Pump Head / GPM and plumbing curve questions

[Dean287]

hi guys, long time lurker, first time posting. Few questions on pump head and flow. i believe my pool is a bit over powered and want to swap impellers to slow the water flow, reduce the pressure/head overall and quiet the pump. when the motor gives up ill be looking into a VS but for now im tinkering with what i have.

thank you for sharing the pool pump spreadsheet! really great asset!

I have taken pressure readings right at the pump housing on the drain ports. Are these numbers enough to get an accurate GPM with out having to consider the specifics of the inline equipment and pipe runs/turns etc? i basically zeroed out the inputs on the psivac2gpm tab for pipe length/multiport etc.

Pool mode:
11 inhg- pump suction side
28 psi- pump pressure side
20 psi- filter gauge
77ft head
calculated plumbing curve 0.01067


spa mode:
14inhg- pump suction side
31psi- pump pressure side
25.5 psi- filter gauge
87ft head
calculated plumbing curve 0.04080

if i swap the impellers to a smaller hp unit from the sp3215ee or even sp3210ee, will the total plumbing curve remain the same?

is there anyway to calculate the % decrease in pressure/head/flow with these impellers?
not sure how much pump i really need with the spa/heater combo.

suction side is three separate intakes at 2inch each merging to one 2inch pipe in front of the pump. two skimmers and one floor drain
return side is three separate pipes at 2inch each, one feeds 5, 3/4inch eyeballs into the pool the other feeds 6, 3/4 inch eyeballs into the spa, last one feeds water fountain coming out of the spa but valve is nearly closed 99% of the time.

 

[cfherrman]

The head loss will change with a lower gpm. I think the best you could do is calculate your head loss at the lower gpm rate.

 

[mas985]

Welcome to the forum!

if i swap the impellers to a smaller hp unit from the sp3215ee or even sp3210ee, will the total plumbing curve remain the same?

Yes, plumbing curves will usually remain the same with flow rates. The only exception is when you have check valves in the system but the change is usually small.

is there anyway to calculate the % decrease in pressure/head/flow with these impellers?

Yes, you just need to determine the intersection of the new impeller head curve with the old plumbing curve. This will give you flow rates. You can then determine changes in pressure/suction with the change in flow rates using the pump affinity equations. The change in pressure ratio is proportional with the square of the ratio in flow rates. However, you need to take into account the elevation of the pressure measurement as this does not change with flow rate (i.e. static head). But if you are using the spreadsheet, it can take this into account. Just use the second column and the new equivalent pump. Everything else is taken care of, assuming you entered the correct elevation.

not sure how much pump i really need with the spa/heater combo.

If you have a spa, and this pump runs the spa jets, you should not change the impeller size or the spa jets will become much weaker. A better solution is to replace the current motor with a two speed motor so you can run the pump on low speed when using it for the pool and high speed for the spa.

the other feeds 6, 3/4 inch eyeballs into the spa

Are these just eyeballs or are they feeding spa jets?

 

[Dean287]

awesome thank you guys for the input.

a variable or 2 speed motor is coming but hoping for another year or 2 from the current one.

i did not realize there are actual spa jets. I have just eyeballs, not jets in the spa. visually they are identical to the pool returns. When i unscrew and remove the eyeball, the pipe is wide open just like the pool. There is an air blower that creates bubbles/extra force that is tee'd in after the pad

so i guess i have a hot tub, not spa right?
does this change your thoughts on needed flow when in spa/hot tub mode?

 

[mas985]

i did not realize there are actual spa jets. I have just eyeballs, not jets in the spa. visually they are identical to the pool returns. When i unscrew and remove the eyeball, the pipe is wide open just like the pool. There is an air blower that creates bubbles/extra force that is tee'd in after the pad

so i guess i have a hot tub, not spa right?

Actually, those are probably spa jets. The jets cannot be seen from the outside. The eyeball sits on the outside of the spa wall that you can see but the jets are deep within the spa wall. But the jet power is heavily dependent on the pump so the last thing you want to do is downsize the pump.

Spa jets look something like this: