Adding C wire for thermostat

TomNorthJerseyOctober 4, 2012

Got a new thermostat that requires a 24v common C wire. The current cable is a 3 conductor that I had previously labeled Rh, W and G when replacing the old round mercury thermostat a few years ago.

Looking for some advice before I get started.

Running the new wire shouldn't be a problem as it's pretty much a straight run through an interior wall and I can see where the wire comes out in the basement.

There is a small panel mounted on a wall which I believe houses the transformer (will double check when there) that has armored cable coming into it from the main breaker panel and going out through armored cable to the gas fired boiler.

The three conductor cable runs directly from the thermostat to the the boiler.

Old house, old wiring but in good shape. Boiler was replaced a few years ago.

Some questions:

Should I replace the existing wiring completely or is it ok to just add a single wire for the C wire?

What gauge wire is recommended?

Where does the other end of the C wire attach to? Somewhere on the boiler with the other thermostat wires or somewhere in that box on the wall?

Anything else I should consider?

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I suggest running a new set of thermostat wires. There are wire bundles made specifically for thermostats. You can use 18 or 20 guage wire.

The C wire is the common ground. It should go on the ground side of the transformer.

The G wire is normally the fan control. What does the G wire do in your set up?

    Bookmark   October 4, 2012 at 3:49PM
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I just looked up what the wire letters stand for. I assumed G was a ground. When I first replaced the old, gold colored, round thermostat (honeywell I think) with a different model programmable thermostat years ago I just remember following the instructions and labeling the wires without giving it much thought. I just looked up what the letters and G doesn't make much sense for my system. This thermostat only controls the boiler. The G wire is inserted into the G terminal. Everything has been working fine.

Looks like on a 3 wire system it should be R, W and Y not R, W and G like I have it.

Later tonight I'm going to check the connections at the boiler and see if G is even connected to anything. If not I can use that for C.

Found this video on a way to use a different wire for C. Any idea if I can do something like that if my G wire isn't usable?

    Bookmark   October 4, 2012 at 4:36PM
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Forgot video link

    Bookmark   October 4, 2012 at 4:37PM
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The end of the cable in the basement has 4 conductors. Red and White (maybe was yellow) were connected. The other two were folded back and not connected. Only saw 3 wires at the thermostat end. One of the must have broken off. COuldn't make out the colors of any of them. 50/50 chance I get it right the first time.

I couldn't make out any markings on the terminal block where the red and white wires were connected. There was one wire coming off the transformer and into that block. I'm guess that was C right?

Will go over over the schematic again and try and take a pic of the terminal block. The face is pointing towards the side, not the front and it's difficult to make out anything on it.

    Bookmark   October 4, 2012 at 6:00PM
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