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Tool Talk Discussion Forum

Re: Furnace pilot light won't stay lit

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Author  [Modern View]

03-11-2013 21:13:54

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Stan, were you able to get the pilot light going with the button pushed down? After awhile did the pilot go out when you turned the knob to let it run?

How big is the flame, it doesn't take much. If its not running when the knob is turned there is one issue, if it stays lit thats good.

If you can get the light to keep burning until the furnace tries to light, that is where you would take out the whole assembly and clean it.

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Stan in Oly, WA

03-12-2013 00:41:55

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 Re: Furnace pilot light won't stay lit in reply to Dalet, 03-11-2013 21:13:54  
No, the pilot flame would go out even before my finger was completely off the button. I take it that it works by the button opening a valve to let gas flow to the pilot. Once the pilot is lit, the flame heats the thermocouple which sends millivoltage to the unit which then holds the valve open so you can release the button. But that never happens. The flame only stays lit until you release the button. Bad thermocouple, I now understand. But why? The furnace has only been in operation since late October, and not that much---we've had a warm winter.


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missouri massey man

03-12-2013 04:17:30

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 Re: Furnace pilot light won't stay lit in reply to Stan in Oly, WA, 03-12-2013 00:41:55  
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The thermocouple works by having the junction of two dissimilar metals heated. When heated they generate that milli voltage. The safety coil in the redundant gas valve will be held in only when the milli voltage generated by the thermocouple matches it. You are bypassing that safety coil by depressing the button long enough for the milli volts to take your place. Typically what you are describing is one of three things: 1) Bad thermocouple 2) Insufficient heat on the thermocouple end...too small of flame or flame is so high it is actually bypassing where it needs to be 3) Defective holding coil in the safety circuit.
Why possibly a bad thermocouple? We are finding poor quality of products flooding the US market in virtually everything, why not thermocouples too... Usually if the junction of the two dissimilar metals (swelled up end) of the thermocouple is intact it will generate sufficient milli voltage, but that is no longer true with the lower quality of materials.

Though a slim possibility, the same applies to the redundant gas valves holding coil. If bad, the gas valve needs to be replaced. The easiest way to check it is with another new thermocouple. This will ring true ONLY if the pilot is adjusted properly and coming in contact with the thermocouple properly.

Make sure you have a strong pilot flame encompassing a minimun of 1/2 to 3/4" of the end of the thermocouple. There should be a nice inner core of blue with a different softer and larger outer ring of blue with NO yellow. Yellow or white in the flame will indicate imminent carbon build up. A tad of orange is all right as usually that will be superficial rust particulate sucking into the flame as you are or have been disturbing the area surrounding the pilot with all your work. That will go away mostly. Please keep us informed on what you do and what you find.

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Dusty MI

03-12-2013 04:29:14

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 Re: Furnace pilot light won't stay lit in reply to missouri massey man, 03-12-2013 04:17:30  
I've checked thermocouples with a DC volt meter and a propane torch.


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missouri massey man

03-12-2013 05:00:51

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 Re: Furnace pilot light won't stay lit in reply to Dusty MI, 03-12-2013 04:29:14  
Yeah, I too have a few tricks up my sleeve and have done the same thing...I used to carry an old "Baso" switch to check the thermocouples with. Just left the pilot assembly in the furnace, unscrew the end into the gas valve and screw into the Baso and relight the pilot while holding down the redundant valve to maintain pilot gas flow. After 10 maybe 15 seconds, depress the Baso switch and if it holds in then the thermocouple was generating adequately. I have been doing commercial and industrial for the last 25 years so this old school technology (if you can call it that) is slowly going by the wayside. I probably haven't touched a standing pilot in a decade.

I was thinking he probably doesn't have a dvom and was going for the basic description of the pilot circuit sequence so the repair can be made with nothing special.
He's gotten a lot of good suggestions so far and I am confident he will figure it out.

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03-12-2013 02:15:42

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 Re: Furnace pilot light won't stay lit in reply to Stan in Oly, WA, 03-12-2013 00:41:55  
> But why? The furnace has only been in operation since late October, and not that much---we've had a warm winter.

Remember, the pilot is on all the time, and was probably on all through the summer. The usual cause of premature thermocouple failure is an improperly adjusted pilot flame. Check the installation instructions for the furnace and see if you missed any adjustment steps. Usually the pressure needs to be checked with a manometer, and the flame needs to be adjusted so it's just big enough to set the thermocouple.

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