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The efficient functioning of smart home gadgets, such as smart thermostats, commonly necessitates a constant stream of electricity, irrespective of their usage frequency. By frequently using a C-wire, you can ensure a stable electricity supply, thus providing an unbroken power flow to the device.

If your system doesn’t have one, you have options, read on!

TLDR: Quick links to “Option 1: Install a C-Wire Adapter” are below.

8.2 Venstar
Top PickA quick and reliable solution to the C-wire problem.
7.9 Honeywell Home
Runner upPerfect for all Honeywell smart thermostats.
5.8 Fyve Global LLC
The easiest option for smart thermostats without adding a C-wire.

What is a C-Wire On a Thermostat?

A “C-Wire,” also known as the common wire, serves as a return path for continuous low-voltage power in your thermostat. This feature is essential for thermostats that require consistent power for functionalities such as Wi-Fi connectivity, backlighting, and digital displays.

In a standard residential HVAC system, the thermostat is connected to the system via a series of differently colored wires. The C-wire, often colored blue or black (though this can vary), provides this necessary return path.

While older thermostat models or non-programmable thermostats may not require a C-wire due to their limited functions and lower power requirements, many advanced “smart” thermostats do.

Not all HVAC systems include the C-wire in their thermostat wiring strip. That’s when you might want to consider possible workarounds such as:

  1. Installing a C-wire adapter
  2. Repurpose an unused thermostat wire
  3. Transform the G wire into the C-wire
  4. Replace the thermostat cable with one that includes a C-wire

As you can see, I’ve listed them from the most convenient to the least, so let’s dive into these solutions.

Thermostat Wire Colors
The C Wire is an essential component of modern thermostats

At SmartHomePerfected we always advise readers not to undertake any work they are not competent in performing and if in any doubt, it is best to seek the assistance of a professional.

Tools Required

If you’re hoping to carry out any of the options listed below, I highly recommend gathering all the necessary tools and fasteners so you can complete this DIY project safely and without having to constantly run back to the hardware store. 

So for installing a C-wire adapter or replacing your thermostat, you will need:

  • Philip’s head screwdriver
  • Flat head screwdriver
  • Wire cutters/strippers
  • Needle-nose pliers
  • Splice connectors or wire nuts/wire connectors
  • Smartphone or camera (to take a picture of the thermostat wires)
  • Non-contact voltage tester (to check for live wires before working on them)
  • Label maker or similar tool (to label the thermostat wires)
  • Electrical tape
  • Appropriately-sized zip-screws/self-tapping screws
  • Double-sided tape

Option 1: Avoid Rewiring with a C-wire Adapter

Instead of dealing with the hassle of installing a new thermostat or running a new C wire from your thermostat to the furnace control board, you can install an adapter to create a virtual C-Wire.

This approach, although simple, requires a certain amount of electrical work so make sure you take extra precautions and have the necessary tools for installation. 

Always remember to shut down the power to the thermostat and the HVAC system either at the electrical outlet, breakers, or for added security: both.

Top Pick
A quick and reliable solution to the C-wire problem.

The Venstar Add-A-Wire accessory is a tailor-made solution for older homes without a C-wire connection. The ACC0410 can help you install your smart thermostat without having to resort to costly measures like installing a new thermostat cable.

Top Features
  • Maintains a stable connection without compromising on your thermostat wiring
  • Compatible with 4-wire and 5-wire systems
  • Can also be used to extend any thermostat wire, not just the C-wire
  • Compatible with almost all thermostats on the market.

The Venstar Add-A-Wire Accessory is the most reliable and versatile solution to fix a thermostat without a C-wire.

This adapter boasts a straightforward installation process and is compatible with both 4-wire and 5-wire systems, helping you literally “Add a wire” to your existing thermostat wiring cable without going through the fuss of getting help from an HVAC company.

However, specific thermostat models with automatic load detection features i.e., advanced thermostats that read information from the HVAC system, might run into interference. So make sure to verify compatibility with the manufacturer.

But if you’re sure that your thermostat only measures the indoor temperature and simply commands the HVAC system, you should be fine for the most part. 


To install the Venstar Add-A-Wire Accessory:

  1. First, turn off the power to the HVAC system at the breaker or the corresponding wall outlet.
  2. Then, unscrew or remove the thermostat panel from its wall mount/base plate
  3. After that, take a photo of the thermostat wiring configuration on the base plate. 
  4. Also, take another photo of where the other end of the thermostat wiring strip connects to the furnace or HVAC system. Usually, this is the control board inside the furnace. You can identify the terminals by their colors and the number of wires. 
  5. If any two (or more) wires look alike, make sure to label them accordingly. If you don’t have a label maker you can simply mark them with strips of electrical tape and create a legend to recognize the wires. 
  6. After organizing your thermostat wires, you can then remove the G wire from the terminal and reinstall it to the C terminal. 
  7. Then, remove the wire connected to the Y/Y1 terminal on the thermostat
  8. Take out the Add-A-Wire diode wire and connect the blue wire on the thermostat with the blue wire on the diode setup using a wire nut.
  9. Connect the green wire on the other end of the diode wire to the “G” terminal on your thermostat
  10. Next, hook up the yellow wire to the Y/Y1 terminal on your thermostat.
  11. After that, you can hide the thermostat wires back into the hole and re-install your thermostat. 
  12. In the next step, open the door to your furnace or HVAC control board (where the thermostat wires meet the HVAC system)
  13. After making sure the power is off (using a non-contact voltage tester), mount the Add-A-Wire reassembly box near the control board so its wires can be directly connected to the thermostat terminals on the furnace control board. 
  14. You can mount it directly onto the sheet metal that’s holding the thermostat control board or stick it on there with double-sided tape.
  15. Then, connect the Red wire of the reassembly box to the “R” terminal of the control board. Don’t remove the existing red wire coming in from the thermostat.
  16. After that, take the common wire (brown wire) of the reassembly box and connect it to the “C” terminal.
  17. Then, disconnect the green wire that’s connected to the “G” terminal in your control board and connect it to the “C” terminal instead, so it matches up with the changes you made to the thermostat in the previous steps.
  18. Remove the wire connected to the “Y” terminal in the control board and install the yellow wire from the reassembly box into this “Y” terminal.
  19. Then connect the green wire from the reassembly box to the “G” terminal on the control board
  20. Finally, use another wire nut and connect the remaining blue wires together.
  21. Close the thermostat and restore power. Check if the thermostat powers on. Initialize any settings and check if the HVAC system responds to the thermostat commands. 
Runner up
Perfect for all Honeywell smart thermostats.

If you've already got a Honeywell thermostat and you’re looking for a way to add a C-wire without going through the trouble of rewiring, then say hello to the Honeywell C-Wire Power Adapter.

Top Features
  • Compatible with all Honeywell thermostats.
  • Compact enough to fit in tight spaces.
  • Easy to set up and install.

Considered our runner-up solution to C wire adapters, the Honeywell Power Adapter is fully compatible with all Honeywell thermostats and helps you create a virtual C wire without the help of HVAC technicians.


The adapter is surprisingly easy to install since you can connect everything on the furnace side and easily reconnect the cables on the thermostat according to the instructions. 

Besides, you can easily stick the adapter box into the furnace; no drilling is required. 

To learn how to set this up, check out the installation guide below.

The easiest option for smart thermostats without adding a C-wire.

The C-wire AC transformer is an ideal solution for those who want to use smart thermostats like ecobee, Nest, or Honeywell without the need for a C-wire installation.

Top Features
  • Works with ecobee, Nest, and Honeywell thermostats.
  • Surprisingly simple installation process
  • 20 feet of cable for ease of use.
  • Minimal wiring required.

The C-Wire AC transformer is the simplest approach to adding a C-wire to your thermostat. 

To set it up:

  1. Turn off the power at the breaker box
  2. Attach one wire from the adapter to the RC and C terminal
  3. Plug in the power adapter
  4. Switch the power back on, and reattach the thermostat's faceplate – that's it!

This straightforward option involves minimal wiring and ensures compatibility with many popular thermostat models.

Beyond that, the adapter has 20 feet of cable, allowing for easy installation even if the closest outlet is far away.

However, be aware that the transformer cable running or hanging along the wall may be unsightly, so I recommend hiding them or getting help from a professional electrician.

Option 2: Repurpose An Extra Wire

If you don't want to go through with setting up an adapter, you can check the existing wiring for a redundant thermostat wire, and repurpose that as the C wire. To carry this out:

  1. Turn off the power to the thermostat and HVAC system
  2. Remove the thermostat from the base plate
  3. Check behind the wall for an extra thermostat wire that's not connected to the terminals and coiled up around the thermostat cable (or in some cases, connected to a redundant terminal)
  4. Pull this cable and install it to the C terminal of your thermostat
  5. After that, head over to the furnace/HVAC system's control board and find where the thermostat wiring strip connects to the control board.
  6. Check for the color of the C wire you just installed and connect that redundant wire to the C terminal on your control board. 
  7. If it was connected to a previous terminal, you might need to check the HVAC system's specifications to confirm whether this wire was actually unnecessary.

Option 3: Substitute the G-Wire As The C-Wire

With no extra wires and the option to set up a new C-wire seeming like a nearly impossible feat, you can also reroute the G wire for the indoor fan/blower as the new C wire. 

However, if carrying this out, your system will lose independent fan control since the fan will only start up when the heating or cooling systems are operational. 

So make sure to take note of these conditions before proceeding: 

  • Make sure the thermostat is wired to a furnace and not a zone panel 
  • Make sure any humidifier, air purifier, or IAQ (indoor air quality) systems aren't connected to your indoor fan
  • Decide if you only want the fan to run during cooling mode (G and Y jumper) or only to run during heating (G and W jumper)
  • Don't mind losing independent fan control of the thermostat.

If you don't mind these conditions or don't have any ventilation systems connected to the fan, you can then proceed by substituting the G wire for the new C wire. Here's how:

  1. First, turn off the power to your HVAC system using the circuit breaker
  2. Open the thermostat panel and make sure the wires are securely connected to your thermostat back plate. 
  3. Then take out the G wire and plug it into the C wire of your thermostat, secure the terminals if necessary
  4. Head over to the furnace control board and locate the terminals where the thermostat wiring strip connects 
  5. Disconnect the G wire from this control board, you might have to unscrew the terminals first
  6. Connect the now disconnected G wire to the C terminal.
  7. Add a small jumper between the Y and G terminals to operate the fan only during cooling. Conversely, you can set up the jumper to run between G and W if you want the fan to run during the heating equipment. You can only pick either of these options.
  8. Reconnect the thermostat and reinstall the furnace door
  9. Turn the power to the HVAC system back on via the circuit breaker 
  10. Try removing the batteries and checking if the thermostat powers on, 

If the thermostat powers on, you’re good to go. That being said, I recommend re-installing the batteries as a backup so you won't have to re-initialize the thermostat every time a power outage occurs. 

Option 4: Install a New C-Wire

Installing a new C wire, or thermostat cable is not so straightforward. You'll have to reroute the cables back from the furnace or HVAC system into your thermostat. 

So depending on your situation, this can turn into a complicated DIY project so I suggest getting help from an electrician or HVAC specialist. 

To make rerouting the wires easier, I recommend connecting the new thermostat cable to the old one (using electrical tape) and pulling out the entire length of the old wire from the furnace side, when this is done, the new wire will have seamlessly replaced your older wire. 

Installing a new thermostat cable, although tedious at first, will make your system more future-proof, allowing you to switch to a different thermostat in the future and possibly upgrade your HVAC system for multiple stages and emergency heating options.


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