I am upgrading my old non-wifi programmable thermostats with Honeywell wifi models. These need a C-wire and my wiring only has 4 conductors and no C-wire (see image). I contacted Honeywell support and they suggest using a C-wire adapter, but it appears my system's air handler units for heating/cooling do not have a control board with a terminal block to connect the adapter to, I opened the side panel. They are First Co units that were installed when the house was built 30 years ago. There's a unit in my basement, and one in the attic. All the wiring connections are mounted on the outside of the units. Only 3 wires go into the unit beside a separate AC power line. Can I still use a C-Wire adapter with this configuration? What are my options to provide a C-Wire to my new thermostats?

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    – Traveler
    Commented May 10 at 1:59
  • Thanks @Traveler, but what do you mean here? Are you saying the brown wire going in to the air handler unit is the C-wire that needs to be routed somehow to the thermostat?
    – NetGeni59
    Commented May 10 at 13:48
  • Yes, you have to get the Brown wire to the thermostat
    – Traveler
    Commented May 10 at 22:20
  • FYI: This worked, thanks.
    – NetGeni59
    Commented May 13 at 18:30

1 Answer 1


If you can, run the Brown wire coming from the transformer to the thermostat. That would be your C wire.

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