My house is in the hills; the water coming in from the city is 170psi. I have a Wilkins 600XL regulator that was installed about 10 years ago. Over the last year or so we have been getting high-pressure surges when a faucet is opened in the house. This lasts a few seconds and then calms down. I checked the pressure in a hose bib that comes off the regulator (about 20 feet away) with a Watts Test Gauge (with the twin needles) and it showed 120psi when placed on the bib (after releasing some water first). The red needle, though, capped out overnight at 160psi.

I bought the repair kit for the 600XL and installed all the new parts (all the old parts were very deteriorated). The pressure then read as 60psi but I was still getting faucet surges. The overnight reading was back at 160psi. I cranked down the regulator and now the “regular” reading is 40psi (and the pressure coming from the faucets seems pretty low), but we are still getting faucet surges and the overnight was again in the 160psi range.

I just purchased a Watts PLT-5 thermal expansion tank (there was not one on the system) but have not installed it yet because the PLT-5 is rated for 150psi max--and is supposed to be set to match my regulator’s 60psi output). I am concerned about the tank catastrophically failing if the overnight pressure hits 160+psi (as it has been doing).

So I thought I'd verify that the water heater is actually causing the pressure build-up when the water heats. I turned off the water heater last night (and ran the hot water in the house out until is was lukewarm). The gauge says the overnight pressure was 160in again, while the regular reading is 40psi.

There is a hose bib (for the garden) near the regulator that is not regulated, and that one reads consistently at 170psi.

Any thoughts on what might be causing these issues? Is there a possibility that the unregulated pipes for irrigation might be somehow mixing into the regulated pipes? What else can I do to get the overnight pressure down? Thank you for any help!


Thermal expansion in a water heater on a feed that has a check valve can indeed significantly raise the water pressure, I had a house that my son was living in that the water company just installed meters with check valves. He complained the TPR on the water heater was leaking. I told him to put a pressure gauge on the drain, open it up and let me know. Sure enough it was over 100PSI. Whats bothering me is why the TPR on your WH isn't popping off? You have one, right? If so, it might not be working right. I'd still install the expansion tank. There is a maximum "working limit" and usually a much larger maximum limit. It's not going to blow at just 10 psi over it's working limit. Unless your PRV is leaking a bit of water thru it, I can't see a reason for the overnight high PSI. I could be wrong and others with more knowledge might have better ideas, but these are mine.

  • Thanks, George! I inadvertently left out two things, one which you pointed out. Yes, there is a TPR valve on the water heater and it has not been discharging. The other is that the plumbers who installed the regulator added a PRV coming off the output of the regulator (with a branch pipe, about 6 inches off the regulator). I don’t know why they did that but maybe the 170psi coming in prompted this? It was discharging a lot so I replaced it when I repaired the regulator. The new one is discharging too.
    – yotimbo
    Jul 13 '20 at 2:41
  • How much is it discharging and for how long? Constant? or a for a while and then stops. If constant, I think your PRV is leaking, if for a while and stops, probably water heater thermal expansion. Still troubling your TPR valve isn't discharging on the WH. Jul 13 '20 at 14:10

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