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thermocouple guestion

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HT in Pa_3
HT in Pa_3 Member Posts: 24
in THE MAIN WALL
I've replaced a lot of thermocouples and my experience is when they are bad, they will not let the pilot light stay lit. Well, is it possible for a supposedly bad thermocouple to start to function again, at least for a short time? I went on a call today where the customer said the pilot would not stay lit. "I think it's the thermocouple", he said. I agreed. When I got to the job, the pilot lit every time! I replaced the thermocouple anyway. Anyone ever been in that situation?

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  • Slimpickins
    Slimpickins Member Posts: 339
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    gotta test 'em

    You don't know if they're bad if you don't test them. You could have a weak pilot, bad gas control, bad fuse able link. 
  • bill_105
    bill_105 Member Posts: 429
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    Just had that

    Is there a real quality thing between honeywell and Johnson controls? When uncle Tim gets up, I'm sure we will know the answer. This guy is awesome. He helped me out on a thing last week on a different topic. He should be riding in on his White horse soon!
  • JK_3
    JK_3 Member Posts: 240
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    Sometimes its not the thermo

    I have seen many times (Especially with the new sealed combustion chambers) that the thermocouple is not bad but a lack of air makes it appear that way. at the end of the heat cycle the burner turning off sucks all the available air from the chamber starving the pilot light and it goes out. There could be a few different causes but the most common lately is partially clogged screens on the air intake.

    Hope this helps.

    There was an error rendering this rich post.

  • Tim McElwain
    Tim McElwain Member Posts: 4,628
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    Yes it is possible

    that the thermocouple can do that. As it gets older and under some heat duress the inner copel lead will begin to deteriorate and will shut down, it will then cool and the lead will re-attach internally and it will work for a short time. This will show up when you take millivolt readings at an open circuit say of 30 to 35 millivolts and a closed circuit of around 2 or 3, when charted out this will recommend a thermocouple change.



    Check out procedures for testing thermocouples at "RESOURCES/LIBRARY TO TIME MCELWAINS CLOSET.
  • Bob C._2
    Bob C._2 Member Posts: 33
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    thermocouple

    Where can I get info. on testing thermocouples and power piles thanks
  • Tim McElwain
    Tim McElwain Member Posts: 4,628
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    Bob C

    Go to the top of this site to "RESOURCES" click and then click "LIBRARY" look for Tim McElwains Closet you will find procedures there. For more information contact me by e-mail at gastc@cox.net and I will send you my catalog of manuals available for training.
  • tim smith
    tim smith Member Posts: 2,765
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    Re: thermocouples and quality

    We have found that the Johnson husky thermocouple has always been the best millivolt producer on equipment. Of course not all pilots will fit the husky wider base design. Then Honeywell.  We always use g bushings screw in testers to test thermocouple or magnets in valves to see where they hold and drop out at. I Learned this over 25 yrs ago and still like to pass it on to our newer guys. Not many old standing pilots around any more except for water heaters and older unit heaters.
  • HT in Pa_3
    HT in Pa_3 Member Posts: 24
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    A BIG thanks for all the comments

    The gas boiler is still running, so I'm inclined to think that the problem must have been a bad thermocouple. A spicial thanks to Tim "the knight" McElwain for explaining in detail how a thermocouple can break down, and then still continue to work for a while. I'm gonna have to learn how to test them.
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