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    Wiring power transformer to furnace

    Asked Jul 5, 2006, 09:20 PM 4 Answers
    I have a lennox furnace & I am trying to replace my transformer. On my old transformer one side has a black wire coming out connecting to a black wire coming out of the furnace. On the same side there is a white wire connecting to a yellow wire on furnace. On the other side there is a red wire coming from transformer connecting to a yellow wire, a green wire coming from transformer connecting to a blue wire. On the new transformer from radio shack one side has 2 black wires and the other 2 yellow & 1 black. Where do I hook these wires up to? Help please...

    Last edited by mav_01; Jul 5, 2006 at 09:44 PM.
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    tkrussell's Avatar
    tkrussell Posts: 9,638, Reputation: 722
    Uber Member

    Jul 6, 2006, 03:39 PM
    Any chance of getting some pictures of these transformers and wires? Difficult to determine what the wires are for without more detail.

    Also need the new transformer part number. The two yellow and one black means the low voltage side may be two voltages, like 12 and 24 volts. Also need to be sure the VA rating of the new is the same or larger than the old one.
    mav_01's Avatar
    mav_01 Posts: 3, Reputation: 1
    New Member

    Jul 6, 2006, 09:15 PM
    Thanks for responding. I am going to attach some pictures of my old transformer. My new transformer is from Radio Shack and the part number is

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    I have never changed this before so I am a little confused because I am not sure what the 2 yellow wire connect to, also I am not sure where I connect the 2 black wires to. I am assuming that the black wire between the yellow wires is the ground. I think I do not need the ground because the new one mounts to the furnace itself and that would ground it.

    Thanks for your help.
    tkrussell's Avatar
    tkrussell Posts: 9,638, Reputation: 722
    Uber Member

    Jul 7, 2006, 04:25 PM
    Ok, on the new transformer the two blacks alone on one side are the 120 volt input, if there is a white line or tracer on one of these, it is the neutral and connects to the white input wire , and the other is black. If not then does not matter.

    The black with the two yellows may not be a ground, It may be called a center tap, which connects in the center of the secondary transformer coil, and supplies half of the voltage between the two yellows.

    Once you connect the line side, measure between each yellow and the center black to see what the voltage is. Even if no voltage, do not ground unless instructed to do so by the manufacturer.
    Between the two blacks should be 25 volts, which will connect to the blue and yellow from the furnace.

    This transformer at 25.2 volt and a max of 2 amps is 50 VA, which should be enough for a standard hvac system control power.

    Be sure to mount to clean metal for a good ground connection.
    mav_01's Avatar
    mav_01 Posts: 3, Reputation: 1
    New Member

    Jul 10, 2006, 09:15 AM
    Thanks for your help. My AC is finally working! Have a good one.

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