When I bought my home it had a 50 gallon gas water heater in the garage without an expansion tank. On the hot water line there is a Watts adjustable pressure relief valve (this NOT the PRV on the tank - it's an additional valve). The water pressure as measured on the hose bibs was frequently around 150 PSI and the Watts PRV or the PRV on the tank would frequently release water. There is a pressure regulator valve on the cold water supply line to the house and the pressure when measured without the water heater heating is 55-60 PSI.
The water heater survived 20 years under those conditions but it finally started leaking. I replaced the water heater with another 50 gallon unit and added a 2 gallon expansion tank vertically on the cold water line feeding the tank. I expected this to solve the high pressure problem but it hasn't. Even with the water heater turned down to around 115F, when it heats, it brings the pressure over 150 PSI and the PRV on the tank opens and stays open until I run the water somewhere to release the pressure. ~30 minutes later, the pressure is back up and the cycle continues.
I'm baffled on what the problem is. The expansion tank is the recommend size for the size of my water heater and line pressure. The only thoughts I have are to get a bigger expansion tank or potentially replace the pressure regulator valve on the cold water supply line. The latter is a bit of a hail mary IMO and will be a major project due to the location.
Anyone have thoughts on what else could be the problem or potential remedies?