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Question DetailsAsked on 11/25/2016

pilot light is lit and cuts out but furnace works Why does pilot light cut out?

I could say the pilot light sputters. What is wrong?

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1 Answer

Voted Best Answer

Reasons in no particular order:
1) rust buildup on/around the pilot assembly, blocking air or the gas
2) the draft deflector is aimed wrong (the curved or crooked metal "finger" up "behind" the flame), so the pilot flame is pulled toward the burner when it is igniting or burning and therefore "flickers" or "walks" - see manual for where it is supposed to be located
3) on ones with adjustable location brackets, pilot flame is too close to the burner flame so it is pulling combustion gases from the burner instead of air, causing it to starve for oxygen. Can also happen if draft deflector is aimed wrong - see manual.
4) gas tube to pilot is kinked so it is not getting enough gas to maintain a steady flame
5) rarely, gas valve going bad - but if furnace main burner is working OK otherwise then that is quite unlikely
6) I have never seen it, but I suppose a thermocouple or thermopile going out or having been sharply kinked could result in a weak enough voltage that the gas control valve for the pilot is cutting in and out. Ditto to the thermocouple not being in the flame properly, so it is cutting in and out due to heating and cooling - but would have to be just so for that to happen quickly enough that the pilot keeps burning instead of being shut off and cooling down enough to keep the gas flow off. Ditto I guess (have never seen it) if the thermocouple tip which goes up into the gas control valve is corroded or not snug to the contact point, making for an intermittent contact. A dirty/corroded thermocouple tip might cause this too - clean with steel wool or extremely fine sandpaper to shiny metal
7) some thermocouples use the outer tube as part of the circuit so if it is in contact with metal along the run that can affect its performance or short it out - ditto with some other types if contacting metal along the run and picking up a static charge, because the normal thermocouple only runs at about 30 millivolts, and thermopiles at less than a volt - so static buildup (which occurs due tot he airflow and gas burning) can cause problems. (Furnaces and water heaters should be grounded or this and other reasons but not all are, especially with plastic pipes/ducting these days).
8) with thermopiles, they commonly use fine wires inside the tubing to carry the voltage - if it has been kinked or the wires pulled on, could be intermittently shorting out
9) if you have a strong draft blowing through the furnace (due to wind outside) which is causing pilot flickering and "walking around" you may need a wind deflector type cap on the exhaust vent pipe
10) excess gas flow to pilot, causing it to "lift off" the outlet orfice or "walk around" - will also have excess yellow flame if that is the case

Assuming this is a new event, and that nothing has been done on the furnace recently to change the draft deflector or gas tubing or thermocouple or such, the thermocouple/thermopile would be the obvious first thing to replace to see if that solves it - commonly last 2-5 years but "good practice" is to replace a thermocouple every year, thermopile every 3-4 years (especially on furnaces where its proper operation is more critical to the house than water heater) as part of annual preventative maintenance so it does not fail during the dead of winter and put you out of heat in the middle of the night or in very cold conditions while you are out of the house.

IF the above do not help you solve it yourself, Heating and A/C is the Search the List category to get a technician in to look at it.

Answered 3 years ago by LCD



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Answered 3 years ago by Member Services

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