I have a Heatilator ND4236 gas fireplace (natural gas). The thermopile (Q313A) fails every two to three years (I have read they should last at least five). What could be causing the too frequent failures?

Factors that might be relevant:

  • The first two times it failed, I hired an HVAC company to come repair it and I don't know if they used the right part (I got the Q313A part number off the part they used)
  • The Heatilator manual only references the part as part of the pilot assembly (2103-010)
  • I tend to leave the pilot on year round (I didn't know how to properly light it until recently and messing with it was low on my priority list)
  • I have never attempted to clean the thermopile with sandpaper to see if that fixes the problem (I have only ever replaced it once myself, yesterday)
  • The fireplace is 19 or so years old and I only know about the last seven years worth of maintenance (basically none besides the two times the HVAC company worked on it and my recent replacement of the failed thermocouple and thermopile)
  • The thermocouple doesn't seem to fail as often (I don't believe the HVAC company replaced it and it only recently failed for the first time since I have owned the fireplace)
  • The specific parts I used to fix it this time are thermopile and thermocouple

The part is only $15 on Amazon (compared $60 - $100 the pilot assembly costs or the hundreds the HVAC company was charging me), so I don't really care that much unless this is a symptom of some larger problem waiting to crop up.

Additional information that now seems relevant: the fireplace is also controlled by a wall switch. It is probable that the builder used a standard 120 volt switch and the thermopile can't generate enough power to overcome the resistance after a few years. Cleaning the thermopile may help, but the right solution may just be to find a better switch. I am going to check the switch when I get home.

1 Answer 1


One issue is not failure of the thermopile, but a covering of corrosion preventing heat reaching it. Certainly, try brushing the outside with sandpaper a bit to see if that fixes the issue.

Another possibility is misadjustment -- check from a repair manual that the thermopile is correctly positioned in the pilot flame.

Also check the connections: thermopiles are very low voltage devices, and a loose connection could prevent sufficient current from holding the valve open.

Finally, check that water or condensation is not dripping on the unit. That could cause failure internally through electrolytic corrosion, though if the pilot light is on all year, that is unlikely, because the thermopile would be too hot for condensation.

BTW, I've never experienced a true thermopile failure. They're simply a bunch of short wires inside a metal tube, with no moving parts to break.

  • Yeah, my bet at this point is that it is a combination of it getting dirty and the wall switch not being the right type of wall switch (and therefore requiring the higher end of the 750 volts the thermopile can produce to trip the solenoid). Sadly, I threw out the old thermopile, so it won't be easy to test it for a couple years, but I will update then. Jan 2, 2022 at 17:59
  • That is not 750 VDC, but 0.750 VDC, only three-quarters of a volt, about half that a single (Zn-C) flashlight cell produces. That's very little "push" to the electrons, and any resistance in series would prevent it from pulling in the valve mechanism. BTW, there should not be a switch in series with the thermopile -- it would serve no purpose, and would increase resistance. Jan 2, 2022 at 23:58

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