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ComfortMaker Furnace Problem

labanslabans Member Posts: 1

My 10 year old Comfortmaker a/c and gas furnace have a problem; here is the issue:

Due to severe electrical surges two transformers blew out on the original Honeywell ST9120 circuit board. The last time it blew the circuit board itself. So i purchased a new board which is a universal replacement and has its own on-board diagnostic. I installed the board and when I fired it up it began to give one flash which means open limiter switch.

I do not know what that is so I just checked all of the switches that measure heat or whatever and they all test the same for values. Also, the new board is not a direct replacement and the wiring diagram for it is slightly different so I am not sure I have the new board wired correctly. I had left off one of the wires because I was not sure where it went---it is the fat white wire coming from the blower, along with the other fat ones, red, orange, black and blue. I thought the white wire should go to neutral on the board, but it show having 120v when the switch is closed for operation.

Anyone who can help me with this rats nest? I could not get local help and the temperature is dropping---just before the last surge, everything worked beautifully. Can submit many photos if needed.

Thanking in advance.......


  • Tim McElwainTim McElwain Member Posts: 3,780
    What is the model number

    of the Comfortmaker?

    Did you replace the ST9120 with a Honeywell ST9120U control? If you used the correct harness then simply follow the directions that came in the box for replacing the ST9120. Once the harness is attached the instructions give you wire for wire how to install the wires.

    Beyond that try sending a picture of how you have the board wired and I will try to help you. You said a light was flashing was it the status light.

    If you give me an e-mail I can send you some instructions, my e-mail is [email protected]
  • EmpireEmpire Member Posts: 2,343
    C Maker

    The flash you see can either be a heart beat, or an actual error code.  Assuming it's a limit trip as you stated, I'm not sure how you checked the switches, but with power off all limit switches should be closed and indicating a "0" on your ohm meter.  Your white wires most likly are common and make sure this is the case.  Common that should apply would be; common for...  line vac, t-former, induced draft motor, blower motor.  Your control wiring must be checked for the open switch or temp switch as you stated.  Unusual that a limit would pop, but check for any disconnected wires that may have been pulled during the installation of the new board. 

         Snap a pic of the new board as it currently sits in the unit.  I'll see if I can possibly help with any obvious connections.


    Mike T.
  • unclejohnunclejohn Member Posts: 858
    Reversed polarity

    Sounds like you have swithch your hot and netural wires. The fat white wire from the blower should go to the neturals on the board. Check back to your power switch and make sure the black wire reads 110 to ground and not the white wire.
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