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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.

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  • Note that a pilot light means you are continuously burning some gas whether you use it or not, like leaving a car idling when not being used. You might want to check the cost of replacing that with a more modern system that uses electric ignition.
    – keshlam
    Commented Apr 16, 2023 at 14:52

2 Answers 2

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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.

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  • 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. Commented Jan 2, 2022 at 17:59
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    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. Commented Jan 2, 2022 at 23:58
  • @DrMoishePippik -- manually controlled fireplaces will have that switch there as that's how its turned on and off Commented Apr 16, 2023 at 15:59
  • @ThreePhaseEel, Thanks, I stand corrected -- and that would definitely be a place to look for higher resistance. A bad switch (i.e., more than a few milliohms) in series could prevent the pilot-light relay from latching. Quick test: temporarily put strong-jawed clip lead, or even temporarily solder a jumper, across such a switch. Commented Apr 16, 2023 at 17:20
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The most common reason I have seen for premature failure of a thermopile is the pilot flame is set too high. You need to look at the specs on your system to find out what the output of that thermopile is supposed to be, and, with your meter leads on the two-wire cable coming off that thermopile, read that output. Adjust the output by adjusting the supply of gas to the pilot flame, to whatever the spec is supposed to be.

This measurement will be in millivolts, the symbol on your meter looks like this: mV. You need at least a reasonably good meter & leads to trust your readings.

A Thermopile has a hot zone (the upper end) and a cold zone. If the Thermopile is overheated (i.e. the pilot flame applied to it is too high) this will cause the cold zone (the lower end of the Thermopile) to be too warm, leading to premature failure.

Up to the time of outright failure, you'll probably see the thermopile fail, and then just cool off and sit there doing nothing. You come along realize it's not working, relight the pilot, and it all works fine... If this continues, with that pilot flame adjusted too high, it will eventually destroy the Thermopile. Is this what's happening?

Many thermopiles can produce up to about 800mV, but that doesn't mean they should be set up that high. In my experience, they, not under load, should produce around 500mV. Under load is generally around 350mV-400mV, and if it's much lower than that adjust the pilot flame and check again. If that doesn't get it: clean the Thermopile, pilot orifice, & electrical connections, then test again. If that doesn't get it replace the Thermopile.

While you're in there you should clean up all the above-mentioned and check everything if you feel there is ANY reason to. This stuff is expensive to have service, but it saves to be thorough.

It is said many thermopiles have a special thin coating on them (probably copper) that can be damaged by excess cleaning, like using sandpaper. Some suggest using steel wool to clean them. All I have ever used is a fine SS wire brush and have never had a problem with stripping off this coating.

It is also remotely possible the coil this thermopile is energizing is partially shorted across itself, &/or to the ground, but that is unlikely. If that were so very likely the coil would eat itself in short order. You could test for this with a decent meter, testing to ground and across the windings of the coil. In order to know the resistance of that coil you'd need that spec from the maker of the gas valve, and good luck getting that out of them... :)

Leaving that pilot flame on 24/7 WILL cost in gas, and in some additional cooling for your house during the warmer days of the year; over time this adds up. You may want to consider leaving the fireplace off during this time, unless you're using it regularly. There is also the very remote possibility of the gas valve failing open...

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