Old house (new to me), one floor, city water. Mostly copper pipes, some pex in remodeled areas, gas hot water. House is 62 years old, plumbing might not be as city water was added after build (I do not know when).
After moving in I noticed lack of an expansion tank, checked pressure and after hot water use with faucets closed got the expected rise in pressure (a bit over 100psi) with immediate relief if you run just a bit of water. All that seemed normal, had plumber install expansion tank, and at the same time plumb for my home assistant pressure monitor. Calibrated it to regular gauge and cross checked with plumbers gauge. At the same time we raised the house feed's regulator from 50 to 60psi. So please assume this isn't measurement error.
Now I get really high pressure at times that does not fall with minor use (not like expansion tank issue) but falls low with heavier use, like watering a lawn or a long shower. It then rises slowly. Below is a plot of pressure for a few days.
Yesterday the pressure dropped while watering the lawn to approximately 50psi, then when watering stopped quickly rose to my set point of 60 (so that seemed normal) then over night kept rising to about 95. In the night I used water (but no sign of it on the chart) and this morning I flushed 5 times (to have a quantitative idea of use, so about 6 gallons) and it fell from 95 to 92 and is now rising again (4 of these flushes are off the shown chart, the first had no affect).
Expansion tank was set to 60psi by the way.
I don't know what to make of this. I know I needed the expansion tank, that's fixed. These rises do not seem associated with water heating at all, just time. The quick rise near 60 after heavy use seems to say the regulator was set properly.
Is there such a concept as a regulator "leaking" so that it slowly allows the pressure to rise too high? The fall to 50 with heavy use I guess could be all the old pipes can support, though it's suspiciously near the default/prior regulator setting. And it all seems to level off around 100psi (street pressure?). But the quick rise when usage stops to near 60 looks consistent with the regulator setting (though also the expansion tank setting).
Could I have the regulator set it too high? (But wouldn't the rise be quicker then? Like immediate after turning off all faucets?)
I don't want to just start turning the screw trying to chase this due to the slow nature of the rise (and lack of fall with minor use -- e.g. when we set it, we left a small flow going on one faucet).
Basically hoping for a theory of what could cause this to help chase it down?
Linwood
Update: I finally got around to replacing the PRV (well, the guts inside). I'm posting two new screen shots, one is the guts, and one is a graph showing the transition (ignore the two near zero it was while doing the replacement, the first attempt I got a bad O-ring in there, and had to do it again.
enter image description here
