Didn't expect the low speed schedule to be the noisy part

Started by Marcus Reed · · 4 replies
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#1
Figured dropping my IntelliFlo to 1000 RPM for the overnight turnover would be dead silent, that's the whole pitch of a VS pump. Instead I get a rhythmic clunk every few seconds right at the check valve on the return side, only shows up under about 40 gpm. Turns out at that flow the valve flapper doesn't have enough pressure differential to hold fully open, so it's fluttering instead of sitting still. Bumped the schedule up to 1500 RPM for that block and it's gone, costs me maybe a few extra kWh a month, not worth chasing further. Wouldn't have guessed a pump running quieter mechanically could introduce a noise problem somewhere else on the pad. Anyone else had a valve start chattering once they went low RPM.

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#2
No VS pump here so I can't speak to the RPM tuning side, but before anything else, is that a spring loaded check valve or just a swing/flapper style. That matters a lot for why it'd flutter instead of just sitting open lazy at low flow, the spring ones usually need a certain minimum pressure to compress and hold, and if yours is a cheap flapper it might just not have enough weight or a stiff enough hinge to sit still under 40 gpm.

If it's the spring style, sometimes you can find one with a lighter spring rate made for lower flow, though I'd bet money the pool store version is marked up stupid for what's basically a piece of plastic and a spring. Might be worth checking if a generic version exists before paying whatever they want for the branded one. For what it's worth, bumping the RPM like you did is probably the actually cheaper fix long term versus hunting down and swapping a valve for a problem that only shows up in one narrow window anyway.
#3
Jason's right about the spring versus flapper distinction mattering, but the one thing I'd add is check where that valve sits relative to the pump on the pad, because mine did something similar years back and it turned out to be as much about the length of straight pipe before the valve as the flow rate itself. Too short a run into it and you get turbulence that makes a marginal valve chatter even at flow that should be fine on paper. I never did chase mine down to a part number, just run my Raypak heater loop at a slightly higher RPM overnight same as you did and called it a wash against the electric. Twenty five seasons and the binder has exactly one entry for a check valve, and it was a full swing type that finally cracked with age, nothing to do with flow. Sounds like you landed on the right answer already, some things aren't worth the parts hunt for a few extra kWh a month.
#4
Marcus, before I'd chase this any further I'd want to know what brand and style that check valve actually is, spring assist or just a gravity flapper on a swing gate. Big difference in how much differential it needs to sit open quiet, and the cheap gravity flapper ones are notorious for fluttering right at that low end no matter what you do upstream. Tom's point about straight pipe run into the valve is worth checking too, but I'd nail down the valve type first before worrying about pipe layout.
#5
Yeah when I redid my plumbing after the liner swap I had a check valve chatter on me right after the pump elbow, turned out dave and Tom are both onto it, mine was a cheap gravity flapper with barely two feet of straight pipe before it, swapped in a spring style and it shut right up even at low flow. Didn't need the branded one either, generic spring check off the shelf did the job for way less than what the pool store wanted.