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List the reasons for this failure

tim smith
tim smith Member Posts: 2,752
Best thermocouple we have found and have mainly used for the last 25 yrs is a Johnson Controls Husky thermocouple #K16BT-36. Best MV production and just built heavier than the rest. Only problem is they won't always fit in the hole in pilot assembly on water heaters mainly as they have a fatter body on the t couple. JMHO, Tim


  • Ted_9
    Ted_9 Member Posts: 1,718
    My customer has asked....

    The gas company replaced a thermocouple last year.
    Why would it fail in another year?

    To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"
  • lchmb
    lchmb Member Posts: 2,997
    a few

    Gas pressure
    improper orifice
    improper alignment
    over heating
    poor construction
    I have also seen where the pilot went out for no apparant reason, it's an inexpensive item so the first thing I replace. I of course do test all functions' to rule anything else out..
  • lchmb
    lchmb Member Posts: 2,997
    a few

    Gas pressure

    improper orifice

    improper alignment

    over heating

    poor construction

    I have also seen where the pilot went out for no apparant reason, it's an inexpensive item so the first thing I replace. I of course do test all functions' to rule anything else out..
  • Ken D.
    Ken D. Member Posts: 836

    Most of the time they aren't bad. Just easy and cheap to replace when doing a service call. It is cheap insurance. What can cause it to go bad so soon? Either defect in materials or workmanship (doubtful), overheating, poor combustion, impurities in the ambient air, soot or scale, pilot flame not hitting it properly (not mounted properly), roll out that damages the mounting, base, or wire, low gas supply pressure, pilot dirty, chaffing, etc., etc.
  • Had that before Ted

    usually at 1am in rental properties. I usually check the MV on the replacement TC and adjust the pilot flame accordingly, sometimes I have had to bend the pilot burners "deflector" to get it right, as they likely make a "million" a day.

  • Guy Woollard
    Guy Woollard Member Posts: 82

    Is the boiler condensing and dripping on the (HOT) thermocouple?

    Guy Woollard
    N.E. Regional Sales Mgr
    Triangle Tube Corp.
  • Thermocouple??? Treating the symptom only

    Maybe the thermocouple was not really "bad" last year, just a bit weaker than a new one.

    What was the MV output of the "bad" one before he replaced it???

    If someone tells me a thermocouple is “bad” I ask what it its rating and what was its tested output when you changed it. If he says it was “bad” and does not know what the output was, he “does not really know” that the thermocouple was “bad”.

    Possible Scenario:

    Pilot is slightly blocked or not exactly contacting the thermocouple properly.

    The flame drops out with the old thermocouple.

    Tech does not "test" the MV output of the oil thermocouple, he just replaces it and things run.

    He said it is a "bad" thermocouple.

    New one runs for a while until, the output of the new thermocouple decreases to the point where the old one was.

    Here we go again.

    He may have been treating a symptom last year, and the treatment for that symptom bought a year of time.

    Food for thought

    Ed Carey
  • Noel
    Noel Member Posts: 177
    How long SHOULD they last?

    Honeywell suggests that they are deteriorated within, after one year, and should be changed at the annual inspection timeframe. The bimetal junction goes south on them. They are a maintenance item, as are filters, nozzles, pump gaskets, and gauge glass gaskets.

  • Chris_82
    Chris_82 Member Posts: 321

    Do a search, Tim has many excellent suggestions for this exact problem in the past.
  • Replace each year


    I agree that they are deteriorated within a year, as per my previous post.

    Is that information in the instructions with the thermocouple, or does Honeywell have that posted anywhere?

    Not questioning your statement, just looking for something to hold in hand from the Mfg, when telling someone that the thermocouple should be changed each year.


    Ed Carey
  • I have a white paper...

    by Honeywell, named "Thermocouple Quality" that I took from their website last year.

    I can't make it come up, today. I can fax it to you Monday, from work, if you like.

  • To Noel

    That would be great.

    I sent you my contact information by e-mail.


    Ed Carey
  • ScottMP
    ScottMP Member Posts: 5,884
  • Ted_9
    Ted_9 Member Posts: 1,718

  • Partially clogged pilot orifice, especially if it's propane. I just had one where I had pilot and a good T'couple but not enough pilot flame to heat it properly. Cleaned the orifice and it's been fine ever since.
  • Glenn Harrison_2
    Glenn Harrison_2 Member Posts: 845

    Is this the bulletin you were referring to?

  • Thank You

    That is a great doc.


    Ed Carey
  • They don't make'em like they used to...

    And THAT'S the TRUTH. I've never changed so many thermocoules in my last 5 years. In fact, i would say I've changed more (multiple times) T-couples in the last 5 years as I have my whole 33 years in business...

    A little birdy told me that the manufacturers have basically 2 choices for T-couples, a 3 year life expectancy, and a 10 year life expectancy. Guess which one the manufacturers prefer to use. Its all about the almighty buck. I told this birdy that I would be willing to pay more for the 10 year T-couple because it was cheaper than running around replacing thermocouples that were just too darned young to die..

    (Here's where I get to sound like an old timer) Was a time that the T-couple would outlast the appliance...

    Heck, the manufacturers could use it as a sales feature if'n they were smart...

    "Our appliances come with a 10 year rated thermocouple, not the three years that everyone else uses..."

    My peace of mind is worth something too, isn't it...

    I was told the primary reason these beauts die is cause they get too hot. (NOOOooo....Are you kidding me???)

    How hot is too hot? And more importantly, how do you measure the flame temperature??? If it is too hot, how you do you cool it down??

    I remember when thermocouples were MUCH more reliable than intermittent ignition. Maybe it's a plot by the boiler manufacturers to turn us against thermocouples... Yeah, that's it, a vast industry wide conspiracy to get us to quit using this archaic method of wasting gas for the sake of lighting a burner.

    Yeah, That's the ticket :-)

  • Testing MV


    A lot of the responses were very good. This was a very informative thread.

    I still think that to truly diagnose the problem with a thermocouple or for that matter the gas valve magnet, you have to test the thermocouple and know how much voltage it is putting out.

    A MV tester is not expensive, and it is an easy test. Otherwise, just changing the parts without testing is, well,,,,, "parts changing". It is a best guess approach to the problem.

    Just my $0000.02

    Ed Carey

  • I'm sure Honeywell would LOVE to see you change the thermocouple every time you serviced a standing pilot system. I've changed relatively few bad ones in the last oh, 30 years. Usually they were pretty old when they failed. I for one prefer to leave the pilot assembly undisturbed until it NEEDS to be disturbed. The less you take something apart, the less chance you have of damaging it in the process and causing further repairs to be necessary. That doc reads more like a sales pitch than anything else as far as I'm concerned.
  • That's the one, Glenn

  • Glenn Sossin_2
    Glenn Sossin_2 Member Posts: 592
    Bad gas valve

    Here what I was taught, -The thermocouple generates a small electric current to hold open the pilot solenoid. If this coil is damaged, or a few shorted windings, there will not be enough of a magentic field to hold the pilot valve plunger open.

    If this coil requires more of a micro current than normal, as the thermocouple ages, so does it's ability to generate electical current -hence the shorter life span.
  • George T
    George T Member Posts: 9
    A similar scenario....

    I had one that "seemed" to go out after a year. What really happened was that the service tech didn't tighten the connection down enough at the gas valve and it apparently loosened up even more over the year (expansion/contraction?)causing a bad connection. I lucked into finding this as I was about to change the thermocouple out and started at the gas valve end.
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