On Sat, Jun 21, 2008 at 12:46 PM, David Scheidt <dmscheidt@gmail.com> wrote:
> We've suddenly got really hot water (170F!) coming out of faucets.
> The water heater is a pretty standard double element electric job. I
> opened the covers, and the settings were where I'd expect them to be
> (right in the middle.) The upper element is hot to the touch. I've
> cranked its setting down, and I'll see if that makes a difference once
> we've used up some hot water. Do the elements ever stick on, or is it
> likely to be that the thermostat has died? (and is there a way to
> test it?)
For the benefit of the archives:
What happened in my case was that the lower element's thermostat
welded its contacts together, leaving the element on constantly, until
the thermal safety switch tripped.
Here's some discussion of the theory of these things, and some
troubleshooting. (I didn't find much in a brief web search.)
Most dual element electric water heaters are set up for
non-simultaneous operation. In other words, only one element can be
on at a time. The theory is that when you start from dead cold, the
top element switches on and heats the top of the tank, which provides
usable hot water sooner. Once the top is heated, the bottom element
comes on, and does most of the work. (I assume there's some sort of
baffle to encourage stratification of the water, but I'm not cutting
my tank up to find out.) So, in this system there are two
thermostats, one for each element. The thermostats read the
temperature through the side of the tank, so they need to be firmly
pressed against it.
The top thermostat is, in fact, two thermostats, hooked in series.
The first of these has the power inputs from the house wiring, and
thermal overload safety. It's reset with a button on the ones I've
got. One of the outputs is permanently jumper to the thermostat it's
in series with. The other output goes to one side of both the upper
and lower elements. The themostat in series has two outputs; they're
the inverse of each other -- when one is on, the other is off. One
goes to the other side of the top element; the other is the input to
the bottom element's thermostat.
The lower element's thermostat is much simpler. It's got one input,
from the output of the upper element's thermostat, and one output, to
one side of the heating element.
The wiring diagram included with the replacement thermostats makes
this much clearer than my words do. It also makes clear the obvious
trouble shooting test. If an element is powered, it's got 240 volts
across it. Putting a voltmeter on the two contacts of the element,
and then turning the knob on its thermostat should cause it to cycle
on and off. (I can actually hear mine click, too.) (The bottom won't
be powered if the top is on, remember.)
--
David Scheidt
dmscheidt@gmail.com
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