I want to install a Smart Thermostat to control my AC unit. The current "dumb" thermostat is battery powered and does not use a C-wire (see current thermostat wiring pic). When I look at the wiring diagram for the control board it seems relatively clear that a brown wire coming from the receptacle is the one that should be the C-wire (see pics). It appears that it is pigtailed to a white wire, however, and I am not sure if I should just attach the blue wire (which I plan to use as the C-wire to the thermostat) to this wirenut or separately connect wire the brown and the blue wires and separately cap off the white wire that it is connected to.

Old Thermostat Connections AC Blower wiring diagram Current connections to thermostat - location of what I think is the brown wire indicated to be C-wire in wiring diagram Wires emerging from the "recp" indicated on the wiring diagram

  • You are correct ! In your case the Brown is the C wire
    – Traveler
    Commented Jul 31, 2023 at 16:24

1 Answer 1


Just connect the blue wire in with the existing brown/white wires

You should incorporate the blue wire into the existing junction of the brown and white wires. That way, the blue wire will provide C to the thermostat without interfering with the C wire return from the condensing unit's compressor contactor, something that's very important to have otherwise your air conditioner won't work.

  • Thanks! Just the information I was looking for.
    – Steve Bell
    Commented Aug 1, 2023 at 13:54

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