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When we moved in, our natural gas fireplace worked well: Flick the switch & it would ignite.

Years later, it started acting erratically: Sometimes, the switch worked, other times, no luck. We'd try turning the switch off & on again a few times ("Hello, IT. Have you tried turning it off & on again?") and occasionally that would get it going.

Most recently, the workaround hasn't, & the burners fail to ignite 100% of the time. The pilot is fine.

Knowing the fireplace doesn't use the house power (we could use it even during an outage), I figured the voltage was low enough for me to troubleshoot. I disconnected the switch and tried to turn on the fireplace by carefully joining the conductors. This worked 100% of the time. So, I believed the fireplace was OK and decided the switch needed to be replaced.

Since I had it open already, I immediately installed a spare switch I had lying around. This worked well at first, turning the fireplace on & off 100% of the time on that first day. However, the next day, it was acting up again and failing most of the time.

I'm thinking it worked that first day since I'd warmed up the fireplace by connecting the wires, and subsequent attempts that immediately followed, using the new switch, could take advantage of the higher starting heat that generated just enough voltage. However, once the fireplace cooled to the pilot-only ambient temperature the next day, it wasn't enough voltage.

What kind of switch, exactly, should I be getting to fix this? I checked our big-box hardware stores but their switches all seem rated for typical 120V+ use. I spotted thermostats in the fireplace section, yet no switches. Where, generally speaking, might I find what I'm looking for? I'd prefer just a wall switch, and I don't want to call somebody in to fix this.

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  • Do you have a model number for either the old switch(es) or the fireplace?
    – K.A.Monica
    Commented Jan 12, 2014 at 21:43
  • Or pictures of the old switch?
    – bib
    Commented Jan 12, 2014 at 22:23
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    The old switch and the one I tried next are both standard, unremarkable 120V wall switches, the same as anywhere else in my house. I'm guessing the home builder didn't bother with the correct kind of switch for a fireplace though I'm not sure why it would have worked better earlier on.
    – Chris W. Rea
    Commented Jan 12, 2014 at 23:50
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    @AlekDavis Replacing the switch didn't solve our problem; some were better than others but all had issues. The answers below about the thermopile are spot on; make sure it is clean and generating sufficient voltage. The battery trick also worked but that was a workaround to a poorly functioning thermopile, and not the solution. Also, make sure your fireplace is in the right "mode". One season we had left the "ON - PILOT - OFF" switch set to "PILOT", which for our fireplace only permitted the pilot to stay lit, while preventing the switch from turning the fireplace fully on.
    – Chris W. Rea
    Commented Jan 3, 2017 at 0:57
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    BTW, I also came across this post at one point and I ought to link to it here: DIY: Gas Fireplace Won’t Light? How to Clean your Thermopile and Thermocouple
    – Chris W. Rea
    Commented Jan 3, 2017 at 1:00

5 Answers 5

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The thermopile in your fireplace puts out millivolts, nothing near the 120V the light switch was designed for. It's probably just a matter of finding a switch with a low enough on resistance. A generic low voltage switch from a electronics store, or ripped out of a toy, would probably do it (for example a 12V SPST). Really here the smaller the better, but beware of anything too cheap as that may put you down the wrong path again.

from discover circuits magazine

There are some fireplace specific forums which could help, particularly with a model number given. An example is http://www.hearth.com/talk/ . The above image is from "Discover Circuits Magazine (December 2009 Issue 4)", which has a guide on this exact topic along with a clever booster solution from one "Mr. Wily Widget". Find it over at http://www.discovercircuits.com/dc-mag/Issue_4/pg-7.htm

You can also try cleaning the thermopile which may have crust of soot. Measure the voltage between those two exposed wires to see if your cleaning makes an improvement.

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I had the same problem, tried everything, replaced the switch and the thermopile, no avail. Then finally I removed the pilot light assembly, the top just pops off and using a straw blew a bunch of dust out of it. The result was a better flame on the thermopile which allowed the valve to open, try that.

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In my previous house I installed an X10 relay to trigger the fireplace from across the room. I never had the switch problem or a problem with the relay.

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  • Interesting. What might make the X10 relay work well? Might it be designed better for varying voltages, including lower voltages?
    – Chris W. Rea
    Commented Jan 15, 2014 at 20:30
  • Ordinary wall switches sometimes get oxidation or other junk on them which; when switching a substantial load, the current flowing through them can help clear off the junk. Switches which are designed for switching smaller amounts of current generally use contacts plated with corrosion-resistant metals to avoid that problem; further, some magnetically-operated reed switches may be hermetically sealed, with nothing inside that could corrode them.
    – supercat
    Commented Jan 22, 2015 at 22:24
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I had the same problem, intermittent flames, sometimes no response no matter how long I leave the switch to on position. Thanks to the knowledge here I measured the voltage on the wire and it was 320 millivolts. So I cleaned the contacts and the wire and reassembled the switch. Voila, works perfectly now. Thanks everybody. Just a little sandpaper on the wires and contacts.

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Does anyone know what voltage a "healthy" thermopile produces? A little Googling does not give me a clear answer; I've seen everything from 30mV to 750mV, but I am leaning towards 750mV being the correct answer.

I have the problem described in this thread and my thermopile is producing 300mV DC from the pilot light, measured at the switch. I've taken an AA battery and lit the fireplace at the switch, and if I turn the switch on while the fireplace is burning the 300mV is enough to hold the valve and keep the fireplace burning, but apparently not enough to open the valve. In addition to looking for answers on the expected voltage from a thermopile, I thought the approach of using a AA battery at the switch to light the fireplace might prove useful to others.

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    You haven't actually said how you use the AA battery. Please update your post to add that information, preferably with photographs or a circuit diagram. Thanks!
    – Niall C. ♦
    Commented Jan 11, 2015 at 17:14

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