A new T&P cost under $10 to take it for granted it is thermo expansion could cost you your life.
why not invest in a New ASME/AGA approved T&P full size 3/4 and also check the near system piping for a proper heat sink (27″ ) and also keep in mind these valves are to open with 150 PSI or 210 degree’s. Dont just put in an expansion tank and think that is the cure all. For the $10 part you will atleast have peace of mind.
Never assume it isnt a part going defective. Most “plumbers” install a T&P with the heater and NEVER look back to check this device. Good luck
[Edited by SylvanLMP on 11 June 2000]