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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?

Thermostat wires going to air handler

Air handler wiring diagram

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  • Where does the blue wire from your t-stat cable go to? Dec 25, 2021 at 23:13
  • The blue wire is connected to the RC terminal at my thermostat.
    – Blackhall8
    Dec 26, 2021 at 1:22
  • Ah, I take it it's connected in with the red wires at the air handler then? Also, what terminal on the thermostat is the red wire in the thermostat cable connected to? Dec 26, 2021 at 1:40
  • Yup, the blue wire from the t-stat is connected in with the red wires at the air handler. The red wire in the t-stat is connected to the RH terminal. Also, white to W, yellow to Y and green to G.
    – Blackhall8
    Dec 26, 2021 at 1:53

1 Answer 1

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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.

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  • So I should disconnect the t-stat blue wire from the red wire bundled at the air handler? Then connect that blue t-stat wire to the terminal on the relay where the brown wire is connected at? With the jumper in place on the new t-stat, the RC terminal will not have anything connected there, right?
    – Blackhall8
    Dec 26, 2021 at 11:32
  • @Blackhall8 yes on disconnecting the blue wire from the bundle of red wires at the air handler and moving it over to where the brown wire is connected at. The only thing that should be connected to RC when you're done is the jumper over to RH at the thermostat Dec 26, 2021 at 15:18

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