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:
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.
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
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
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
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
You gottit right, Jerry.
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 shits 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. <G>
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