System is a AO Smith power vented 50 gallon gas water heater about 8 years old. 2 gallon Amtrol ST-5 expansion tank (charged to 65 psi) on cold water supply side was installed in December 2022 to replace 8 year old 1.25 (?) gallon tank. House pressure is currently set to 40 psi. Temp set to 125F.
Last year, I replaced the hot water heater's TPR valve after it started dripping/partially opening. I drained and flushed the tank while I was at it. After a few days, I remembered to check it again, and I noticed it was still doing that. After some internet "research", the symptoms pointed me to the expansion tank, which was probably installed at the same time as the heater. I put in one a size bigger due to wanting the temp higher for therapy reasons. I installed the new one (it was precharged to 50 psi). Again, I checked it after a few days, and the drip had resumed.
This drip gradually increased to a steady flow of water from the TPR valve over the course of a month. I increased the precharge on a new tank from 50 psi to 65 psi. It returned to a drip. I replaced the TPR valve again, and the tank stopped dripping for a week.
So I started paying closer attention to both the peak and current water pressure as measured by a dial gauge on the tank valve on the bottom of the water heater. Although I have the house pressure set to 40 psi, I was recording spikes every day over 130 psi. The spikes are present on both cold and hot water side. I noticed with further inspection that the spikes are coinciding with the last few minutes of the heating cycle, and the TPR valve discharges are occurring then as well.
Is this pointing to a bad PRV for the house, a bad water heater (maybe scaling inside?), or something else?
EDIT for clarity: this is for the potable water for washing and such and not in a closed loop heating system. The supply from the street has a check valve installed by the city to prevent backflow.