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  1. #1
    Join Date
    Aug 2001
    Posts
    212
    I have been working on a G27M5-120A-2 serial#6399b12127
    the furnace will not stay on high fire. Low fire runs fine gas pressure is 1.70 inducer pressure is .70'w/c on low. Furnace trys to ramp up to high fire pressure goes up to .84'w/c then drops out second stage circuit board relays start to chatter. This all happens in about 10 seconds. I have changed out circuit board on second stage and gas valve and no luck. Has anyone had this problem thanks Grove

  2. #2
    Join Date
    Apr 2003
    Location
    Iowa
    Posts
    763
    are you sure the pressure switch isn't causing board to chatter?

  3. #3
    Join Date
    Aug 2001
    Posts
    212
    It seams that the Inducer does not want to stay at the higher speed for very long. That would cause the pressure switch to not make. So I wonder what would cause the motor to not stay in the higher speed?

  4. #4
    Join Date
    Apr 2004
    Posts
    7,680
    Capacitor, motor winding. What's the high fire pressure switch setting?

    Also check vent sizing and HX for blockage and while your at it, make sure the drain is proper (assuming 90+).

  5. #5
    Join Date
    Aug 2001
    Posts
    212
    High fire pressure setting is .68'w/c furnace is 80% flue size is sized correct.

  6. #6
    Join Date
    Oct 2003
    Location
    west chester pa
    Posts
    343
    is the furnace sealed combustion

  7. #7
    Join Date
    Aug 2001
    Posts
    212
    The customer has had this problem for three years. Another company that has finally gave up had us see if we can figure it out. I really want to help this family out because when it gets cold they freeze because the furnace only runs on low fire.

  8. #8
    Join Date
    Feb 2003
    Location
    garland, texas
    Posts
    666
    what position is the furnace? upflow,down,horizontal? Stat-single or two stage? Two stage board jumper set for one,two or timed stage?

  9. #9
    Join Date
    Aug 2001
    Posts
    212
    Furnace is a upflow with a single stage stat T87. Circuit board is on timer for 8min.

  10. #10
    Join Date
    Apr 2003
    Location
    California
    Posts
    634
    ever consider calling Lennox technical support and have a service rep come out and assist you.

  11. #11
    Join Date
    Aug 2001
    Posts
    212
    That is where I am headed towards on Mon I thought I would see if anybody had any ideas over the weekend.

  12. #12
    Join Date
    Feb 2003
    Location
    garland, texas
    Posts
    666
    Ok you've got a single stage stat. Is the 2 stage board set for single stage? On single stage setting furnace will always fire high heat only, the 8/12/15 timer is not active in this setting. The 8/12/15 timer is only active if the board is set to timed operation while using 1 stage stat.

    [Edited by nvr2old on 02-06-2005 at 12:00 PM]

  13. #13
    Join Date
    Aug 2001
    Posts
    212
    The board is set on the 8 min setting for 1 stage. It does time out on the 8 min setting ramps up on the inducer for high fire gas pressure drops and second stage board starts to chatter and then drops out. Starts all over again on low fire same thing happens.

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