Hot Water Suddenly too Hot
One of the filaments (elements) is not shutting off and the water is overheated, in this case to the boiling point.
You cannot leave the heater running in this condition without the danger of building up so much (steam) pressure as to cause an explosion. Summarily replacing both thermostats and also the pressure relief valve would be a good move on your part. Before that, switch off the power when no one is using hot water. Turn it on only when needed, an hour in advance while trying a hot water faucet every now and then to check for unusual pressure in which case turn it off again.
Someone with electrical know-how can test the old thermostat by boiling it in a pot of water.
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Last edited by AllanJ; 05-06-2009 at 01:41 PM.