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  1. #1

    Help troubleshooting a home gas furnace module and induced draft system

    Hello,
    I have an old Amana AC unit with Gas Furnace.
    It has a White Rodgers 50A51-225 module.
    I was about to trash and replace the entire unit but managed to get the AC working.
    The problem is that the gas furnace won't turn on.
    I don't want to spend the $200. on this since I'm borderline about replacing it,
    so I'm not sure that a new module is necessary, or if it is, that I would buy one for this.

    I understand that the sequence is that the fan must come on first,
    which activates the pressure (vacuum) switches, which in turn enables everything else to power up and ignite the burners.

    The problem is that the induced draft fan does not turn on when the thermostat is switched to heat.
    I get either two or three flashes, indicating pressure switch open or short.
    The induced draft fan has two speeds and the motor runs when I give it direct 120vAC on the high and low wires.
    There are two different pressure switches that tap off of the single tube.

    Can somebody answer the following questions?
    Why does the induced draft fan have two speeds.
    What speed does it usually run at?
    Why are there two pressure switches? (My suspicion is that the vac has to be enough
    to trip one, but not enough to trip the other..)

    If the fan doesn't turn on when switched to heat....
    is the module bad?
    I'm thinking that there has to be a relay on the module board that does the switching....
    can that relay only be replaced?

    Is there a way to bench test the module?

    I hope that's enough info..
    Thanks in advance for any help....
    thanks
    tim

  2. #2
    Join Date
    Mar 2009
    Location
    Mid-Mo
    Posts
    3,596
    Nothing personal but we don't give out DIY advice here.

  3. #3
    Join Date
    Jun 2012
    Posts
    116

  4. #4
    My apologies for posting.
    sorry for the inconvenience.

  5. #5
    Join Date
    Apr 2003
    Location
    Maple Grove, MN
    Posts
    1,431
    Some furnaces operate differently than others. Some will check to make sure the pressure switch(es) is open before allowing anything to start, and others won't care and will send power to the inducer motor first. I don't know which logic sequence your unit uses. My guess is that your furnace either needs a new pressure switch, or a new control board.

    As far as why you have a 2 speed inducer motor, usually they are used in 2 stage furnaces. There should be a wiring diagram in your furnace somewhere that will indicate how it is supposed to be wired.

    I am in no way advocating that you try to repair your furnace yourself, I am just trying to give you some info on how it may or may not work

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