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Two controls within, upper and lower
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Posted by CrNiFe [Email] (#408) [Profile/Gallery] (more from CrNiFe) on Sat, 19 Apr 2008 20:29:02 Share Post by Email
In Reply to: Gas Water Heater Question, ursaw, Fri, 18 Apr 2008 14:36:36
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In a typical water heater there are two sensors, an upper temperature limit control and a lower temperature limit control, which are read by the thermostat. They feed back the tank water temp, and the logic controller determines when to fire the burners.

If the upper temperature control, located in the upper section of the tank, malfunctions, then the tank firing control will not let the burner run long enough to heat the water up, at least to the point that the thermostat says, whoa, it is hot enough.

If the lower temperature control, located in the lower section of the tank, malfunctions, then you can bring cold water in to the bottom of the tank, and the firing control will not detect a change, and send the message to the thermostat to fire up the burner.

Since you describe a situation where the water eventually becomes piping hot, it would have to be the upper one that has failed, because the lower one is working - it calls for heat once the cold water feed lowers it temperature.

Sorry, but you need a new tank, since it is not worth it to try to change on of the two control sensors.

By the way, I do investigations of hot water tank failures as a forensic engineer. You would not believe (maybe you would) the mess you can get when an older tank perforates, or if the temperature and pressure relief valve gets plugged up. But, they DO NOT EXPLODE, because the TPR valve is redundant. The thermostat and control sensors logic has too fail first, and the the TPR has to not work. You could win a lottery and get struck by lightning on the same day with those odds.

The little tag on the TPR valves says Check me once a year! Nobody does.

The anode stick to prevent corrosion of the tank wall should be inspected every two years, if you have a water softening system. Nobody does.

Keeps me in business.

CrNiFe
Toronto


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