How To Add C Wire To Thermostat?

Thermostats control the temperature of your house and keep you comfortable. While looking for a thermostat, you will come across a term called C wire or the common wire. The wire gives the thermostat a constant power flow. So, your thermostat should have this wire. 

If you need to add the C wire separately, you can use a 5-conductor thermostat wire that contains the C wire and run it from the HVAC system to the thermostat. If your basement has a boiler, snake the conductor to the basement and connect the wire to their terminals.

C wires are mostly present in smart thermostats that use WiFi to provide constant power flow to the thermostats. This guide will help you know the importance of the C wire, how to add the wire to the thermostat, and the cost of adding one.

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What is the C wire in the thermostat?

C wire is considered an extra wire that provides a return path for the constant 24-volt power. 

It is usually the return path to power the thermostat. 

The C in the wire stands for common. 

You will often see this wire labeled as C behind the thermostat backplates. 

Remember that the wire is not necessarily labeled as C and has no specific color code for identification. 

C wire does not alone provide power to the thermostat. 

The Rc (cooling) and Rh (heating) are the wires known to provide power. 

These wires provide a source of 24-volt power from the HVAC control board. 

Sometimes these wires work separately and need separate connections for operation. 

Other times, they are the same wire where it will be labeled as Rh/c. 

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Why do we need a C wire in the thermostat?

In modern days, thermostats have improved. 

They have multiple features and work wirelessly when connected to WiFi. 

Wireless thermostats need more power than calculator screen programmable thermostats. 

A WiFi router is one of the power requirements nowadays for these smart thermostats. 

Maintaining the router connection will drain a lot of batteries within days. 

Many smart thermostats have a large, full-colored touch-screen. 

This feature also required a constant power source. 

The C wire can provide sufficient power needed by the thermostats. 

Some thermostats do not have C wires, for example, the Emerson Sensi WiFi thermostat. 

But connecting a C wire can improve the WiFi connectivity and battery life. 

Not having a C wire will affect the battery life. 

If your thermostat loses power completely after some time, it won’t turn on again. 

So, it is better to have thermostats with C wires. 

If you do not have one, consider adding one to the unit. 

How to know that your thermostat has a C wire?

To know whether your thermostat has a C wire, you can check the thermostat closely. 

Here are some steps to check if your thermostat has a C wire or not:

Turn off the power supply.

You must turn off the power supply before starting work on the thermostat. 

Turning off only the thermostat switch won’t be enough. 

So, you have to turn off the power on the breaker box. 

Verify that the system is off.

Adjust the thermostat’s temperature setting to check whether you have properly switched off the power. 

If you do not find any system running within 5 minutes, it is confirmed that the power is off. 

In the case of a digital display, the screen will go black if the power is off. 

So you do not have to wait for confirmation. 

Remove the existing thermostat.

Remove the thermostat from the wall, but leave the wires connected. 

You can remove some thermostats by grasping and a gentle pull. 

For others, you must unscrew the screws, buttons, and clasps. 

Look for C wires on the wall plate.

Your thermostat will have a C marking on the wall plate. 

It means that your thermostat is using a C wire. 

You will need a C wire if you have installed a Resideo WiFi thermostat. 

Check for loose wires.

Sometimes the installers run the C wires from the control board to the thermostat but fail to hook them up. 

You will have to take off the thermostat backplate, pull out the wires, and look for extra wires. 

Maybe the wire remained unused for a long time. 

Check the control board.

If you are still unsure about the pressure of the C wire in the thermostat, check the HVAC control board. 

Check and see if you find any wire attached to the C terminal. 

How to add a C wire to the thermostat?

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If you want to add a new thermostat or replace an old one with a smart thermostat that runs on WiFi, you will have to add a C wire. 

You do not have to worry if your thermostat already has the C wire. 

But, if it misses, you need to add it separately.

There are two ways to add a C wire to the thermostat:

  • Using a C wire adapter
  • Installing a C-wire transformer

You can use the same procedures to add one to your existing thermostat without a C wire.

Here are the steps to add the C wire to the thermostat:

C wire adapter

A C-wire adapter is a small device that repurposes the existing wires between the thermostat and the furnace and adds a new wire to the system. 

This new wire will be the C or common wire that will provide constant power to the thermostat and save bills. 

Installing this adapter will require 30 to 60 minutes. 

You can always hide the adapter inside the furnace circuit box. 

You do not need to run any wires through the rooms or affect the house’s aesthetics. 

The installation process is quite a job. You have to open the furnace circuit box, make some wire connections, attach the kit to the box, and then end up with more connections. 

The steps to install the C wire adapter divides into two main parts:

  • The steps at the thermostat
  • The steps at the furnace circuit box

Steps at the thermostat

  • Turn off the power at the breaker.
  • Open the wall plate of your thermostat. 
  • Drag the wire on the G terminal and connect it to the C terminal.
  • Remove the wire on the Y1 terminal and pair it with the blue wire on the wire pair. Close the wire openings with a cap. 
  • Connect the green wire to the G terminal and the yellow wire to the Y1 terminal. 
  • Now, close the box. 

Steps at the furnace circuit box

  • Cut off the main power to the furnace before starting.
  • Connect the adapter to the furnace circuit box. You might have to drill some screws into the mounting hole. 
  • Link the red wire from the device to the R terminal of the circuit board. 
  • Connect the C wire from the adapter to the C terminal on the circuit board. 
  • Remove the Y1 terminal’s wire from the furnace board and connect the yellow wire from the device to the Y1 terminal. 
  • Connect the green wire from the adapter to the G terminal on the furnace board. 
  • Now, one wire is left on the adapter, the blue wire. Pair this wire with the wire you previously removed from the Y1 terminal, and close the openings with the caps. 

The process is over, and you have successfully added a C wire to the thermostat. 

Please turn on the main power and the switch on the thermostat to check its functionality. 

C wire transformer 

The transformer is a 24 Volt step-down adapter plugged into the main outlet. 

The device will have 2 output wires which you will connect to the thermostat.

If an outlet is near the thermostat, you can connect one wire to the C terminal and another to the thermostat terminal.

The process is easy as you only need to plug the wire into the individual terminals. 

The problem is you need to have an outlet near the device. 

Otherwise, you will face running long wires. 

Besides, it can damage the look of your house.

The steps to install a C wire transformer are very easy. 

  • The transformer will have two output wires. 
  • Connect one wire to the Rc terminal and the other one to the C terminal of the thermostat. 
  • Now, plug in the transformer to the main power outlet.
  • Switch on the thermostat to check its performance. 

You have successfully added a C wire to your thermostat without replacing it. 

Is there any other way to add a C wire?

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If you do not want to install the adapter or transformer, you can add a C wire in another way.

Sometimes, some thermostats will have an unused wire hidden in the wall. 

In that case, you can find it out and use it as the C wire. 

You will then not require any C wire adapter. 

You can follow the following steps to find and identify the wire. 

If you feel uncomfortable doing it yourself, hire an expert. 

  • Turn off the power and bring out the thermostat. 
  • Take a picture of them if you cannot memorize them. 
  • Label the wires on the wall plate with stickers, tape, or a pen. If the wire has 2 labels, label the others with bold letters. 
  • Note down the wire colors connected to the terminals in the wall plate. You need additional wiring support if some wires are not listed on the wall plate. 
  • Pull out the wires and check the place from where the wires come out of the wire bundle sleeve. If you want more space, remove the screws from the wall plate, pull them out, and check for unused wires. 
  • You can use this wire as the C wire by connecting it to the C wire terminal at the thermostat.
  • Also, Label that unused wire as the C wire with the sticker, tape, or pen. You can use a wire stripper to expose ¼ inch of the wire. 
  • Go to the furnace, find the unused wire’s other end, and connect it to the C terminal. 
  • Close the furnace cover after you have finished working. 

You have successfully added an unused wire as the C wire without any adapter or transformer.

What happens if I install a thermostat without a C wire?

The C wire provides continuous current flow by finishing the circuit from the hot wire to the HVAC control board. 

If you have a C wire, the thermostat will always run both during the day and at night. 

But without a C wire, your thermostat will soon lose its power and stop working, especially if you use a smart thermostat. Call a professional and get one installed. 

You will require the C wire settings in the thermostats with WiFi settings. 

Do all thermostats need C wires?

All the thermostats do not require C wires. 

Ordinary thermostats can run without a C wire because they run on batteries and do not have as many features as smart ones. 

Smart thermostats also have batteries, but their multiple features cannot rely only on them. 

Some smart thermostats do not run on batteries but only on a WiFi connection. 

C wire is a must for them.

However, some thermostats have rechargeable batteries and can recharge themselves. 

You have to wait for your house’s HVAC system to get activated. 

However, this process is not recommended. 

Since the batteries take power from the HVAC, the thermostat keeps turning the system on and off, reducing the system’s lifespan. 

What will be the cost to add a C wire to the thermostat?

It is always best to let the professionals do this job as it involves working on the electrical wirings. 

However, if you have experience, you can do it yourself. 

If you are uncomfortable, leaving the job in the hands of the experts is the best option. 

$96 and $218 are the cost of the thermostat only. 

The labor cost will be between $218 and $265. 

The average price to add C wire will be around $172. 

The cost can reduce to $119 if your home already has a C wire. 

The access points and placement will affect the cost range. 

Sometimes, the price may increase if you need to repair the drywall around the thermostat. 

The minimum cost for the thermostat will be $96, the wire installation will be $157, and the labor cost will be $218.

The maximum price for a thermostat will be $112, wire installation will be $188, and the labor cost will be $265. 

The average price for a thermostat is around $104. $172 is for the wire installation, and $241 is for the labor cost. 

A small guide about all the wires in the thermostat

Except for the C wire, there are many other wires, and you will have to deal with them while working on the C wire. 

Here is a brief list of the common wires and their color codes you might see while taking out the thermostat:

  • Black or blue wire – C wire
  • Red wire – R / Rc wire
  • White wire – W1 / W2 wire
  • Orange wire – O wire
  • Dark blue wire – B wire (O/B)
  • Green wire – G wire
  • Yellow wire – Y1 and Y2 wires

Other wires include:

  • AUX NO
  • AUX C
  • AUX NC
  • X/AUX
  • E
  • BK
  • RS1
  • RS2
  • ODT1
  • ODT2

These wires may not have an exact color code. 

To know their function and color codes, you need to know it from the technicians when they install the thermostat. 

You can also check the thermostat manual to learn about the wire functions and their color codes. 

Final thoughts

C wire provides constant power to the thermostat to function properly without stopping. 

If your thermostat does not have the C wire, the device will lose energy faster and stop working. 

Since modern thermostats have multiple features and are run by WiFi, they cannot rely only on the batteries. 

C wire can save energy and provide enough current for them to run day and night. 

You can use the thermostats that need batteries if you do not want to install a C wire. 

Some thermostats steal power from the HVAC system and charge their batteries themselves. 

But that is unsafe as it reduces the system’s lifespan. 

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Arthur Smith

Howdy! I am Arthur Smith, an electrical engineer who is extremely passionate about electronics. I have lived in different parts of the US and currently reside in Wisconsin. I am one of those rare children who knew what he wanted to study and do in life right from the start. I was a curious child who wanted to know how switches work and how the AC works, and I would always observe my dad whenever he would handle the wires and fix things around the house. I currently work as an electrical engineer at a reputed company and write for this blog. And I read loads of books or play video games in my free time.

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