High Water Pressure After Rebuilding the Regulator
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 &quot;working limit&quot; 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.