SylvanLMP
Rather the take wild guesses lets think like a professional.
some stumble bums throw in expansion tanks Other low life’s just increase the T&P pressure rating
How about We refrain from guessing and you go out and place a pressure gauge on your incoming cold and hot water discharge piping and check the delta.
A temperature /pressure relief valve can be defective Or non ASME/AGA/NFPA rated
Once you know your pressure and temperature (compound gauge) then you can make a qualified decision on to what avenues are open.
You could very well have some type of back flow preventer isolating this tank thus causing excessive pressure
