Thanks. I agree that the thermostat would cause this problem, but in my case the thermostat was okay. I do not leave the hot water heater turned on all the time, but only close the breaker when I need hot water. I then look at my AC ammeter and can see when the heater kicks on, and about 10 minutes or so later it turns off. When the current drops to zero is my indication that the heater tank has reached the proper temperature and I can take a shower. Thus, in my case the thermostat was functioning.

Thanks!

HI Steve. Your thermostat could be bad and that would cause the heating element to not cut off. That would cause the element to continue to heat the water until the pressure gets so high that it forces water out the relief valve.