It is most likely the lower heating element is shot. The lower thermostat could be malfunctioning or there is a poor connection to the lower heating element. But in all likelyhood, the lower element has gone out.

A bad element would not affect the high temperature shutoff. Flushing out sediment is not a bad idea. But you have a stuck thermostat.

Flushing is not a bad idea for sure. A bad lower element will indeed cause the upper high temp shutoff to trip.

maryland_irisman : Explain how a bad element can cause overheating. In my experience, a bad element does not heat at all!

Thank you both for your replies. I have some more info to add.

This is in a rental. The renter just said that after hitting the reset, the heater heats, but it heats the water up to scalding hot, or near scalding. That would mean then that the cut off switch is doing its job, but that a thermostat is bad...right?

tia again.

Sue

I think you will find it's the element. The way it works is that on a 240v element and the thermostat is only switching one pole of the 240v. The element burns out, usually because it's buried in muck at the bottom of the tank and it burns in two, shorting out to ground. The result is that the section of the element heats until the reset (which cuts power to both sides of the 240 circuit) shuts it off. I've seen it often.

BTW: The Reset is a safety device, don't keep resetting it and hoping it works.

I hope the original poster will let us know when the problem is resolved. I disagree that the element is the cause. If the element grounds through the water while the thermostat is off, the element receives only 120 volts and thus heats, if at all, at only 1/4 or less of the original wattage. But do let us know.

BF has just left to go and get it fixed, whatever it is.

Yes, I will be back later once he has for sure gotten the problem taken care of.

Again, thank you all for your input.

Sue

I've seen it happen quite a few times where the element shorts out and over-heats the tank. It's not a theory, I've seen it and verified it with my meter. The wattage is cut in half because it's 120v but it stays on continuously, until the overheat stat shuts it off.

Redo the math. A given resistance will produce only 1/4 the wattage at 120 volts as at 240 volts.

Point taken on the math, but it doesn't change the fact that I have seen this type of failure repeatedly.

It's always the bottom element because it's encased in sediment. The thermostat will be open but the current will be flowing through the element on one leg.

You will pull the element out and it will often times be burnt in two.

And, to mess with your math just a bit, since the element is shorting somewhere usually in the middle, the resistance would be cut, nearly in half which would result in half the wattage.

The point is, it's a really common mode of failure for electric heaters. It may not be the problem in this case, but that's what I would look for first.

Your observation of the element may well be correct. But the conclusion is wrong. If the bottom element has failed, the heater will continue to function on the upper element alone. The stored hot water will be about 1/3 the capacity of the heater as a whole with the top element only. If the lower element failed and then the upper thermostat failed in the closed position, the combination would appear as you claim to have observed. And that is entirely possible. If the claim is that only the bottom element had to be replaced to restore the heater without any further incidents of the tripping of the manually-resettable thermal overload, then I do not believe it. Further, the claim that the element will heat effectively with water as one of the conductors does not withstand scientific examination. If my math is correct, and it can be checked at the site linked, copper has a conductivity of 59,600,000. Sea water has a conductivity of just 5. And drinking water has a range of 0.0005 to 0.05. Neither conducts enough amperage to create any meaningful heating of the water in the heater. But they will conduct enough to be deadly to humans.
My original post was intended solely to assist the original questioner.

Here is a link that might be useful: Conductivity

Electric water heaters typically cycle the upper element first and then when that thermostat is satisfied they switch to the lower element. Thus if the lower element fails in the completely open position, the typical complaint is that they run out of hot water very quickly.

However, if the lower element fails by shorting, usually to the outside metal jacket of the element and (not to the water) then the lower element will overheat the water as I described. The failure is precipitated by the sediment that keeps the water from getting to the element to cool it off.

I'm pretty handy with my Fluke 83 digital multimeter and have confirmed what I was seeing by testing the current flow as well as the confirming the fact that the lower tstat was open.

There is a possibility that the thermostat has failed. But I have seen the shorted bottom element enough that if I were going out on a job with those complaints, I'd plan on replacing the lower element and would be surprised if it turned out to be a tstat. If the heater were newer, I'd be less inclined to blame the element.

I'm with Jake on this one having experienced this same problem as a young man in a remote location. I took the easy way out (without tools) and changed the stats first, problem remained and finally located wrench and it was the lower element.

A failed grounded element will do it.

Update.

He didn't do any testing first before just replacing both elements as they both looked bad, the bottom one looking a lot worse of course.

I will stop back by later in a couple of days, to let you know for sure if that took care of the situation.

Again...thanks to all.

Sue

Last update.

Again all is well. I think is is safe to say the problem has been taken care of.

Sue

New update.

I spoke too soon on that last update. I thought it might have been a little premature to assume it was fixed. The WH tripped the reset button, and he ended up changing one of the elements. Let's hope that does it.

Sue

Are you saying that three elements have been installed in efforts to solve the overheating problem?

Are you saying that three elements have been installed in efforts to solve the overheating problem?
Opps...sorry. I should have been sleeping as opposed to posting so late. Thanks for catching that.

He initially had replaced both elements. When that did not do it, he replaced one thermostat. Hopefully it was the right one, as I don't think he has tried to do any testing before just blindly replacing.

Sue

Testing cold thermostats is difficult, to say the least. And testing with heat is not easy with ordinary tools. Replacing one thermostat offers a 50-50 chance of getting the correct one. The bottom thermostat is more likely, in my reckoning. It functions much more than the top one in typical use. And the bottom one is the least expensive since it lacks the thermal cutoff switch. For professional service people, replacing one at time is not a viable option due to repeated service calls.
As far as the other explanations about bad elements being the cause, the described failure that could cause overheating is highly unlikely to be the one that occurs most often. For example, if the element developed a ground fault at the end where the 120 volt supply is constant, the overcurrent device would actuate and open the circuit rather than the heater overheat. And the element can fail at every other point along the element length. So by my reckoning, the incidents of overheating due to failed element constitutes a very statistically small percentage of such cases. Your mileage may vary.

Bus Driver,

You have consistently tried to theorize and dismiss what several people have said they have personally experienced.

I appreciate applying reasoning and what we know about electrical theory to a problem as much as the next guy. However, you aren't listening to people. Instead you are offering up baseless theories and claims such as "Overheating due to failed element constitutes a very statistically small percentage..." when you have absolutely nothing at all to back that up other than your theories of how the failure happens -- which have been faulty up to this point.

Now, I admit, as I have all along, that it may end up being the thermostat. However, during my eight years as a maintenance manager for over 400 apartment units with electric water heaters, I've seen the source of the problem be the lower heater element grounding out often enough that it's my first guess on an older heater.

Actually I have a great deal to back up what I have posted, formal education, trade licenses, and experience. But since you characterize it as "baseless", that settles the matter as far as you are concerned. My first post suggested the thermostat and the original poster says that was the problem. Theory combined with experience is a good combination, my opinion. Res ipsa loquitur.

Bus Driver,

The point is that you have said repeatedly, very authoritatively but incorrectly, that it couldn't be the lower element. Whether this instance turns out to be the thermostat or the element isn't the issue. Rather than listening to others' experience, you dismissed them and gave references to the conductivity of water instead. Even when a couple of posters said the same thing and elaborated on how it happens, you insisted that it couldn't.

Again, you have absolutely no basis for your conclusion that "element failures constitute a very statistically small percentage..." and you aren't doing anyone any favors by saying that.

The fact that a thermostat could be bad is probably the obvious place to look. Where this website comes in handy is to take advantage of the experience of other people who have come across a similar problem.

Although I admit to not having to use Ohm's law very recently-- as I'm sure you noticed-- I'm no slouch on electrical theory. I was pursuing my Ham license when I was 11 years old which at that time required a lot of basic AC and DC theory (I could never pass the Morse code test).

As I said, I really do appreciate people who apply technical knowledge and sound reasoning to these kinds of problems. But when you misuse that to sound authoritative when you aren't is unfair.