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

    Mar 8, 2005, 02:48 PM
    70's Day and Night Gas Furnace Common wire?

    I have a furnace that I need a common wire to hook up
    A new thermostat that req's a common wire-
    The Company that originally made the furnace, BDP - merged
    w/ Carrier I think-

    I have a Red and Blue coming out of the top of the transformer
    And black & white @ the bottom - 115v looks like according to
    The schematic-
    Took a reading w/ a voltmeter of the red/blue - hold a 24volt

    Can I use the Blue as a common wire- I don't have a terminal block to
    Connect to -

    Thanks for any help

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

    Mar 8, 2005, 05:10 PM
    My wiring FAQ, Usually there is a 24 volt AC transformer in the furnace with the secondary winding connected to a red wire running to the thermostat and a blue wire to the gas valve, A/C relay, and fan relay. From the thermostat there will be white wire to the gas valve, yellow to the A/C, and green to the fan. The thermostat is wired to switch the power from the red to the white, yellow, and green as needed with the blue completing the circuit. It may be wired to have the control wires return to the furnace and its controls and then a second wire goes to the A/C unit. Internal wiring may replace the green wire if the thermostat does not give you the option of fan only or continuous fan.

    If the thermostat is powered, it will need both the usually hot red wire and the usually blue common wire. There may already be a blue wire in the cable to the thermostat. Just connect it at both ends. Often the only blue wire used outside the furnace is out to the AC relay. My original furnace, installed in 1970, was wired up that way.
    dallison's Avatar
    dallison Posts: 2, Reputation: 1
    New Member

    Mar 8, 2005, 05:35 PM

    Thanks so much for the reply-

    Looks like my red and white go to the thermostat and then I have
    A blue that goes from the transformer to the gas valve
    And branches to the fan switch-

    It's single stage heat only so I don't have the AC wiring
    I had earlier verified that the red/blue wires hold 24V with a voltage meter and a 1k 1/2 watt resistor connected btw-

    I plan on using the Omnistat RC80 thermostat w/ the furnace
    w/ RS-232 control via a PC - The Omnistat req's a common so
    I'm sunk w/out it

    Thanks again

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