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