Where to connect common wire from thermostat to First Co. air handler?

Question

I'm trying to install a new Honeywell WiFi 9000 thermostat which requires a common wire. When I removed my old thermostat, I can see there are two extra wires in my wall (black and orange wire) that are wrapped around the bundle of wires. Can I use one of those as my c-wire?

However, when I go to my First Co. air handler, I can see that the black and orange wires are not connected to anything. The air handler wiring diagram says the brown wire is common. Could I just connect that black or orange wire to the relay terminal where the brown wire is connected to?

Answer 1

Move the blue wire on the thermostat cable to join that brown common wire at the air handler and to C on the thermostat, then use a short jumper wire to join RC and RH on the thermostat

It appears that your installer lost the jumper between Rc and Rh that normally comes with older two-transformer-capable thermostats, so they used separate power wires in the cable for the two R terminals, which is silly. So, we're going to fix that silliness now instead of perpetuating it further, by adding a jumper between Rc and Rh at the thermostat, then swinging the blue wire in the thermostat cable over to C on the thermostat and the brown common wire on your air handler.

That way, you can stick with the de-facto standard color coding instead of doing something non-standard to work around previous non-standard-ness.