The C-Wire: What is it? Do I need one? What are my options if I don't have a C-Wire?

What is a C-Wire?

A common wire (C-Wire) provides Wi-Fi thermostats continuous power by connecting it to a heating and cooling system (e.g. furnace). C-Wires are required on most Honeywell Wi-Fi thermostats, with the exception of the Smart Round Thermostat (formerly known as the Lyric Round).

How do I check if my thermostat is using a C-Wire?

1. Turn off power

To protect your equipment, turn off the power at the breaker box or switch that controls your furnace or heating equipment.

Note: Turning off your thermostat won't turn off the power to the furnace or heating equipment.

C-Wire


2. Check that your system is off

Change the temperature on your existing thermostat so that your system starts heating or cooling. If you don't hear or feel the system turn on within 5 minutes, the power is off.

Note: If you have a digital thermostat that has a blank display, skip this step.

C-Wire


3. Remove your existing thermostat from the wall plate

On most thermostats, you can take off the thermostat by grasping and gently pulling. Some thermostats may have screws, buttons, or clasps.

Note: Don't remove any wires from your thermostat at this time.

C-Wire


4. Do you have a C-Wire?

Look at the existing wall plate's wiring: is there a wire in the C-Terminal?

YES – This means your thermostat is using a C-Wire.

NO – This means your thermostat isn't using a C-Wire. Please view next question to learn what to do if your thermostat is missing a C-Wire.

C-Wire

What do I do if I'm installing a new thermostat, but my existing one is missing a C-Wire?

If you're installing a new thermostat, but your existing one is missing a C-Wire, you have the following options:


Check for Unused C-Wire

Some homes have an unused C-Wire hidden in the wall. To check for an unused C-Wire:


1. Turn off power

To protect your equipment, turn off the power at the breaker box or switch that controls you heating and cooling equipment.

Note: The thermostat off switch won't turn off the power to the equipment.

C-Wire


2. Check that your system is off

Change the temperature on your existing thermostat so that your system starts heating or cooling. If you don't hear or feel the system turn on within 5 minutes, the power is off.

Note: If you have a digital thermostat that has a blank display, skip this step.

C-Wire


3. Remove your existing thermostat from the wall plate

On most thermostats, you can take off the thermostat by grasping and gently pulling. Some thermostats may have screws, buttons, or clasps.

Note: Don't remove any wires from your thermostat at this time.

C-Wire


4. Do you have a line voltage system?

Line voltage systems have thick, black wires with wire nuts, or are labeled high voltage (120V or higher).

YES – Your system may not compatible. Contact Support at 1-855-733-5465 to find a pro-installer in your area.

NO – Continue to the next step.

C-Wire


5. Take a picture of your existing wall plate's wiring

Make sure you get a good view of all terminals. You may need to reference this image later.

C-Wire


6. Remove any jumpers

A jumper is used to connect two terminals. It may look like a small staple or colored wire.

Note: Don't discard any jumpers.

C-Wire


7. Label wires

Label each wire on your existing wall plate with the stickers provided with your new thermostat. If you don't have stickers, you can use tape and a pen.

Note: Don't label jumpers.

C-Wire


8. Write down the colors of the wires

Check the boxes and write down the color of the wires connected to terminals in the existing wall plate. Check all that apply (not all will apply).

Note: If there are wires in terminals that aren't listed, you will need additional wiring support. Contact Support at 1-855-733-5465.

C-Wire


9. Disconnect the wires and remove the existing wall plate

You may need a screwdriver to release wires from terminals.

Note: Wrap the wires around a pencil to prevent them from falling into the wall.

C-Wire


10. Do you have a C-Wire?

This is to confirm that your system isn't using a C-Wire. Look at the thermostat wiring checklist from Step 8: is the C-Terminal checked? Alternatively, you can look at the photo from Step 5: is there a wire in the C-Terminal?

YES – This means your thermostat is using a C-Wire, and you don't need to install a C-Wire Adapter. You can continue installing your new thermostat.

NO – This means your thermostat isn't using a C-Wire. Continue to the next step.

C-Wire

C-Wire Go to your furnace or heating system

This system is often located in your basement, attic, or garage.


11. Do you have a zoning panel?

Your zoning panel is often mounted to the ducting on your furnace.

YES – C-Wire Adapter installation is more complicated for zoned systems. Contact Support at 1-855-733-5465 to find a pro-installer in your area.

NO – Continue to the next step.

Return to your existing thermostat

C-Wire


12. Do you have an unused wire?

You previously removed your existing wall plate. Look at the bundle of wires coming from the wall.

Note: You may have to pull the bundle of wires out from the wall to find the unused wire.

YES – You can use this unused C-Wire. Continue to Step 13.

NO – Since you don't have C-Wire, you need to use a C-Wire Adapter. Continue to the Use C-Wire Adapter section.

C-Wire


13. Label unused wire

Label your unused wire with the "C" sticker provided with your new thermostat. If you don't have stickers, you can use tape and a pen. You may need to use a wire stripper to expose at least 1/4 inch of wire.

Note: If you have multiple unused wires, then label only one wire and make note of the color.

C-Wire Go to your furnace or heating system

This system is often located in your basement, attic, or garage. Bring a flashlight and screwdriver.

C-Wire


14. Remove cover from furnace or heating system

Open the heating and cooling system's cover to find the control board. The control board may be at the top or the bottom. You should see the same terminal labels that are on your thermostat.

Note: If you don't see a control board, your system may not be compatible. Contact Support at 1-855-733-5465 to find a pro-installer.

C-Wire


15. Find the other end of the unused wire

In Step 12, you found a bundle of wires near your existing thermostat. Locate the other end of this bundle near your furnace or heating system, and then find the unused wire.

Note: The unused wire should be the same color as the one near your existing thermostat. See Step 13 for the color you wrote down.


16. Connect the unused wire to the C-terminal in the control board

Note: If there are existing wires in the C-terminal, make sure they are still connected after connecting the unused wire.

C-Wire


17. Close the cover to the furnace or heating system

Be sure the cover is completely closed. Some systems won't power up if the cover isn't fully closed.

Congratulations, you've connected an unused C-Wire! You won't need to use a C-Wire Adapter. You are now complete.

C-Wire


Use C-Wire Adapter

A C-Wire Adapter replaces the need for a C-Wire. The C-Wire Adapter is included in newer Honeywell Wi-Fi thermostats. Contact Support at 1-855-733-5465 to learn how to obtain a C-Wire Adapter.

To use a C-Wire Adapter:


18. Do you have a G and Y wire?

In Step 7, you labeled the wires of your existing wall plate. Look at those wires and make sure you have both a G and Y labeled.

YES – This means your system is compatible with the C-Wire Adapter. Continue to the next step.

NO – Your system is not compatible with the C-Wire Adapter. Contact Support at 1-855-733-5465 to find a pro-installer.


19. Relabel your G-Wire to "C"

Relabel your G-Wire with the "C" sticker provided with your new thermostat. If you don't have stickers, you can use tape and a pen.

C-Wire


20. Relabel your Y-Wire to "K"

Relabel your Y-Wire with the "K" sticker provided with your new thermostat. If you don't have stickers, you can use tape and a pen.

C-Wire Go to your furnace or heating system

This system is often located in your basement, attic, or garage. Bring a flashlight, labels, phone, and screwdriver.

C-Wire


21. Remove cover from furnace or heating system

Open the heating and cooling system's cover to find the control board. You should see the same terminal labels that are on your thermostat.

Note: You may need to unscrew the cover. The control board may be at the top or the bottom.

C-Wire


22. Take a picture of the control panel wiring

Make sure you get a good view of the following terminals, since you may need to reference this image later:

  • G
  • C
  • R
  • W or W1 (not in all systems)
  • Y or Y1

Note: If you have wires connected to two R-Terminals, then you have a two-transformer system and need additional assistance. Contact Support at 1-855-733-5465 to find a pro-installer.

C-Wire


23. Label wires

Label each the following wires with the stickers provided with your new thermostat:

  • G
  • C
  • R
  • W or W1 (not in all systems)
  • Y or Y1

If you don't have stickers, you can use tape and a pen.

Note: If multiple wires are connected to a terminal, only label the wires running to your thermostat *.

C-Wire


24. Disconnect the labeled wires

Disconnect only the wires that you labeled. Don't disconnect any unlabeled wires. If unlabeled wires disconnect, reconnect immediately.

Note: You may need a screwdriver to release wires from terminals.

C-Wire


25. Connect labeled wires to the C-Wire Adapter

On the C-Wire Adapter's "Thermostat" side, press down a tab to open the corresponding terminal. Insert the labeled wires to the following terminals:

  • W-Wire --> W-Terminal
  • G-Wire --> C Terminal
  • Y-Wire --> K-Terminal
  • R-Wire --> R-Terminal

Note: This will convert G into C, and Y into K.

C-Wire


26. Make sure the wires are connected

Gently tug on the wires to make sure they are connected to the C-Wire Adapter.

C-Wire


27. Connect C-Wire Adapter to control board

On the C-Wire Adapter's "Equipment" side, connect the wires to the corresponding terminals on the control board.

Note: Be sure to remove the plastic tip on each wire before connecting to a terminal.

C-Wire


28. Mount C-Wire Adapter

Use the included mount and adhesive pad to attach the C-Wire Adapter inside your furnace or heating system.

Note: Make sure your wires aren't over-extended or strained.

C-Wire

C-Wire


29. Close the cover to the furnace or heating system

Be sure the cover is completely closed. Some systems won't power up if the cover isn't fully closed.

Congratulations, you've connected a C-Wire Adapter! You are now complete.

C-Wire


Check if your System is Heat-only or RedLINK

Both heat-only and RedLINK thermostat systems have only two wires, and neither of those wires are C-Wire. To check if your thermostat is heat-only or RedLINK:


30. Turn off power

To protect your equipment, turn off the power at the breaker box or switch that controls your furnace or heating equipment.

Note: Turning off your thermostat won't turn off the power to the furnace or heating equipment.

C-Wire


31. Check that your system is off

Change the temperature on your existing thermostat so that your system starts heating or cooling. If you don't hear or feel the system turn on within 5 minutes, the power is off.

Note: If you have a digital thermostat that has a blank display, skip this step.

C-Wire


32. Remove your existing thermostat from the wall plate

On most thermostats, you can take off the thermostat by grasping and gently pulling. Some thermostats may have screws, buttons, or clasps.

Note: Don't remove any wires from your thermostat at this time.

C-Wire


33. Do you only have 2 wires?

Check how many wires you see connected to terminals.

YES – Continue to next step.

NO – You don't have a heat-only system.


34. Are the 2 wires connected to the R and W terminals?

YES – You have a heat-only system, which is more complicated. Contact Support at 1-855-733-5465 to find a pro-installer in your area.

NO – You may have a RedLINK system, which is more complicated. Contact Support at 1-855-733-5465 to find a pro-installer in your area.

My Wi-Fi thermostat uses batteries. Do I still need a C-Wire?

If your Wi-Fi thermostat uses batteries, you don't need a C-Wire.

Some Wi-Fi thermostats are dual-powered by either batteries or C-Wire. These devices only require either batteries or C-Wire, but not both.

Last updated: September 24, 2018
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