Fifth wire

Discussion in 'Electrical' started by paulmars, Feb 9, 2018.

  1. paulmars

    paulmars

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    My HVAC thermostat is 4 wire system W,R,Y,G. I just bought a new thermostat that uses 5 wires, w/extra Common connection. I was considering running a 5th wire, no easy task. then i removed old thermostat and pulled the wires out of the wall and there is a 5th black wire that was not used. So, i go to the furnace in the garage and look at the control board and there are the same four wires hooked up to the board and the fifth black wire not hooked up. So, all i need to do is connect this wire to common or furnace ground, right?

    Before i did that, I checked several things.

    At thermostat end of the wires with the thermostat disconnected I get 28 volts AC between R and each of the other three, but between R and the black I get volts starting at 7, 8, or 9 volts then it climbs up rapidly at 1st then slowly and by the time it gets to 12.x it is increasing very slowly. Now i go to the furnace and I get the same 28 volts between R and each of the others, except black. Between R and Black I get zero.zero volts. Between the black and ground is also zero.zero volts.

    ive gone back and forth several times and get the same.

    How is this possible and what does it mean?

    tks,
    pa
     
    paulmars, Feb 9, 2018
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  2. paulmars

    paulmars

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    paulmars, Feb 13, 2018
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