Electric water heater boils?

Question

Our Kenmore electric water heater is about 5 yrs old. First it just stopped heating. I tried to play with the upper thermostat and found out that if you decrease the temperature setting to the minimum, it starts working (can hear the hissing sound inside). However after half an hour it started spewing very hot water out of the pressure valve. I figured something's wrong with the top thermostat and replaced it with the new one. Same thing. Looks like it never turns off the top heating element. What else can I do to find the problem? Thanks.

Update: Checked both thermostats again, seem to be working properly, first upper element heats, then lower one. Reduced the setting on both thermostats to the minimum and now everything seems to be ok, except the water is not as hot as I'd like it to be. I wonder if I should replace the lower thermostat as well, just to be on the safe side. Can it be that the lower thermostat never turns off on medium temperature setting and causes overheating?

Answer 1

A new thermostat won't help when the temperature sensor itself is dead or corroded.
What most likely happened is that you turned the thermostat so low that whatever signal was coming from the broken sensor triggered the thermostat to turn on. But since that signal never changed, the thermostat never "knew" that the setpoint had been reached, so the water continued to heat well past the safety limit.
This is bad for a couple reasons: there should be a high-temp safety cutoff somewhere (I can't vouch for your particular unit) which, if it exists, is also failing.