those extra GPM that did flow through would still be much warmer than the pool water and effectively raise the pool temp just the same. Possibly even faster due to the sheer volume of warmer water.

We played with this while in the pool many times but never were able to decide on a winner. At very low RPMs the return temp in the pool is toasty warm. At full throttle it was not nearly as warm but the surrounding water had less of a bite to it so overall the general area was warmer, not just the return stream.

 

[Rancho Cost-a-Lotta]

This is a great question and one I’ve always wondered myself.

Me too. If power consumption were an issue I would probably run the lower speed. I think I'll follow the theoretical approach and run at the higher speed.

Thank you both for the input.

 

[PoolGate]

That calculator looks useful does it have a field for ambient temperature? I would think that matters. I get 1-2 degrees per hour with my heater. I never use the solar cover it is about to go in the trash I used it like 5 times before getting tired of it. I get it up to @ 88 and we are good to swim. But since we all like heated water coming out of the waterfalls and returns, I frequently run the heater the entire time we are swimming. Got it up to 95 once! Wow was that nice! My wife told me I have totally ruined her for ever swimming in a "cold" pool again! Ha!

 

[JamesW]

Example:
Air temperature at 80, water temperature at 80.

Air temperature in the combustion chamber goes up to about 1580 degrees Fahrenheit.

As the exhaust passes through the heat exchanger, about 336,000 btu/hr transfers to the water. (hd models are 82% efficient vs. 84% for the regular model. So, only 328,000 btu/hr is transferred to the water.)

64,000 btu/hr is lost as the exhaust leaves the heater at about 320 degrees. (hd models lose 72,000 btu/hr and exhaust temperature is about 350 degrees).

The flow rate through the heater should be between 40 and 80 gpm.

At 40 gpm, the temperature rise from inlet to outlet is about 16.8 degrees. Inlet 80 degrees, outlet 96.8 degrees.

At 80 gpm, the temperature rise is about 8.4 degrees Fahrenheit. Inlet 80 degrees, outlet 88.4 degrees.

The heat transfer depends on the temperature difference between the exhaust at 1580 degrees and the water temperature in the exchanger.

When you look at the average temperature in the exchanger, the lower flow results in about 99.719% of the heat transfer compared to the higher flow.

So, the difference is negligible.

Also, the flow through the heat exchanger is limited by the internal bypass.

So, even at 80 gpm, you're not going to get 80 gpm through the heat exchanger. You will probably get less than 60 gpm.

As long as you follow the minimum flow guidelines in the manual, going higher will not provide any significant benefit as far as efficiency.

I would try to stay as close to 40 gpm as possible.

 

[Rancho Cost-a-Lotta]

Thank you James...very helpful!

That calculator looks useful does it have a field for ambient temperature?

No field for ambient temp. I rarely use the heater unless we need a quick bump to get to 85, but these cool temps have left the water cold. Once I move the water 20-25 degrees or so, we should be able to maintain warmer water temps assuming the weather cooperates.

 

[JamesW]

One way to check for any efficacy difference would be to check the stack flue temperature at different flow rates.

For example, if the stack flue temperature is 320 degrees at 40 gpm, and 310 at 60 gpm, I would estimate that you're getting an extra 2,700 btu/hr.

I would estimate about 270 btu/hr per degree of temperature difference in the stack flue temperature.

If it's the same temperature (allowing for minor variations) then that would imply the same efficiency.

The ideal flow rate is 40 to 60 gpm. So, there's probably not going to be much difference.

 

[Rancho Cost-a-Lotta]

I heated the pool overnight. Starting water temp was 65 and I set the thermostat to 85.

In 10.5 hours, temps rose 18 degrees (83). I should hit 85 at 8:00 am after 12 hours.

Ambient air temps ranged from 60 to 53 degrees. Pool was covered.

I have no way to check flow rate, but the pump ran a combination of two speeds, 1,800 and 2,400 rpm, about 50/50.

Stack Flue Temps:
1,800 rpm: 303 degrees
2,400 rpm: 300 degrees

We're supposed to get up to 88 degrees today, up 30 from two days ago

 

[ajw22]

A bit less then 2 degrees an hour. About what I calculated.

 

[PoolGate]

Are hand-held instant read laser style thermometers accurate for measuring the exhaust temps?

 

[Rancho Cost-a-Lotta]

A bit less then 2 degrees an hour. About what I calculated.

Yes...you 'da man! Please don't calculate the costs for me

Are hand-held instant read laser style thermometers accurate for measuring the exhaust temps?

I had to look it up, but for the Pentair I was able to check from the control panel by holding the "Pool" key.

Thank you!

 

[JRB100]

My temperature rise averages 2 degrees per hour during nighttime (cloudy and/or colder) conditions. No cover, 400k heater, 15k pool. The rate will be higher is in daylight/sunny conditions, but the 2 degrees/hour rate has been a dependable predictor.

 

[JamesW]

Are hand-held instant read laser style thermometers accurate for measuring the exhaust temps?

I don't think that you would be able to get a reliable reading like that.

It would be interesting to see how the reading compared to the stack flue temperature sensor.

The stack flue temperature readings of 303 vs. 300 are close enough that it's probably the same efficiency.

Maybe a minor difference, but the ambient air temperature and other variables introduce some normal fluctuations.

If the stack flue temperature was significantly different, that would indicate an efficiency difference.

Note that heating rates have to account for heat loss to the ground, to the air, to evaporation and to radiation.

 

[Rancho Cost-a-Lotta]

I don't think that you would be able to get a reliable reading like that.

It would be interesting to see how the reading compared to the stack flue temperature sensor.

The stack flue temperature readings of 303 vs. 300 are close enough that it's probably the same efficiency.

Maybe a minor difference, but the ambient air temperature and other variables introduce some normal fluctuations.

If the stack flue temperature was significantly different, that would indicate an efficiency difference.

Note that heating rates have to account for heat loss to the ground, to the air, to evaporation and to radiation.

Thanks for the lesson!

 

[JamesW]

If we assume an 84% efficiency at an exhaust temperature of 300 degrees Fahrenheit, and approximately 270 btu/hr per extra degree of exhaust temperature, that would indicate an efficiency of 83.798% at an exhaust temperature of 303 degrees Fahrenheit.

So, approximately 0.2% difference or about 810 btu/hr.

 

[Rancho Cost-a-Lotta]

If we assume an 84% efficiency at an exhaust temperature of 300 degrees Fahrenheit, and approximately 270 btu/hr per extra degree of exhaust temperature, that would indicate an efficiency of 83.798% at an exhaust temperature of 303 degrees Fahrenheit.

So, approximately 0.2% difference or about 810 btu/hr.

So higher speeds are slightly more efficient, but not worth the extra power required to achieve the higher speed.

Thank you.

 

[JamesW]

810 btu/hr is about 237 watts.

So, from a power perspective, if the pump power increase is less than 237 watts, then it would be worthwhile to increase the speed.

You would also have to check the cost per unit of energy.

0.810 cubic feet of gas x 1.4 cents per cubic foot is 1.134 cents.

0.237 kwhr x 18 cents per kwhr is 4.27 cents.

So, increasing the speed costs more than it's worth.

Overall, as long as the flow rate is between 40 and 60 gpm, that is going to give you optimum performance.