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Can weak thermocouple cause weak main flame (nat. gas)?

Motorapido Member Posts: 307
If a thermocouple is gradually failing, and generating the bare minimum millivolts to keep the gas valve open, can it result in a lean, weak, fluttering flame on the main burner? Or if the thermocouple is going bad, does the flame simply shut off completely? So in other words, is it a black/white issue? Thermocouple healthy = gas flame lit and burning fine. Thermocouple fails = flame simply shuts off.

Or can you ever have the following: Thermocouple weakening along the road to death = main burner flame continues to burn but burns lean and weak and fluttery. In other words, if the millivolts are weak from the thermocouple and at the bare minimum to open the main gas valve, is it possible that the weak millivolt output opens the main gas valve only partially? Or is the main gas valve a full-on/full-off valve without the ability to partially flow some gas?

(this is a natural gas, freestanding, direct-vent heater stove)


  • Sully1266
    Sully1266 Member Posts: 13
    If it”s a stove/ heater I assume it’s a millivolt system with the pilot generator producing enough milivolts to hold in pilot magnet on gas valve and enough to open the main gas on a call for heat going through all the limits. A weak thermopile ( which is a larger thermo couple that produces more milivolts then a standard lead) will cause a valve not to open unless you give it a little tap with a wrench. I’ve never seen a milivolt valve that opens only slightly but it is possible, milivolt systems are very temperamental even the lose of a few percent Voltage will cause a valve to not open. A good Powerpile producing 750mv is what you are looking for you need at least 250mv once connected to the circuit going through all limits and wiring to open the gas valve. A weak powerpile is always the first step before you look at the gas valve.
  • mattmia2
    mattmia2 Member Posts: 9,850
    edited December 2022
    No. A thermocouple holds the safety valve open or switch closed. If it doesn't produce enough current it snaps to the opposite state and must be reset with the button.

    It might be possible with a thermopile and certain valve designs.
  • MaxMercy
    MaxMercy Member Posts: 514

    If a thermocouple is gradually failing, and generating the bare minimum millivolts to keep the gas valve open, can it result in a lean, weak, fluttering flame on the main burner?

    I'm old enough to know that no answer is absolute, but the odds of a weak thermocouple causing that issue must be long indeed. Every thermocouple I've seen has been binary - it's good or it ain't. Thermocouples are cheap and easy, so go ahead and try one.

    Other than that, if the gas supply is sufficient, the valve sounds like your problem.

  • HomerJSmith
    HomerJSmith Member Posts: 2,483
    edited December 2022
    Of course not. I have never seen it. The thermocouple produces enough voltage about 25 to 30 MilliVolts to keep the pilot valve open. It will not open the pilot valve. That has to be done manually when you press down the red button with the knob on pilot setting.

    The main gas valve is opened by the thermostat 24V transformer. The flame from the burner tubes should be as the manual shows.

    A thermopile produces about 750 Millivolts which is enough to open the gas valve. However if measuring the voltage at the valve it will measure 350 to 400 MV.

    If you have fluttering flame on the burner tubes look to dirty burner tubes or high gas pressure to the gas valve which should be about 3.5"w/c output with a 7"w/c input or improper air shutter adjustment. A high gas pressure can cause flame lift off which may appear as fluttering.
  • Mad Dog_2
    Mad Dog_2 Member Posts: 7,208
    edited December 2022
    What I have seen, and it drove me MAD, was a standing pilot AND main gas burners on a new, gas steam Burnham IN boiler just GO OFF in the middle of a full fire call. I changed alot of parts until Timmie Mc Elwain helped me focus in and figure it out. Long story Short, the Watts 9D Backflow preventer nut had a small drip. I tightened it and forgot about it and kept troubleshooting. I isolated the problem to the Argo Relay box - everything else was replaced. (The husband was very nice and understanding but the wife wasn't even though I was going back for free every time). Looking at the top of the box where the knockouts are, I noticed a white chalky area about the size of the US dime right under the nut that dripped. Pulling the cover off, I looked underneath that area and saw more crust. It had dripped on to the cube relay and obviously damaged it. The maddening part was that it was so intermittent. It would be fine for a week then, bang! Off again. Plumbing can be a real B! and heating is on another level. I still never fully understoodit but basically Timmie said a short in the low voltage circuits COULD interrupt the millivolts of the gas circuit. Many told me it was impossible, but that was the end of the problem. Changed the whole argo box for good measure. I have a saying, if you don't love this business, you'll hate it! Mad Dog