HVAC Adding a Common

hvacthermostat-c-wire

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Got a new Honeywell WiFi thermostat and need to add a common. Currently the 3 common wires per the diagram (BL, BK, GY) are wired to one side of the AC Unit. Is there an issue if I piggy back a common off those wires (add a new wire to the wire nutted bundle).

Thanks!

Best Answer

Go for it man

That's precisely the right move in your case -- that's where the system wiring diagram expects you to tap your C wire at.

Related Solutions

Where is the common wire on the unit

You'll want to focus your attention to the wiring at the bottom of your last photo, that's where all the control wiring is.

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Don't touch the wiring on the primary side of the transformer, as it's at line voltage and could cause a nasty shock.

If you clip the zip tie holding the bundle of wires together, you should be able to get a better look at what's going on. I can't tell exactly what's going on from the photo, so I'll explain what would typically be seen.

You should see a cable with a red, blue, green, yellow, white, and possibly brown wires. This is the cable that runs between the air handler and the thermostat. You should see another cable with a red and white wire (it may include other colored wires as well), which runs between the air handler and the condensing unit. According to the schematic, there should be a red and brown wire coming from the secondary side of the transformer.

The red wire from the transformer should be connected to the red wire from the thermostat, this is your Rc wire. The yellow wire (Y) from the thermostat cable, should be connected to a the red wire from the condensing unit cable. The blue wire (C) from the thermostat cable, should be connected to the white wire from the condensing unit, and the brown wire from the transformer. The green wire (G) from the thermostat cable, should be connected to the black and green wire from the air handler.

At the new thermostat you should have two cables, one from the air handler and one from the boiler.

Remove the jumper between Rh and Rc, if one exists.
Connect the red wire from the boiler cable to Rh.
Connect the white wire from the boiler cable to W.
Connect the red wire from the air handler cable to Rc.
Connect the yellow wire form the air handler cable to Y.
Connect the green wire from the air handler cable to G.
Connect the blue wire form the air handler cable to C.

NOTE: This is all based on typical wiring, your wiring may vary.

NOTE: Some thermostats may only use the Rh and C wires for power, so supplying an Rc/C combination may not work.

Wiring – Is it possible to add a “C” wire to the electric heater

On the schematic, the BL wire in the section labeled "Low Voltage Field Connection" near the bottom of the diagram. That's the C wire.

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It appears as though the "C" wire is bonded to the chassis, so you should be able to connect the C wire from the thermostat to the chassis as well.

In the last photo, if you follow the yellow wire from the thermostat cable. It connects to a yellow wire from another cable, which I would assume leads to your A/C system. The blue wire from the A/C cable is attached to another blue wire, that blue wire ties in with all the other "C" wires. So you could connect the C wire from the thermostat, in with those blue wires as well.