Eating Thermocouples


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Old 04-03-08, 10:59 AM
Z
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Question Eating Thermocouples

Seems to be a somewhat common problem on these forums. I have a gas fired boiler (Weil-McLain) and for the past few years I need to replace the thermocouples about every 4 months during the heating season.

Of course the reason that I'm posting is that I replaced one this morning that I just installed in the beginning of January. I checked the voltage output (NG) and also tried numerous times to light it (as I do every time yet another one burns out). When it is lit and happy the pilot flame hits it as it should 1/2" from tip, nice blue flame, surrounds tip.

The question I have, when the burners kick on, the flames from the BURNERS are engulfing the thermocouple. Does this sound right? It would seem to me that this is the reason that my thermocouples are burning out so quickly and ONLY during the heating season. Otherwise, with just the standing pilot, the thermocouple is happy the enitre non-heating season (i.e. 8 months at a time or more).

Any comments or suggestions?

Thanks in advance.
ZS
 
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Old 04-03-08, 11:19 AM
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Wink

Out of the box here. Have you talked to Weil-McLain ??

Are burners clean ?? Burn right soft but blue, air mix ok??? W/C (psi) of the gas ok?? Burners in right set right venturi of burners clean ??.
 
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Old 04-03-08, 11:38 AM
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Weil-McLain told me to "contact my local plumbing and heating contractor". Gee thanks for that expert help.

Burners are clean, blue flame, air mix OK, burners in right.

I guess the simple question is, should the burner flames be engulfing my thermocouple???? or should they be no where's near it?
 
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Old 04-03-08, 12:37 PM
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I personally have never heard of such a problem before. Has anyone else?

Maybe the thermocouples are not 'burning out'. Maybe something like the threaded hex that goes in the gas valve is loosening some?, or maybe you aren't careful unrolling the new thermocouple coiled in the package? I don't think the flame has anything to do with it. After all, they always sit right in a flame.

But I'm all ears on this one.
 
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Old 04-03-08, 12:59 PM
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The connection to the gas valve is tight when it stops working. I unscrew and check with a VOM with the pilot burning (obviously need to hold down the button on the pilot lighting control for this), a good thermocouple gives me about 30 mV and these "dead" ones give me notta.

I'm curious, what are you getting at about not "carefully unrolling the new thermocouple coiled in the package"? Is there something I should know? The new ones do work fine after installation for a while (up to 4 months).

I am very careful to make sure that the thermocouple is not touching any metal as it snaked through the tubes and out to the gas valve so as not to possibly change any resistance, although that doesn't seem to matter.

The reason I ask about the burners possibly burning out the thermocouple is that the standing pilot is fine the entire offseason and then has problems once the system kicks on and has been running for a monthor two. The one that I removed this morning was no longer copper colored at the business end but looked more like a white/grey color, like something that has been burned.
 
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Old 04-03-08, 05:33 PM
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Yes, it is the main flame that is destroying the thermocouples.

Ideally the pilot burner should be located so that the "wings" of the pilot flame will reliably light the main burner but the main burner flame will not hit the thermocouple.

Can you post a few pictures of how the pilot burner is mounted in regard to the main burner?
 
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Old 04-04-08, 07:25 AM
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Thanks Furd, as I suspected.

Don't know if I can borrow a digital camera to take the pics but I will take a really close look at the height of the thermocouple and see if I can't drop it down and still get the correct pilot flame position.

I've been using "universal" replacement thermocouples and installing them, with their universal sleeves and such, isn't an exact science.

Thanks so much for the corroboration of my burner theory.

Guess I'll be back if/when the newest thermocouple burns out.

Thanks again to all for your help. Certainly more help than Weil-McLain offerred me.
 
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Old 04-04-08, 06:49 PM
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Thermocouple

As Furd said, the main burner flame should not hit the thermocouple. I suspect either the main burner is not burning properly or the pilot burner is improperly positioned. Maybe bent when replacing a thermocouple at some point?

Most pilot burners have a hood which directs the flame left, right, or both. That hood should be positioned in such a way as to direct the flame to the main burner but be far enough away to keep the thermocouple out of the way of the main burner's flame.

A bit out of the box here but you could have low gas pressure or a partially clogged pilot orifice thus causing a pilot flame too small to reach the main burner. Someone, sometime may have compenstated by moving the pilot burner closer to the main burner.
 
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Old 04-08-08, 07:55 AM
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Thanks Grady, I think you're right on the money. I looked back in my notes and discovered that my problems with the premature thermocouple burn-out started shortly after I had a licensed HVAC guy do some work on my system. Not pointing fingers, but the timing does seem rather suspicious.
This is the main reason why I now do everything, that I possibly can, myself. At least I know what I did.

I've adjusted the height on the thermocouple and now it's not getting hit by the main burner flame. Time will tell.

Thanks to all for helping me solve my problem.

ZS
 
 

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