Pump speed to Maximize Heating Questiion

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Pump speed to Maximize Heating Questiion

Postby bwilkins » Mon 29 Dec, 2008 19:08

I have a 4 speed Intelliflow Pump and a Pentail Model 400 propane heater.
What is the most efficient speed to run the pump at when heating the pool?

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Postby chem geek » Mon 29 Dec, 2008 20:37

The answer depends on whether you want to maximize the rate of heating or minimize your electricity usage. Faster flow rates are more efficient for heat transfer, but are more expensive in electricity due to the higher pump speed. There is a lower limit to the flow rate allowed for your heater. I believe the installation and operation manual is here. Under "Water Flow Rate" for the Model 400 (which has 400,000 BTU/hr input), it says the minimum flow rate is 40 GPM and the maximum is 120 GPM.

A BTU raises one pound of water by 1F in temperature. One gallon of water weighs about 8.33 pounds so at 40 GPM the temperature rise, assuming 100% efficiency, is 400,000/40*60/8.33 = 20F. A flow rate of 80 GPM would have a 10F rise in temperature. The lower temperature loses heat more slowly which is why higher flow rates are somewhat more efficient.

The electricity usage of your pump vs. flow rate will depend on your specific plumbing situation since larger diameter pipe and shorter runs have less water resistance (lower dynamic head) so use less electricity to pump at the same flow rates. You'll probably find that being closer to 40 GPM will be much more energy efficient for the pump even though it is somewhat less efficient for heating.


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