Help with recurring thermal cutoff failure on electric clothes dryer

[Passepartout]

Good Job! You probably saved a few $hundred$ to spend on vacation.

 

[T_R_Oglodyte]

Good Job! You probably saved a few $hundred$ to spend on vacation.

Sometimes taking a shower leads to inspiration. I had been wracking my brain, trying to figure out what could be causing the cutoff to fail without having the first shut down the system. The cutoff should only fail if the thermostat fails, since it is the safety backup to the thermostat. Since the heater hadn't failed, and actually tested to spec, I hadn't been thinking about that.

Then in the shower it suddenly seemed clear. There had to be a temperature differential between the two locations. And that would only happen if the heater element was not heating uniformly. Ergo, the problem must be in the heating element. Then when I got the old element outside I could see that it was oxided and likely wasn't too far from outright failure.

 

[csxjohn]

Replacing the heater element solved the problem, even though the element hadn't failed and actually was exactly on specification.

This supports my hunch that there was a hot spot on the heater element located between the high limit thermostat and the thermal cutoff. Being downstream of the high limit thermostat, the thermostat didn't pick up the overheating.

Glad you got it worked out. My theory may still hold some water, that the heat was high enough fast enough that it tripped the high limit thermostat but still had enough heat to melt the thermal cutoff before the power to the heating coil could be cut and the coil allowed to cool. Once the coil cools the high limit thermostat resets itself and there is no evidence that it tripped.

So I'm thinking if it's an air flow problem the HLT will cut off before it gets too hot. If it's the heating coil giving off way too much heat they will both trip and the one resets.

Either way, other TUGers will know to replace the heater coil if there is not an air flow problem.

 

[Elan]

There could have been electrical heating of the thermal cutoff as well. Doesn't really matter at this point, problem solved. Nice job, Steve!

 

[T_R_Oglodyte]

There could have been electrical heating of the thermal cutoff as well. Doesn't really matter at this point, problem solved. Nice job, Steve!

I'm not sure what you're saying here. Are you suggesting that the electrical current passing through the thermal cutoff heated the cutoff to failure?

If that's what you are indicating, my answer is that's why I paid more to get OEM parts. Those types of failures are common in the cheapo aftermarket products that are listed on Amazon and eBay.

 

[Elan]

I'm not sure what you're saying here. Are you suggesting that the electrical current passing through the thermal cutoff heated the cutoff to failure?

If that's what you are indicating, my answer is that's why I paid more to get OEM parts. Those types of failures are common in the cheapo aftermarket products that are listed on Amazon and eBay.

Yeah, that's what I was thinking. That when the heating element turned on, the impedance changed in such a way to cause excessive current through the cutoff. That, coupled with the ambient temp from the element, caused the cutoff to open (Ti + Ta > Tcutoff). Either way, the issue would be with the heating element itself (not a defective cutoff), which is what you ultimately replaced to fix the issue.

As I said, this was my thought initially, but when you said you were going to replace the element, I was pretty sure you'd fix the problem, so it didn't matter.

 

[T_R_Oglodyte]

Yeah, that's what I was thinking. That when the heating element turned on, the impedance changed in such a way to cause excessive current through the cutoff. That, coupled with the ambient temp from the element, caused the cutoff to open (Ti + Ta > Tcutoff). Either way, the issue would be with the heating element itself (not a defective cutoff), which is what you ultimately replaced to fix the issue.

As I said, this was my thought initially, but when you said you were going to replace the element, I was pretty sure you'd fix the problem, so it didn't matter.

Yeah - but to the extent there is any electrical heating of the cutoff that is considered in the design, at least in the OEM part. In the secondary market, it's entirely likely that there could be added thermal heating, but not likely.

BTW I checked resistance of the elements under full load, and all of the impedance in the heater circuit is in the heater element. None of the other components measures even as much as one ohm.


I didn't measure impedance of the old heater unit under load. Had I done so, I think I would have found that while it was on-spec in cold state, under load the impedance would have been much greater.

In any case it all comes down to a failing heating element causing uneven heating in the heater box, with the area near the high-limit thermostat being in a cool zone and the thermal cut-off located in a hot zone.