A C wire adapter, also known as a common wire adapter, is an accessory used in HVAC systems to provide a dedicated power source, specifically a C wire or common wire, to a thermostat that requires it. It is designed to address situations where a C wire is not readily available in the existing wiring setup.

How does a C wire adapter work

A C wire adapter (common wire adapter) works by adding a new C wire to the thermostat by repurposing the existing wiring between the thermostat and the furnace.

A typical C wire adapter has 4 terminals at the input to connect the wires from the furnace, and five wires at the output; to connect to the thermostat. This fifth wire will serve as the C wire or common wire to provide the necessary power to the thermostat.

Which wires are repurposed to add the new C wire

The specific wires that are repurposed to add the new C wire with a C wire adapter can vary depending on the wiring configuration of your HVAC system. However, commonly repurposed wires are G, Y and W;

  1. G wire: In many HVAC systems, the G wire, which is typically used for the fan or blower control, can be repurposed as the C wire. The C wire adapter connects to the G terminal at the furnace and provides a new connection for the G wire at both the furnace and the thermostat.
  2. Y wire: In some cases, the Y wire, which is typically used for cooling control, can be repurposed as the C wire. The C wire adapter connects to the Y terminal at the furnace and creates a new connection for the Y wire at both the furnace and the thermostat.
  3. W wire: The W wire, used for heating control, can also be repurposed as the C wire in certain setups. Similar to the above scenarios, the C wire adapter connects to the W terminal at the furnace and establishes a new connection for the W wire at both the furnace and the thermostat.

Are C wire adapters universal?

C wire adapters are universal, they can be used to substitute C wire for any thermostat. However, the wiring to and from the adapter will vary depending on the type of HVAC system you are using. This is often described in the user manual that comes with your system.

Also, C wire adapters work only on 24 Volt systems. It cannot be used on millivolt or high voltage systems.

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 to install a c wire adapter.

How to install a 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 a transformer). I am using Venstar – Add a Wire adapter to demonstrate the installation.

Common wire adapter
Venstar Add a Wire Kit

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

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

Installation steps at the thermostat:

The Venstar Add a Wire kit has the Venstar device (which is to be attached to the 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 split 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 to 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 the furnace circuit board
  3. Connect the ‘common wire’ (in brown color) from the device to ‘C’ terminal on the furnace circuit board
  4. Remove the wire on ‘G’ terminal (in the furnace circuit) and connect it to ‘C’ terminal on the furnace circuit
  5. Remove the wire on ‘Y1’ terminal (in the furnace circuit) and connect the ‘Yellow’ wire from venstar device to the Y1 terminal.
  6. Connect the ‘Green’ wire from venstar device into the ‘G’ terminal on the 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 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.

What is a C wire transformer?

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 come 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 the 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 quite easy to set up 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 your thermostat location. Another issue is the long cable running inside the room (not hidden inside the wiring conduits), which tampers with 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 to the furnace box, and finally make some more connections on the thermostat terminals.

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

How to install a 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 models. You’re free to buy any other model you feel good about.  Take a look at the Fyve C wire transformer image given below.

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

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

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 the C terminal of your thermostat. Once you finish this connection, plug the transformer into 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.

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