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gas fireplace pilot outage

zepfan Member Posts: 315
I was working on a fireplace that has an issue with the pilot going out mostly when the main burner is on. The system has a robertshaw mv gas valve, and thermopile. No spark ignition,or external power source. The gas valve, and thermopile are brand new, the pilot was cleaned, and the batteries in the remote, and receiver are good. The open circuit voltage of the thermopile after one minute of holding the knob down is 625 mv. The closed circuit voltage with just the pilot on is 425 mv. When the main burner is on the closed circuit voltage drops to 160 mv, and stays there. Sometimes the fireplace stays on for four or five hours without the pilot going out, sometimes it is on for only twenty minutes. The closed circuit voltage is the same if the remote is in the circuit, or those wires are removed from the valve and a jumper is installed across those terminals on the valve to open it. The pilot flame is the same weather the pilot, or main burner is on, it envelopes the top 1/2" of the thermopile. Adjusting the pilot valve screw has had no effect on the condition, and the incoming gas pressure is 6" w.c. I am looking for suggestions on why with a brand new valve, and thermopile the closed circuit voltage drops so drastically when it is attached to the valve, and the main burner is on. Happy holidays, and thanks to all.


  • SlamDunk
    SlamDunk Member Posts: 1,072
    edited December 2017
    duraflame logs always burn. Never explode. Better yet, WPIX TV yule log on a flat screen in front of fire place.

    Mark Eatherton
  • pumbaa
    pumbaa Member Posts: 8
    natural or propane?? vented or non?? ODPS??
  • Big Ed_4
    Big Ed_4 Member Posts: 1,754
    Seems low on the millivolts ...Thought they drop out around 700mv ... Try bringing up the pressure to 7" WC ...
    I have enough experience to know , that I dont know it all
  • lchmb
    lchmb Member Posts: 2,997
    I know you said the pilot was cleaned but replace the pilot orifice and verify it's the right one. I would expect with a new thermopile at least 800 mv so something is off, although I usually dont time them but hold til it steady's out..
  • zepfan
    zepfan Member Posts: 315
    the system is vented, natural gas
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,837
    In order for a thermopile generator to work correctly, there has to be a differential in temperature across the thermopile. When the gas log turns on, the radiant energy from the main burner causes this differential in temperature to be less, and subsequently, voltage output is less, to the point that the gas valve drops out. Shielding the thermopile might help. I've also seen the gas valves drop out due to this same exposure to the intense radiant energy,

    Gas logs are a PITA in my opinion. They waste a lot of gas and generate HUGE amounts of carbon monoxide. And when they aren't generating CO, it's about as much fun as staring at the burner in your oven...


    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • lchmb
    lchmb Member Posts: 2,997
    but the gas companies love them Mark...;)
  • HomerJSmith
    HomerJSmith Member Posts: 1,336
    So, it has a receiver in the bottom of the fireplace controlled by a remote that turns the fireplace on or off. No thermostat.

    The thermopile just doesn't connect to the gas valve, but the voltage moves in a circuit with safety switches, usually thermodiscs.

    Rarely does one get 700mv out of a thermopile and 450mv is quite good in a circuit. This is the solution to your problem. The thermopile is good, your gas valve is good. You have resistance in your thermopile circuit, which is what is dropping your voltage to 160mv which isn't able to keep the pilot valve open. Where is the resistance?

    I had this problem on a newly installed wall furnace. I replaced 2 thermopiles, 3 blocked flue thermodiscs and a gas valve, all courtesy of the manufacturer. The furnace was still intermittent.
    All the same voltage problems that you have. Guess what it was?
    It was resistance in the thermopile circuit. I finally figured what caused the high resistance albeit it was intermittent. It was--drum roll--please, the blocked vent thermodisc. I had replaced 3 of them, who would have thought. All 3 of them were defective, the company had a whole stockpile of defective thermodiscs. The defect was in the connection of the 1/4" spade tab to the disc itself. It was loose. The rivet didn't bind the tab to the disc tightly so that the resistance went from 0 to infinity creating an intermittent operation.

    That was a time consuming, costly learning experience.
  • HomerJSmith
    HomerJSmith Member Posts: 1,336
    A voltage drop always implies resistance in the circuit. High voltage drop--high resistance.
  • Suzook
    Suzook Member Posts: 221
    edited December 2017
    Try adjusting your pilot a little higher. Ask me how I know?? I bet the flames are blowing out the pilot then the valve shuts off.

    Edit, sorry didn't see you adjusted pilot.