We had a problem with our water heater over the last few days. Seemingly overnight, the temperature jumped from around 115 degrees up to 135, nearly burning my wife one morning in the shower. Needless to say, we haven't touched the thermostat. When I checked it this morning, it was still at it's lowest setting, but I was able to turn it just slightly below "low." We'll see if that does anything, it will take awhile for the tank to cool down (if it does). While it's doing that, does anyone know what would cause a hot water heater to do this? It's only 6 years old, and we're on very clean city water. I must admit, I've never drained it, but I don't know if that could be causing this problem or not. Ideas?
If you have one it's probably mounted in close proximity to the water heater and likely looks like these:
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Its function is to "extend" the amount of "hot enough" water you can get out of a given size water heater by letting you set the thermostat on the heater to a temperature higher than you really need.
The tempering valve has it's own mechanical thermostat inside which mixes cold water with that overly hot water to control its output water temperature to the maximum you want, adjusted by the knob on it.
When the mechanical thermostat bulb inside one of those tempering valves konks out, they don't mix any cold water in, so you get your hot water at whatever temperature the heater is set for.
Capice?
If you don't have one of those, then we'll let some of the other guys here take a shot at what might have happened.
The thermostat may have stuck in the on position. If that's the case then turn the power off, run the thermostat up and down a few times to clean any crud,then set to 120 deg., and turn the power back on. This may buy you a few more weeks while you find a new thermostat. It will eventually stick again.
Years ago, had an electric water heater where one of the thermostats (believe it was the upper) stuck in the on position until the overheat button on the thermostat popped.
For a while, we had very hot water, then no hot water. Replacing the thermostat fixed the problem.
Might want to check your thermostats, see if they're working correctly.
I have always been a little confused as to how they're supposed to work, somebody correct me if I'm wrong. I believe the upper kicks in when you have high demand, when the upper catches up, it switches to the lower.
So if the upper thermostat gets stuck on, the upper element is just going to heat until the bimetal spring in the thermostat pops, or the temp/pressure relief valve vents.
The "hot water" exits from the top of the heater tank and the cold water is inputted near the bottom of the tank by the "dip tube" pipe which extends down from the cold water inlet.
The upper thermostat will turn on the upper heating element when the water near the top of the tank starts getting "too cool". That action also removes power from the lower thermostat/heating element.
With no water flow, as you said, when the upper thermostat temperature is satisfied, it removes power from the upper heating element and sends it down to the lower thermostat and heating element, which will power up the lower element as needed to maintain the temperature of the water near the bottom of the tank.
Since warm water is less dense than cold, the water being heated by the lower element will rise to the top of the tank, where it keeps the upper thermostat satisfied when there's no water flowing.
The upper thermostat is typically set to close and connect power to the upper element at a slightly lower temperature than the lower thermostat is set to.
So, with no water flow, the lower element does all the heating and it's thermostat does all the controlling.
When water flows for a significant time, the water temperature at the upper thermostat lowers enough to make it "take over" and switch power to the upper heating element in an effort to give you hot water for as long as possible, but as we all have discovered, it typically won't keep up with a continuous strong water flow forever and eventually we'll "run out of hot water."
Years ago some electric hot water heaters came equipped with neon pilot lamps connected across the upper and lower heating elements so you could observe what was happening.
Anal soul that I am, I installed a pilot lamp across the upper element of our electric water heater just for s**ts and grins. I've rarely observed it on when I've been in the vicinity of the heater, sove for when our daughter was still living with us and felt compelled to baste herself in a hot shower until the water got cold enough to make her want to quit.
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