If you’re looking to replace your old thermostat with a new smart or WiFi thermostat, you may face the problem of adding a new C wire/common wire (if your current thermostat wiring doesn’t have one in place). We’ve clearly explained everything about C wire in our detailed guide. If you don’t have a c wire in your current wiring (between the furnace and thermostat), the best solution is to use a C wire adapter – like Venstar Add a Wire kit or use a 24 Volts C wire transformer.

A C wire adapter (common wire adapter) is basically a small electronic device, which re-purposes the existing wiring between your thermostat and the furnace, to add a new wire to the system. This new wire will serve as the C wire or common wire to provide necessary power to the thermostat. Installing an adapter is pretty easy and can be finished within 30 minutes to 1 hour time. Anyone who knows how to change an electric bulb, will find it easy installing a c wire adapter.

A C wire transformer is basically a 24 volts step down transformer adapter, which has to be plugged into a mains outlet. This device has 2 output wires which should be connected to the thermostat. If you have an outlet point somewhere near the thermostat (most transformers comes with a 25 foot cable) , you can easily solve the C wire problem by connecting one wire to the C terminal and the other wire to Rc terminal of the thermostat.

C wire adapter or transformer ?

The installation method is entirely different for a C wire adapter (like Venstar) and a C wire transformer. A C wire transformer is quiet easy to setup as you just need to plug in the device and connect two wires to the thermostat. The problem with a C wire transformer is, you need to have a plug in outlet near to your thermostat location. Another issue is the long cable running inside the room (not hidden inside the wiring conduits), which tampers the visual beauty of your room interiors.

On the other hand, a C wire adapter is always hidden inside the furnace circuit box. There will be no wires running through the rooms or any such things that affect the interior aesthetics. A c wire adapter re-purposes the existing wires between the furnace and the thermostat. But the installation takes more time & a little effort compared to a c wire transformer. You’ll need to open the furnace circuit box, make some connections (according to instructions provided in the adapter kit), attach the kit onto furnace box, and finally make some more connections on thermostat terminals.

I’ll explain how to install both the devices using images and videos below. Let’s begin with a C wire transformer and finally move on to C wire adapter.

How to install C wire transformer ?

I am using Fyve C wire transformer as an example to demonstrate the installation process. Fyve – is one of the best selling and popular c wire transformer model. You’re free to buy any other model you feel good.  Take a look at the Fyve c wire transformer image given below.

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The installation steps for Fyve transformer (same for any other model) is detailed below.

24 volts c wire transformer
Fyve – C wire transformer – Source

The installation process is very simple. A C wire transformer has two output wires; connect one wire to Rc terminal and the other wire to C terminal of your thermostat. Once you finish this connection, plug the transformer to your mains power outlet. The installation is complete now and the thermostat will be powered upon switching ON the power outlet.

C wire transformer connection – Source

Watch the video given below and you’ll get a better idea of the installation process.

How to install C wire adapter

As I mentioned before, installing a C wire adapter is a perfect option as it leaves nothing externally visible (like those wires in the case of transformer). I am using Venstar – Add a Wire adapter to demonstrate the installation.

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Steps of installation: The installation is divided into two sections; 1) steps at the thermostat 2) steps at the furnace circuit board

Common wire adapter
Venstar Add a Wire Kit

Note: Make sure to cut mains power to the furnace and the thermostat before you begin installation.

Installation steps at the thermostat:

The Venstar Add a Wire kit has the Venstar device (which is to be attached on furnace circuit board) and a wire pair (which is to be connected to thermostat terminals). This wire pair is simply a wire splitter, where a blue wire is splitted into yellow and green wires

  1. Open the thermostat plate and make the following connections
  2. Remove the wire on ‘G’ terminal and connect it to ‘C’ terminal
  3. Remove the wire on ‘Y1’ terminal and pair it with the ‘Blue’ colored wire on the wire pair. Enclose the open pairing with the caps provided.
  4. Connect the ‘Green’ wire (on the wire pair) to ‘G’ terminal and ‘Yellow’ wire (on the wire pair) to ‘Y1’ terminal
  5. Close the thermostat box.

Installation steps at the furnace circuit box

  1. Attach the venstar device onto the furnace circuit box (you can drill the screws provided into the mounting hole)
  2. Connect the ‘Red’ wire from venstar device to the ‘R’ terminal of furnace circuit board
  3. Connect the ‘common wire’ (in brown color) from the device to ‘C’ terminal on furnace circuit board
  4. Remove the wire on ‘G’ terminal (in furnace circuit) and connect it to ‘C’ terminal on furnace circuit
  5. Remove the wire on ‘Y1’ terminal (in furnace circuit) and connect the ‘Yellow’ wire from venstar device into the Y1 terminal.
  6. Connect the ‘Green’ wire from venstar device into the ‘G’ terminal on furnace circuit.
  7. Now there is one wire left on the venstar device, a ‘Blue’ colored wire. Pair this ‘Blue’ wire with the wire we have removed earlier from ‘Y1’ terminal, and close it with the caps provided.
  8. The installation is finished, close the furnace box and you can power the furnace now.

The video given below clearly explains how to install Venstar Add a Wire adapter kit.

So we have seen how to solve the C wire/common wire problem using a 24 volts transformer or a C wire adapter.


1 Comment


    So when using this, will I still be able to run “fan” only, separate from heat or cool?

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