Compatibility Nest Thermastat

[Mikeme]

Can I use these extra unused wires as a “common” wire. Or even use the blue, orange, or black wire?

[sporfrog]

One of the unused wires in the thermostat cable can be used for C (common) as long as it is connected to C or "common" at the furnace or HVAC unit. It looks like Blue is unused. It is often the color used for C (common). 

[Ryan_G]

Hi everyone,

Thanks for being here at the Google Nest Community Forum.

@sporfrog is right, thanks for the help here. You can also check the connection at the terminal block from your HVAC and verify the wire connections.

If you still need help, get back to us by replying to this thread.

Best regards,

Ryan

[Mikeme]

What terminal should the common wire be connected to on the HVAC? If the blue, black, orange and brown wires are not connected to the current thermostat would they even be connected to the HVAC?

[Ryan_G]

Hi Mikeme,

Thanks for the reply. 

The common wire color for a C wire is blue, but we could verify that by looking at the main terminal from your HVAC unit.

Regards,

Ryan

[Mikeme]

Do I need to cut power at the breaker, just to remove the cover on the HVAC to look at the wire connections?

[sporfrog]

Yes you should cut power. Use a circuit breaker at your electric panel or if the furnace has something like a light switch as a service disconnect you can use that.

Look to see if one of the wires in the thermostat cable not connected at the thermostat (blue, black, brown or orange) is connected to a terminal labeled C, or common. If there is a C terminal and there is no wire connected to it, connect blue (or any other unused color) to it.  Feel free to send a picture of the wiring terminals in the furnace if you are unsure.

Obviously only proceed if you are comfortable with this.

[Ryan_G]

Hey folks,

Thanks for visiting the Google Nest Community. 

Since this thread hasn't had activity in a while, we're going to close it to keep content fresh.

If you have additional questions, feel free to submit another post and provide as many details as possible so that others can lend a hand. 

Hope this helps!

Kind regards,

Ryan