I believe it is one of two things...

1) The thermostat is not working properlty (causing overheating conditions) or

2) Stacking Temperature is probably happening. The "stacking-temperature" conditon takes place when the home users frequently/often use small damands of hot water. This keeps the thermostat cool enough to run the burners longer than needed.

The result is that the cold water entering the tank cannot absorb the diret heat output (down by the burner). The result is that the water surrounding the heated surfaces picks up more heat all the way to the top-- stacking the temperature while creating a fair amount of pressure along the way.

One other question for your... since there is a possibility of frequent short use of hot water, is the home inhabited by a large family? Is there "any" possibility that the toilets could be cross plumbed to provide hot water into the water closet tank? If so, that could be a source of 'frequent' quick use of hot water use.

Richard

__________________
"If con is the opposite of pro, is Congress the opposite of progress?"
Richard Rushing, HCRI
Duncanville, Tx.