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    rongreen's Avatar
    rongreen Posts: 2, Reputation: 1
    New Member

    Jan 25, 2005, 05:20 PM
    Wiring up a gas furnace
    Hi, my first post here. I just bought a used Lennox gas furnace for my workshop. The electrical cable was just cllipped off, leaving 6 inches of cable coming out of a fused on/off switch. It appears to be 12-3 with ground. There's red,black, white and ground. I would expect this is 220, with the red and black both being hot, but I'm not comfortable wiring it this way until I get some advice. Am I making the correct assumption? The sticker inside the furnace says its 110, single phase.

    What's up? Thanks,

    labman's Avatar
    labman Posts: 10,580, Reputation: 551
    Uber Member

    Jan 25, 2005, 06:56 PM
    I would open up the on/off switch and see where the wires go. It is possible the blower motor was replaced or rewired to use 220. The controls would still use 115, and need the white wire to complete the circuit. It is also possible somebody had some 12-3 laying around and used it in place of 12-2. Post back if you have more questions.
    rongreen's Avatar
    rongreen Posts: 2, Reputation: 1
    New Member

    Jan 25, 2005, 07:33 PM
    Looked in the disconnect switch
    The black wire is fused. (There was a 30 amp fuse in there!) The red wire heads on in and is connected to a terminal of some sort. The wire on the same terminal heads over to a purple block with about 10 tabs for wires to plug in to. I have no idea where it goes from there.

    The blower fan is labeled 110V.


    labman's Avatar
    labman Posts: 10,580, Reputation: 551
    Uber Member

    Jan 27, 2005, 08:35 AM
    Aruuuuuuuuuuugh! Sounds like it has been rigged. I hate trying to figure out messes like that. Where does the white wire go? If it doesn't connect to any of the circuits, it is possible, illegal I think, that the red wire was used as the neutral in a 110 circuit. Does the furnace have a schematic? Does it show the black and red connected to the L1 and L2 terminals? If so, I think you can wire it up, connecting the black to L1 and the white to L 2.

    If not, you will have to go through the furnace identifying each wire and what it does. The black wire should connect to one side of the 24 volt control transformer and also to the relays and switches controlling the blower. The white wire will connect to the other transformer terminal, the blower, and any relay coils operating on 110. Let me know what you find, and I will try to help you work through it.

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