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    rsavellone's Avatar
    rsavellone Posts: 5, Reputation: 1
    New Member

    Aug 10, 2011, 05:48 PM
    Connecting batteryless thermostat with a "C" (fifth) terminal to my furnace
    Thank you MyDad'sGirl77. Though I never thought of my problem as a plumbing problem, your response was most helpful, and it spawns new questions in my mind. My transformer leads of course are connected somehow to those four (R,G,W,Y) terminals on my furnace. My multimeter shows 24VAC between R and each of G,W, & Y at my furnace. There is no voltage reading between any of the other terminals. I'm guessing R is the hot transformer output. If this is so, why could I not jump from R to C at my thermostat? The thermostat installation literature says "C" should be connected to 24VAC. If not a jump from R, how about a jump from Y?
    wmproop's Avatar
    wmproop Posts: 3,749, Reputation: 91
    Ultra Member

    Aug 11, 2011, 11:44 AM
    If you jumper red to y, c or anywhere else you probably going to fry the transformer
    The only wire that's supposed to be hot is the red which is power coming from the transformer then when the tstat is set to cool it then sends the 24V power to contactor and to fan control
    Same way if tstat is set to heat ,the tstat then is sending 24V to furnace startup relay
    Setting the tstat to fan only sends 24V to fan control
    The c/common post is kind of like a ground for the tstat so it can operate off the 24V coming from the transformer so there will be no need to put batteries in the tstat itself,, don`t know of any other way to explain

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