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Lennox g43 not lighting

carlc
carlc Member Posts: 4
Hey guys, I have a Lennox G43UF furnace (90% efficient) that is intermittently not lighting. The ignitor will come on (I tested the ignitor at 16.5ohms). The gas valve is getting 24v when it should be. I also have a manometer hooked up to the gas valve and even when its failing, the gas valve is putting out 3.5 inches w.c. I cleaned the orifices as there was a little debris on them, I have also checked to make sure my intake and exhaust are clear. Any help would be much appreciated!

Comments

  • HVACNUT
    HVACNUT Member Posts: 4,590
    Assuming this furnace has been in working order before, 3.5" manifold is NG. It's not LP right? That's usually 10" manifold pressure.
    Is it a 2 stage gas valve?
    And its a hot surface igniter, or spark ignition? Most Lennox furnaces use HSI.
    If your getting 3.5" manifold for NG, there's nothing but the gas train and the orifices after that. If they're clear, it seems to be a bad igniter.
    If it was anything else, the gas valve would not open.
  • carlc
    carlc Member Posts: 4
    Yes, it has been in working order and is only not lighting one out of every 5-10 trials for ignition. It is natural gas, single stage gas valve with a hot surface igniter.
    I haven't yet had the issue again after clearing out the orifices. hopefully that took care of it.
    Thanks
  • JUGHNE
    JUGHNE Member Posts: 9,750
    If the igniter is also the flame sensor, it may ignite but not always sense flame, from my experience.
    If it is a combo device I would change it.
    If separate flame sensor, clean or replace it.
  • Brp814
    Brp814 Member Posts: 22
    What is the dash number for your furnace? If it is a dash 1 the ignitor resistance should be 10.9 to 19.7 ohms. Dash 2 and later units used a different ignitor. The resistance for those should be between 50 to 450 ohms. All G43 models use a 120v ignitor, and the control board ramps the voltage down each time there is a call for heat. On the 256th try the board is at its lowest voltage, then starts to ramp back up. This prolongs ignitor life. I worked on Lennox furnaces for 16 years, and rarely was there an ignitor failure with this system. Could be the control board is failing too. hope this helps.