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I have a 9-year-old Bradford White gas water heater, model MI403S6FBN4. The pilot light went out today, and when I went to re-light it, it stays lit while the pilot button is held down, and it stays lit after I turn the switch to the ON position. However, it goes out immediately when I replace the outer door (which covers the viewing window).

The metal of the door seems to be tripping what is, according to the online manual, the Nat. Gas Resettable Thermal Switch. I've read a bit online and this seems to be a safety device, so I've turned the gas off to the water heater and left it unlit, but what causes that switch to trip, and what needs to be done to fix the issue? Or does the switch just need to be replaced?

My furnace is also gas and also has a safety device to snuff the pilot light in case it detects gas outside the combustion chamber, but it hasn't tripped. I called the gas company, and they came out and didn't detect any gas in the air (I thought I smelled it but it was actually some urethane smell or something), so it wasn't the presence of gas that was causing it.

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    My water heater has a plastic mesh to filter out larger things from getting into the combustion chamber. I've heard that these can get dirty to the point where there is insufficient air flow to allow the pilot to keep going. Do you have a similar mesh filter at the bottom of your water heater? Oct 31, 2013 at 12:59
  • Not that I know of. Also, the pilot does stay lit as long as I leave the outer door off, it's only when I replace it (and the thermal switch is tripped) that it goes out.
    – Dan C
    Oct 31, 2013 at 14:14
  • This is to the original poster. I experienced the same thing today, with a Bradford White water heater. Whenever I would put the cover back on the pilot light would trip. Did you figure out what it was or did it happen again? I'd love to hear about your experiences since then. My solution was to slowly and carefully put it back on and it stayed lit. Thanks!
    – user30100
    Dec 28, 2014 at 7:46
  • I have this exact same model water heater and came across this question while looking for an answer to the same problem. The pilot light runs and the water heater works fine as long as the door is off, but when putting the door back on, the flame shuts off as soon as the door touches the back of the thermal switch (which I understand to be the reset button). Is the door even supposed to touch the thermal switch? Is touching the button supposed to turn it off? Does it mean the switch is bad?
    – Herohtar
    Jan 22, 2023 at 23:34
  • If the answer to this question, @Herohtar, hasn't helped you, please ask a new question. (Also, indicate why the answer to this one hasn't helped, or it's at risk of being closed as a duplicate.)
    – FreeMan
    Jan 23, 2023 at 16:33

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Thermal switches (depending on models) can be really tricky. If yours is really causing the problem I would suggest replacing it. However, before replacing the switch, ensure you have proper airflow into the combustion area. (Not much stacked around unit, any insulation properly installed, etc.) This really sounds like more of an air flow issue.

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