GPM flow rate too low on 1 of 2 return pipes

by Troy

I work with a 58,000 gallon indoor pool that uses two Pentair pumps and two large Sand Filters. The required GPM for the pool is 161. I have a consistent issue with the flow rates for both GPM gauges. One used to be 140+ GPM but now I can barely get it above 110 GPM. The other return pipe has a flow rate of about 60-80 GPM.

They are now both problematic but the primary one I have a concern with is the one that is 60-80 GPM. Repeated Backwash processes are slow to bring it to the required 60 GPM mark as the other pipe is usually good for 100 GPM though it was better before. The low GPM rate is gauging water coming from the main pool heater and there is a big U shaped piping that drops down from the heater outtake and comes back up to the level of the pipe where the gauge is located.

Maybe I am describing this in too much depth but my local supplier/ repair vendor has been unable to resolve this one issue for over 5 years. I would think that the GPM would be the same in both return pipes but this is not the case.

Is this normal? Cleaning out the gauge has not worked. Replacing the gauge may work for a bit but then the issue crops up again -usually within a couple of days. Any ideas on how to tackle this issue? Thank you

Hi Troy. It sounds like you've done everything you can, but I'll throw in my 2 cents. Low pressure on the return side can come from a few things. I'll list them here:

Clean out skimmer and pump basket

Back wash for the correct amount of time or when the water turns clear, then

Valves that are partially closed

Clogged multi-part valves

Clogged impeller

Improperly working motor

Not doing a rinse after backwashing

Clogged GPM meter. Sand gets stuck in the small hole and can give a false low pressure.

Lid o-ring cracked or missing

Impacted laterals

Greasy water. If this is a public pool I gaurentee people are swimming all greased up. This grease gets trapped in the filter and turns the sand into sandy lard. Water cannot properly pass pass through the sand. Showering before swimming must be required.

Not having enough sand in the filter. Check the measurement from the top of the sand to the top of the filter. Pic is enclosed. Not having enough sand can cause decreased filtration.

Low pressure normally can be traced back to 2 areas. A restriction in the filter or lines, or the motor not working correctly.

I'm not sure the kind of GPM meter you have, but the kind I used is a flow meter in the pic. The male end goes into the pipe that measures the flow but sand gets trapped in the small hole, decreasing the flow of water to the meter and giving a false low GPM.

To clear it, turn the motor off and twist the red cap off. Place a towel over the top but allow for a small pocket, like a balloon. Get a tight grip around the towel. You'll need to compensate for the water pressure. Have someone turn the pump motor on for a second or two, then off. This will blow any debris out of the hole. Carefully remove the towel. The little metal piece will be in it. Find the metal piece, put back into the cylinder and screw the red cap back on.

Works every time.

Good luck and have a great Summer.


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