Single speed vs VS pump -- relating to pool HEATING and temps

jesse-99

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May 2, 2021
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Just checking here for some input. I have a 2.5HP single speed Jandy pump right now as my main filter pump. Can I safely assume that, if I replace it (which is my plan) with a VS pump and run it at a lower rpm (say, for example 1750rpm) that when my heater is on, that the water coming out of the heater will be WARMER because the water was passing through it more slowly and therefore had more time to warm up inside the heater?

And can I also assume then, that using a VS pump would also heat my pool faster as a result and also save on my my gas bill/therms? I'm thinking for example that if with a single-speed filter pump water was coming out of the heater at 2 degrees higher than what it went into the heater, that with a VS pump and running more slowly, maybe the water coming out of the heater might come out at 4-5 degrees higher than what it went into the heater? Completely guessing here.

Anyone with knowledge of this?
 

guinness

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May 3, 2019
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California
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Long story short, that's not going to work. The only way you can heat a pool faster is by getting a heater with a higher BTU rating. The other alternative is to get a cover for your pool to reduce heat loss from evaporation. You still want a variable speed pump for energy saving though.
 
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JamesW

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Mar 2, 2011
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1668473487666.png

This shows the expected temp rise (inlet to outlet) (Y-axis) vs. Flow in GPM (X-axis) for a 400,000 btu/hr heater at 84% efficiency.

The minimum required flow is 40 gpm.

So, the maximum temp rise should be 16.8 degrees Fahrenheit.

Every heater has a minimum required flow rate.

So, you should never go below the minimum required flow rate.

In any case, the total heat added to the pool is the same either way.

By slowing down the flow, all you are doing is concentrating the heat in less water.

However, the heat goes into the main body of water and gets distributed based on the total pool volume.
 
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DavidArmenB

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Sep 7, 2022
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I do not believe it will heat faster because you will be putting the same amount of heat energy in the pool. It does not matter how hot the water entering the pool is because that water will disperse in the pool anyway. The important thing to think about is total energy.
 

JoyfulNoise

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May 23, 2015
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Also, most modern pool heaters nowadays utilize a high-volume internal bypass configuration in the header manifold where a large fraction of the water simply goes from inlet to outlet and a smaller fraction gets heated. That way hot water is mixed with cold water to limit the output temperature and safely deliver water to the plumbing that is not scalding hot. The amount of mixing is regulated by the heaters thermal governor which opens and closes in response to temperature.

So, long story short, you might sense a slight difference in outlet temperature but it will not heat your water any faster nor save you on any gas use. That is strictly a function of the size of the heater in BTUs/hr.
 

Toxophilite

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Feb 23, 2022
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As long as your heater core is completely covered, full, of water, it's heating "pool water". The cooler that water is in the core, the faster heat will transfer to it. Slowing flow down reduces efficiency of heat transfer.
 
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