Locating C -fifth wire

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  #1  
Old 02-09-18, 02:51 PM
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Locating C -fifth wire

fifth wire

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
 
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Old 02-09-18, 05:22 PM
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The C (common) connection is like the negative in a car. Everything in the furnace is connected to C. At the thermostat..... the R wire is 24vac ( like the positive in a car).

For heat.... R supplies 24vac to W.
For cooling.... R supplies 24vac to Y to start the compressor and 24vac to G to start the blower.

So measuring from R to one of the letters is going to show some voltage as that device is connected to C.
We need to either have your furnace model number or a picture of the wiring connections at the furnace. How-to-insert-pictures
 
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Old 02-09-18, 08:36 PM
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many thermostats especially the old mechanical units did not need a common wire at the thermostat.
 
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Old 02-09-18, 08:40 PM
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Ok.......and ??
How does that help here ?
 
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Old 02-12-18, 08:09 PM
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"It is called phantom voltage. The digital meters are overly sensitive and pick it up. When a "floating" wire runs in parallel with a hot wire it picks up some voltage due to electromagnetism. Normally nothing to worry about. "

from: https://terrylove.com/forums/index.p...th-wire.74050/
 
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Old 02-12-18, 08:21 PM
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I thought you were trying to find a C connection ?
 
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