Name:   Osms - Subject:   Plumbing/Water Heater Question Date:   4/22/2014 6:36:15 PM

One other possibility is the gravity pressure build up from the water supplier's pressure check valve to your house.  When we built, as soon as we hooked up the water heater the pop off valve started leaking, which led us to pressure check the water supply at the house.  Pressure was set at 60 psi at the meter but by the time the water got the 80ft vertically down to the house the pressure exceeded 120 psi.  I had to put another pressure control valve down at the house level to solve the problem.  No problems since.  You might want to  pressure check at the house.