PRV troubleshooting...

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by Runs with bison, Sep 2, 2009.

  1. Runs with bison

    Runs with bison Member

    Aug 23, 2009
    Okay, I've got what I believe is a problem with the old Watts PRV in my home. It is still functioning and not leaking through to full supply pressure, but it can't seem to maintain dynamic pressure within reasonable range of the static pressure. As soon as I open even the slightest flow the pressure drops off about 10 psi...and by slightest I mean 1/4 gpm...bucket tested. At 1-2 gpm it loses about 17 psi.

    Typical data set is: 1.2 gpm flow (bucket test)
    Municipal supply ~118 psig
    Static pressure (no flow) 75 psig.
    Pressure with flow 58 psig.

    PRV is a Watts N35B, 3/4" with female threads, about 15 years old. Spec curve can be found at The curve suggests that at 1.2 gpm the pressure drop should only be about 1-2 psi.

    The reason it is a problem is I seem to be getting low supply pressure to an upstairs shower head that is already giving up nearly 8 psi in vertical rise. Other fixtures on that level can handle it, but perhaps not the shower head which is underperforming. I raised the static setting to 82 psig at the PRV, measured 66 psi dynamic at 1.25 gpm flow (bucket test.) I've tweaked it a little past this now and will retest with bucket in the morning, but this is at the adjustment limit and outside of specs anyway. It's a low flow showerhead, but 1.25 gpm is at least .25 gpm lower than expected and appears to be detrimental to performance. The old 2.5 gpm wasn't starved for water, so the 1.5 gpm should have even less problems. Could be a defective showerhead, but the PRV performance has my attention at the moment.

    Am I wrong to think that it should be possible to get very little dynamic loss at such low flows if the regulator was functioning correctly? It's strange that I've had no flow issues in the home with multiple things running despite the PRV dropping the pressure so much below set point. I did note that before I started adjusting originally (shortly before the showerhead was replaced) the PRV was at the limit of adjustment, upper 80's psi. I first adjusted it down to 75 static and everything seemed happy enough, although I could tell pressure was lower. Then I installed the new showerhead.

    There is no appreciable lime buildup in the lines I've opened so far, (and I've replaced several plumbing fixtures in the home.) There are no leakers. Unfortunately, my understanding is that the N35B is obsolete and a bear to service in place, plus it might not be repairable. So I'm considering replacing it with the Watts 25AUB in 3/4". Am I missing something obvious?
  2. jimbo

    jimbo Plumber

    Aug 31, 2004
    San Diego, CA
    Pressure drop under load is the classic failure mode of a pressure regulating valve. It is gummed up internally and cannot do its job. Need to replace it.
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  4. Cass

    Cass Plumber

    Nov 12, 2005
    It is in need of replacement....they do make repair kits but don't bother with one just replace it...I may be wrong but I thought 70 PSI was the max. they were adjustable to.
    Last edited: Sep 3, 2009
  5. Runs with bison

    Runs with bison Member

    Aug 23, 2009
    Thanks guys, that's what I figured but wanted a sanity check from experienced hands. As far as replacements go, how is the 25AUB for field service/cleaning? I liked that it appears to have a separate access port for the supply side strainer. Any idea what one of these costs wholesale in the 3/4" size?

    Cass, yes the spec range is 25-75 psi rated, but it appears that this one can do a little more than that on the top end. This could be due to its age, but it wouldn't surprise me if they had some excess range on either end when new--afterall, you don't want to come up short due to small manufacturing differences. Then again, my new Watts pressure test gauge could be off and reading high, I've not checked it against any calibrated gauges. I need to prove this instrument out anyway...
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