Hello. So I found the expansion tank has failed so I replaced it yesterday. I charged it to 2 PSI above the main water pressure as stated in the manual. It seemed fine but when I checked this morning, about 2.5 gallons came out of the T&P valve overnight and the gauge I left on a cold tap in the basement read 140 PSI.
I opened a cold tap and bled off pressure. The gauge went down to 45 PSI. I checked the air pressure in the expansion tank which showed 48 PSI.
Is it possibly the PRV? The expansion tank appears to be set up as recommended. Thank you for any replies.