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Heat just stopped working after working 3 days ago. All circuit breakers are looking good, gas is flowing, and furnace seems to be operating fine. Pilot is on and thermocouple is holding the flame, but it looks like a little rusty/busted.

Is there a chance - without getting into too much repair costs - that the thermocouple isn't doing "its job" by signaling voltage so everything looks OK and I just need to replace the TC? Is there a way to tell (I dont have a multimeter)?

Is it worth trying $15 on a new TC just in case, or would it be a waste of money?

  • My guess would be it’s a different safety device. For example: my brother called me one cold wintery morning with no heat. So after some troubleshooting it turned out to be a dead bird that had come down the flue and gotten lodged in the flue draft inducer fan preventing its squirrel cage from turning. Because this could potentially fill the house with carbon monoxide the furnace would not run without it. Took it apart, removed dead bird, put it back together furnace fired up. – Tyson Oct 25 '18 at 14:27
  • If the pilot is on then it appears the thermocouple is working properly. Your description is a bit unclear, you state the "heat stopped working" but then state "furnace seems to be operating fine" Can you be specific as to what you mean by "heat stopped working". – Platinum Goose Oct 25 '18 at 17:33
  • @PlatinumGoose i thought that the TC isn't about keeping the pilot on - its also about the voltage communicated. The pilot works great. The furnace boilers don't come on even though the pilot is on and the TC tip is fully inside the flame. I know it seems strange but I default to thinking the TC is always the problem. – dama_do_bling Oct 25 '18 at 23:18
  • The thermocouple sends an electric signal to the gas valve telling it that there is a flame at the pilot and it opens the valve that feeds gas to the pilot to keep it lit. I suppose you could have a bad thermocouple but then that would mean your gas valve is bad and it's allowing gas to the pilot without an electric signal from the thermocouple. Sounds like the gas valve is not opening to light the burner. That could be a few different things. If you have a multi meter then start another thread and post the model number of the furnace and someone should be able to help you troubleshoot this. – Platinum Goose Oct 26 '18 at 13:44
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It is absolutely possible for a thermocouple to "die slowly" where it will work intermittently for a pretty long time. But, what makes you think it's the thermocouple? Granted, that is a pretty common failure point in gas fired units, you don't mention anything that actually points to a thermocouple failure (or really, any specific component) as a possible culprit.

If you are really convinced that the thermocouple is causing the pilot to turn off, you can try rubbing it with some sand paper or one of those green 3M scrubbers. You want to take off any rust/corrosion/crud and get it as smooth and shiny as possible. Use the finest grit sandpaper you can find. If you do this and it fixes the problem, it's still very likely that it will come back within a few weeks, although you might get lucky and have it come out more or less as "permanently" fixed.

You really do need a multimeter to properly test it though. I found some instructions (which I will not link to) detailing how to "test" a thermocouple with a paperclip, which just seems like a bad idea.... please don't do that.

Personally, I would try the sandpaper first and see what happens, and maybe spend the $15 on a cheap multimeter (or borrow one from a friend). If you are in an area that is already getting very cold, you might not want to risk it. In that case, the TC is usually a reasonable bet. Good luck!

  • thanks for this! My thought about it being the TC is just because of what you said - it is just always the failure point in my (amateur home owner) experience. But indeed, the pilot is on and the TC is hot, so it doesn't seem obviously that. – dama_do_bling Oct 25 '18 at 17:45
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A thermocouple is a pair of dissimilar metals / wires connected together. As the junction is placed in heat a voltage is generated the more heat the higher the voltage. If the tip is not in the flame and or has buildup on it it can put out a lower voltage and below a specific threshold it will not open the valve so your thermocouple is good based on the information you provided. There can be other issues like water in the gas line. If you have a drip leg just prior to the furnace turn off the gas, open the drip leg and see if it has water in it. A drip leg is usually just prior to the burner after the shutoff. A t in the line with a stub pointing down that is capped. This is where moisture and solids fall and get trapped. If you don't have a drip leg or it is full 1 tiny drop of water can cause intermittent pilot failures.

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