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

    Nov 17, 2007, 09:48 PM
    RTH7500 - old thermostat has 8 wires - how do I wire it?

    I recently purchased an RTH7500 and went to wire it. My old thermostat is a honeywell T7512. I suspect it is from 1994 when the house was built. I can't find anything that will tell me how to wire this... Since I have 8 wires coming out of the wall, and all 8 were connected...

    On my old thermostat, I have

    E Y O R C G W2 B attached. Any idea how I wire the new one? The guide says if you have O C and B - stop... call a contractor...

    I believe I have a heat pump, I do not know if it is multistage/etc.

    Thank you!
    Stratmando's Avatar
    Stratmando Posts: 11,188, Reputation: 508
    Uber Member

    Nov 18, 2007, 07:34 AM
    If comfortabe in air handler, I would open up and see wires used. Maybe only 4, and installer may connected unused wires to unused terminals. Inside unit will show what wires are used and for what. You may get lucky and find a schematic.
    jbanczak's Avatar
    jbanczak Posts: 2, Reputation: 1
    New Member

    Nov 18, 2007, 10:20 AM
    Thanks - good advice. So I climbed up in the attic and opened it up. Indeed, the blue wire was not being used. Here is what was up there:

    Red - R
    Black - C
    Orange - O
    Y/Y2 - 2 yellow wires
    G - Green
    E - Brown
    W2 - 2 white wires

    The unit is a Carrier, model fk4bnb005000aaa

    I'm going to try the honeywell site again - but any ideas would be appreciated!

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