water heater problems

installed a 40 gal. hwh soon after tp valve started to leak intermittently. checked to make sure no trash in valve. problem persisted,changed tp valve same problem. installed an expansion tank. city water pressure is static at 70psi,working at 40 psi set tank pressure at70 psi. line pressure still rises to 150 psi and leaks out of tp valve. need help in solving this problem any suggestions helpful

Until you know WHY the pressure is going that high, and nothing you have told us indicates what the problem could be, there is no way to diagnose it. BUT, one possibility is that you already have a pressure reducing valve and it has gone bad. That would cause your symptoms even with an expansion tank, and a new T&P valve.

do not have pressure reducing valve. do I need one for 70 psi static pressure to house? before I changed hwh I replaced 1" galv. water pipe with 1/2" cpvc pipe. galv. pipe was almost closed up with build up. could the change in pipe size cause thew problem

there is no pressure reducing valve. city water supply is at 70 psi.