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Question DetailsAsked on 12/16/2017

Pilot stays lit for two weeks then goes out with the thermocouple brand new.

My furnace pilot light keeps going out and I have replaced the thermocouple two weeks ago I wondering if the gas flame from the main burner put it out when the pilot lit the burner

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Seem to be a lot of these sort of questionis in the past few months - wonder if a particular brand is involved ?

You can infer one of two things - either the original thermocouple was bad AND the new one is too (or is grounded out or not making good contact in the gas valve), or the problem was elsewhere and still exists. Or the original thermocouple was the problem and the newone was not installed correctly.

Because it is easiest to rule try to rule out the first, check manual for draft diverter (bad finger behind pilot flame) location relative to the main burner (normally but not always between it and the pilot flame), the flame height, thermocouple location in the flame, and flame height (adjustable - use SMALL increments on screw) by small adjustment screw on the gas control valve - be sure to get the right one, the pilot flame gas adjustment, not the output gas pressure screw.

I recommend trying another thermocouple (checking it is the right model for your appliance) - cheap way to be sure that is not the problem, especially with a LOT of defective ones coming in recently - if that solves it return the first "new" one for a refund, if not you have a spare which is not a bad thing to have in reserve - especially for furnaces.

Also - check in one of the following links where I describe how to properly insert the thermocouple into the gas valve - and first check with a flashlight (and with mirror is easier thatn turning upside down under it) to see the silver or copper "button" in the center ofthe thermocouple recess is intact and not corroded and that there is not a broken off end button from another thermocouple up in there. Then properly insert the end button on the thermocouple until you feel a firm seating and that it is not jamming in the threads (you can also confirm thermocouple penetration depth against how deep a wood sliver goes up in) to be sure it is all the way up in, THEN slide the mounting nut up the thermocouple tubing and thread it into the hole. have the threaded nut up against the gas control valve or starrt it first and the tip of the thermocoupel can end up hanging up on the threads inside the recess and not go full depth.

Also - check the thermocouple/thermopile is firmly held in the pilot light mounting bracket (some need a good ground there to work right) and that it is not touching metal along its path to the gas control valve. Also - some models use an insulated sleeve at the pilot light - read instructions that came with it if that is the case, because some use the outer tubing as part of the circuit so uninsulated contact at the pilot bracket or touching the furnace frame or boiler tube along the way can interfere with the millivolt energy the thermocouple is puttint out, making it erratic or non-functional.

Now - with it going out only about every 2 weeks - unless for some reason your utility is having service pressure issues (real rare), sounds like the pilot is being blown out. Four common ways for that to happen, aside from a defective thermocouple or the rare gas control valve getting so old its springs are getting weak so not properly resisting the gas pressure to hold the gas flow on.

1) burner firing up drafting it enough to blow it out - usually only if the pilot is not in the edge of the gas flow from the main burner so it builds up a bunch of gas in the firebox before enough reaches the pilot light to ignite - so would normally be igniting with a "whoosh" if that were the case.

2) being blown out by wind (have strong winds been occurring when it goes out ?). Is the wesather cap on the flue intact (assuming this is a gravity furnace).

3) being blown out by drafting when another appliance on the same flue fires up - observe the furnace pilot light as someone turns the water heater thermostat up to see if the water heater firing up causes a significant flicker in the pilot light

4) "flue gas damping" - flue gases are not being evacuated properly so they are building up or backing down into the firebox, possibly snuffing both the main burner and pilot lights. Can be to partly blocked flue (bird or squirrel nest, etc), too small a flue pipe for the appliances connected to it (though unlikely your cause unless you have changed furnace/water heater recently), or with direct-vent appliances the flue damper is not opening properly or the eductor fan is not kicking on properly each time. Can also happen when the flue ducting and the draft hood are not properly installed or dimensioned, so as it fires up (creating flue gases which are oxygen deficient so they will snuff the burner and pilot) - used to be REAL rare because fireboxes were open to the air, but with more modern units with enclosed fireboxes and a small filtered combustion air inlet can starve for air and end up snuffing the flames. Ditto if the air filter on air inlet is really blocked. Ditto in small enclosed utility rooms with inadequate fresh air intake so they pull a partial vacuum in the room, effectively causing backdrafting.

Answered 2 years ago by LCD

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