I am installing a new honeywell WiFi thermostat but the old battery operated thermostat had batteries and no "C" wire. I look at both the thermostat and the air handler and found an extra un-used blue wire but no "C" terminal at the air handler control panel. From reading other post and looking at my wiring diagram I guess my "C" would be the "B" on the terminal block but there is already a white wire on it. Can I hook up 2 wires on this "B" terminal? If not where do I hook up the blue wire coming from the "C" on the thermostat?


terminal block

1 Answer 1


The schematic image you've attached is difficult to see, so I can't say for sure. Based on the wiring, I would say that the B terminal would work as C.

enter image description here

Since I'm not there and can't test things, I have to make some assumptions. Based on the wiring, it looks like the one cable goes to the thermostat and another goes to the outdoor air conditioning unit. Since the red wire from the A/C cable is grouped in on the Y terminal, and the white wire from the A/C cable is on the B terminal. One can reason that the B terminal should be connected to one side of the transformer coil, while R is connected to the other.

You can test this theory by setting a multimeter (or voltmeter) to AC volts, and testing between the R and B terminals. You should read ~24 volts.

You can either connect both the white wire that's already on the B terminal, and the new C wire to the screw terminal. Or you can connect the white wire and your new wire together with a spare bit of wire, using a twist-on wire connector. Then connect the other end of the spare bit of wire under the B terminal screw.

  • Thank you. That's is what I thought but did not know if having a white wire on that terminal already would cause any issues. Hooked it up and seems to be working ok!
    – user37468
    Commented May 12, 2015 at 2:25
  • 1
    If you're an Electrician, connecting two wires to a single screw terminal is wrong. If you're an HVAC technician, it's common practice.
    – Tester101
    Commented May 12, 2015 at 2:32

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