How To Identify A Thermostat Common Wire (C Wire) & What It Does For Smart Thermostats

In this video we teach you how to identify a common wire also known as a c wire and what it is for when installing a smart thermostat.

How To Identify A Thermostat Common Wire:
1. Turn off the power to your existing HVAC system
2. Remove the existing thermostat from the wall.
3. Inspect the existing wiring to see if you have a c wire also known as a common wire. If you have a common wire it will be connected in the C wire terminal of your existing thermostat.

It's important to note that even if you don't have an C wire currently wired to your existing thermostat you may still be able to wire up a smart thermostat that requires a C wire. There are several options.

1. Check for extra wires
Check your existing thermsotat wiring for extra wires. Sometimes there is an unused wire in the thermostat bundle that can be used as a C wire and wired up to the new thermostat and it will also need to be wired up to the existing HVAC system control board.

2. Run a new common wire
You can run a new C wire from your thermostat to your HVAC system.

3. Use your G wire as a C wire ( Common Wire)
You can use your G wire which is the independent fan control as a C wire. You just rewire the g Wire to the C terminal on both the thermostat and at the HVAC system control board and then you have a new common wire. The only downside to this method is that you no longer have independent fan control.

4. Install A Common Wire Adapter

[meo]

Best video explaining C-Wire that I've found. Tks.

[kylemlsantos]

Thank you for the explanation, it took a long time to find out what C actually was.

[walleyeman]

Thank you, even somebody as dumb as me understood this. Very helpful!

[bm]

Very clear explanation. Thanks for sharing

[sgee]

In case if it helps someone, if there's no C terminal on the thermostat, don't attack the blue wire to anything. Tape it and put away.

[spiritadvertizing]

Excellent video in every respect. Only one situation NOT covered. How can a person install a C wire to an OLD SYSTEM NOT HAVING A MAIN MODERN MOTHERBOARD ON THE FURNACE???

[chuckbiller]

Best explanation I've heard. Thanks!

[MaquinasDePalomitas]

I had to log in just to like n subscribe, Great explanation, thank you so much!

[jlaughlin]

this was very informative but I dont have an extra wire so I was going to move G to C, as explained. However, you say change it to C at the HVAC system. I have a split system so are we changing it at the heater, condensor, or both?

[amjadarafah]

Thanks a lot for the great video. I have AC (cold/hot) and there are 3 wires only. The existing thermostat has a digital screen but no back light even. Any suggestion on how can I install a smart thermostat? The AC is a split unit where there is an external unit and ceiling mounted internal unit.

[macdameron]

I would double like this video if I could

[ptg]

I vaguely recall.... After moving G wire to C, don't you need a jumper wire between G terminal and Y terminal ?

[captainawesomeq]

This was ridiculously informative, tysm

[javierreyes]

Hi, can i used first generation nest with a heat pump sistem??? If so do you have a video explaining how to install
The wires

[grride]

When a disconnected C Wire is found at the thermostat, do you assume it’s connected at the HVAC Unit? Is there a way to determine this at the thermostat?

[CyrusT-fboj]

I'm not a professional, but I wouldn't be comfortable getting rid of my ground for any wiring (even low voltage) running through my walls. That just seems like asking for trouble.