On particularly windy days like today, the pilot light in my furnace likes to go out.

To try to confirm it is the wind blowing out the pilot light through the flue, I set up a camera and recorded this footage:

Pilot light going out with a clunk

The loud clunk is what is throwing me off. What could be causing that noise and how does that correlate to the pilot light going out?

  • 2
    I wonder if it's a flapper in your flue closing. Jun 21, 2023 at 2:59
  • @AldusBumblebore It isn't monetized, Google plays ads on all videos they deem "safe" now. See here and here. Since I'm not a partner, I can neither monetize nor "remove ads" on this video.
    – Resorath
    Jun 25, 2023 at 1:04
  • Oh, I see, I didn't realize it was like that. Deleting my comment! Jun 25, 2023 at 3:41

5 Answers 5


Fairly common on modern pilot lights to have a thermocouple heated by the pilot flame and a valve controlled by the thermocouple (with a manual thing you have to push and hold when lighting the pilot, until the thermocouple heats up enough to hold the valve open.)

So I assume the noise is the valve closing.

  • As much as I was adamant it wasn't the Thermocouple (I replaced it and it was still going out), it seems the "universal" replacements don't fully fit on my gas valve (it seated and mostly worked, but there was potentially bad electrical contact) and I had to order a specialized one. So far so good!
    – Resorath
    Jun 25, 2023 at 1:17

If the thermocouple that is heated by pilot flame does not report pilot flame is on, the main gas valve will shut down.

That is done for safety reason. So check and clean or replace the thermocouple.


Try snuffing the pilot manually: blow on it directly, blow through a straw or tube, or use compressed air. (These suggestions are all intended to simulate wind drafting through the flue.)

Supposing that the loud clunk occurs, you can now light and extinguish the pilot repeatedly while searching for the source of the noise. Use your hand to feel for its vibration, listen with a mechanic's stethoscope, etc.


It's puzzling that the clunk occurs instantaneously when the pilot goes out. A thermocouple usually takes a few seconds to cool off and close the valve with a click rather than a loud clunk.

I think you may have a photoelectric flame sensor, which is typically a cadmium sulfide photocell connected to a circuit board. As soon as a lack of flame is detected, the circuit board logic turns off the gas valve.

These are typically used on electric ignition furnaces without a standing pilot. If you can find a schematic for your make and model of furnace, it would show a flame sensor if there is one.


It could be that the draught is pushing the flame away from the thermocouple, or chilling it. So the valve could trip while the flame is still there, and kills the pilot light itself.

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