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Lennox Furnace G27M Series – Unreliable operation: pressure switch or else?

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  • Lennox Furnace G27M Series – Unreliable operation: pressure switch or else?

    I own a Lennox G27M3-75A-2 two stage furnace and have the following question. The furnace has been installed in 2000 and has been running fine for many years. But lately this furnace has given me trouble on and off over the last year or so. I live in Alaska and have to figure out this latest issue with winter already here!

    I’ve had several HVAC technicians repair several problems over the last year. The main control board has been switched out and replaced with a Lennox 83M00 Sure Light replacement kit last Nov 2007 because the furnace was misfiring and the board had black spots on the back and was therefore diagnosed to be faulty. The flame sensor has been cleaned already several times to resolve some problems.

    I recognize this furnace contains two circuit boards: the main board and another board that controls the two stage mode switching. The furnace has been permanently set in one stage mode. I am far from an expert here, but you get to know your furnace real well when you’re sick of paying over $200 for every visit.

    This time my furnace throws the following problem at me. Here is the scenario: the furnace works fine and provides heat. At some point in time the call for heat has been satisfied and the thermostat switches the furnace off. But somehow the exhaust blower (combustion air blower) sometimes stays on, leaving the two pressure switches energized (closed), setting the furnace up for failure next time there is a call for heat. Because when there is a next call for heat, the furnace recognized that the pressure switches are stuck closed and therefore throws an error code (LED#1 Off, LED#2 Slow Flash). This situation happens sometimes (once or twice a week), but that would be enough to freeze my home when I am away!

    By now the main board has been replaced (Nov 2007), and I just replaced both pressure switched myself, although nothing appeared to be wrong with the ones I took out. I tested them by sucking on them and my meter showed the relays close and open fine.

    The furnace has not failed on me yet, but I do not trust it. I have to know what causes this and I am not so sure it’s the pressure switches alone. I believe my exhaust pipe is clean and free from debris, but have not checked this. I will try to attach a copy of the manual to this post.

    1. Does someone recognizes this situation?
    2. Do you believe two new pressure switches solve my problem?
    3. The manual tells me that one of the causes can be that a pressure switch is stuck close. Isn’t the pressure switch stuck close because the exhaust fan is on? What switches the exhaust fan? The main board? Can the relay be stuck closed that switches the exhaust fan? Last weekend the failure appeared again and I tapped on some relays on the main board and the exhaust fan switched off!
    4. Can it this be a problem with the main control board again? I checked the back and it does not seem to have black spots. I have not been able to check the other board, the two stage control board.

    Your help will be greatly appreciated!

    Thank you,
    Attached Files

  • #2
    Re: Lennox Furnace G27M Series – Unreliable operation: pressure switch or else?

    Just worked on one of these a couple weeks ago. The 2 stage board and wiring harness were burnt up. POS!

    The pressure switches do not turn the inducer on or off so replacing them was unnecessary. The inducer gets is signal from the board. Sorry to say, but you need another new board. Welcome to the wonderful world of Lennox.



    • #3
      Re: Lennox Furnace G27M Series – Unreliable operation: pressure switch or else?

      I have been changing out 2-3 Robertshaw / Lennox spark ignition modules a week on the older G series units...

      Back on topic, Ruddacguy is right, There is a fault in the main control board... I would have an electrican take a good look @ your homes electrical system, I'm willing to bet some surge protection may help with some of these issues.