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What is the c wire on a thermostat -You must know in 2022

What is the c wire on a thermostat | What does the c wire do on a thermostat

When you install a thermostat, you need a C wire. It becomes even more important when you are going to install a smart thermostat at your home.

Now the question comes, what is the c wire on a thermostat? Before I answer this question, I would like to also explain you some of the important information regarding the C-wire.

In order to do that, in this blog post, I am going to answering the following questions mentioned below-

  • What is the c wire on a thermostat?
  • What color is the c wire on a thermostat
  • Why do you need a C Wire or Common wire?
  • what does the c wire do on a thermostat
  • Why do you need a C Wire or Common wire?, and so on.

Without wasting any time let’s get started.

What is the c wire on a thermostat?

The C- wire is an extra wire that is used in the low voltage heating system ( let’s say 24 Volts), which carries continuous power to the thermostat. Nowadays it is quite common in the smart thermostat, making them compatible with heating and cooling system for the installation of smart thermostat.

Now the question comes- what color is the c wire on a thermostat?

The common wire is generally of blue color, but you can also find different colors of c-wire in market such as black, brown or purple.

Wiring in a Thermostat.

According to the smarthomelab, typical home wiring system is as following-

  • First, R or Red wire for Power.
  • G or Green Wire for Fan.
  • Y or Yellow wire for air conditioning.
  • W or While wire for heating, and
  • C wire or common wire which is blue or black in color- used for 24 volts Power to the thermostat.

If you find out different color wires then checkout the labeling on them.

what color is the c wire on a thermostat | What is the c wire on a thermostat?
image credits- Homedepot


Do you know? The wires that provide the power to the thermostat are called the hot wires and labeled Rc (for cooling) and Rh (for heating). These power wire provides power to the thermostat do as per their name.

These power wire may be separated or might be the same in some cases. Hence it can be labeled as Rh/c.

So, what does the c wire do on a thermostat?

Smart thermostat requires C-wire which completes the circuit that further helps to power the smart thermostat or in charging the thermostat battery.

You can consider the C-wire as it provides a return path so that the smart thermostat can be powered without affecting the functioning of the other wires present with it.

Please note that- some of the smart thermostat does not requires C-wire, while some of them needs one. So make sure you checkout the product description or the thermostat manual to find out.

Why do you need a C Wire or Common wire?

As at the start itself, I told you about smart thermostats requires more power as compared to the standard thermostats. They needs up to 24 volts power connection.

The reasons for more power usage in a smart thermostat could be-

  • 24/7 WiFi connection to the thermostat.
  • High resolution Touchscreen consumes power.

Ultimately you will face fast battery drain in your smart thermostat. But Wait, do I told you that this all headache you can avoid by using a C-wire.

If you use a C-wire in the installation of your smart thermostat then you never face any power shortage or fast draining of the thermostat. It will not only provide power to your thermostat, it also helps in recharging the batteries.

There are some of the brands which says they don’t require the C-wire during their installation. For example- ecobee Smart Thermostat and  Google Nest Thermostat are the wifi thermostat comes with no c wire.

While on the other hand- Honeywell Home RTH9585WF1004 Wi-Fi Smart Color Thermostat requires C-wire during their installation.

How do you know if I have a C-Wire?

If you ever had a thermostat at your home, whether it is a standard one or the smart thermostat, you already have some of the wiring in your installed thermostat.

Now the question comes- do you already have a C-wire or not?

To check out this, I suggest you to follow the steps mentioned below-

  • Check the labels.
  • Find out any extra loose wires, and
  • See the Control board wiring.

Check the labels.

Checkout the labeled terminals, you will find out some of the wires which are connected to different labels. There you can find out a label name- “C”. If any wire is connected to it then that is your C wire or Common wire.

If no wire is connected to it, then you might not have a C-wire. In such cases, follow the other steps mentioned below.

Find out any extra loose wires.

In our introduction part, I explained you about what is a C-wire. Their I told you that, C-wire is an extra wire that is used in the low voltage heating system, which carries continuous power to the thermostat.

Hence now all you need to check out the back plate of your thermostat and pull out the wires and find out weather their is an extra wire is present or not.

If you find out one, congratulations luckily you already have the C-wire. Still Confused checkout your thermostat’s control board wiring.

See the Control board wiring.

Go to the HVAC Control Board wiring and check whether their is an wire attached to the “C” terminal of the control board. If you find out one attached to it then probably you have one.

You can also checkout the color of the wire connected to that terminal. However if you find a wire connected to it, then ultimately you have a common or C-wire.

If you find no wire connected to the C-terminal then you don’t have the C-wire.

How to install a c wire in your thermostat?

After checking whether you have a c wire, you might come to know that you don’t have one. Now what to do or in other words- c wire installation is the next thing which you can do.

Now the question comes how to install a c wire in your thermostat?

You can do c wire installation via the methods given below-

  • Use an Add-A-Wire Kit.
  • Look for a 24 Volts C wire adapter.
  • Installation of another wire.
  • Use a thermostat which uses Power Stealing.

Use an Add-A-Wire Kit.

When additional wiring cannot be run to your programmable smart thermostat, we use Add-A-Wire accessory to add a additional wire such as C-wire to the thermostat.

We can install Add-A-Wire Kit to the HVAC control board. Shut off the power and remove the access panel to your furnace to locate the HVAC panel and install the Add-A-Wire Kit to it.

You can checkout the YouTube Video which helped me a lot in adding a c-wire in a furnace mentioned below-

Still not sure about doing it by yourself, ask for the professional to do so.

Now the question comes which Add-A-Wire-Kit you should checkout for your thermostat?

Venstar ACC0410 Add-A-Wire Accessory is one of the great Add-A-Wire-Kit you should checkout for your thermostat.

Look for a 24 Volts C wire adapter.

Before you use a 24 volts C wire adapter, I would like to ask you about do you know what is a c wire adapter for a thermostat?

A C-wire Power adapter is a device which you can use with the thermostats that does not have a C-wire or common wire. It makes such thermostats compatible even without having C-wire.

The C-wire Power adapter or transformer comes with the two wires which you will plug into the thermostat to power it. And the other end of the C-wire Power adapter you can plug it into a regular outlets.

C-wire Power adapter | What is the c wire on a thermostat


Check out the best 24 Volt Transformer and C Wire Adapter Thermostats on Amazon. 

Installation of another wire.

If you need something, then add it by yourselves. From the name itself, you can install a wire from the HVAC control board to your thermostat.

But the main issue with this you have to deal with the extra wire which might be long ( length of the wire depends on the distance between the HVAC control board and the thermostat.

Use a thermostat which uses Power Stealing.

If you don’t have a C wire, then the last option comes is to use a Power stealing thermostat. You might be thinking that what is a Power Stealing Thermostat?

The thermostat which uses the existing circuits to steal or take a small amount of power to charge the thermostat’s battery and further powering the thermostat.

Power stealing techniques are commonly used in the smart thermostats which does not need a C wire, for example- Google Next Learning Smart Thermostat. 

To understand the working of the Power Stealing thermostat, you have to understand the existing circuits working.

The existing circuits have a certain threshold current running through the wire, which represents the switch points.

Hence it is said that if the current flowing through the wire is above the threshold, then it is said as Switched ON, while if it is below the threshold then it is said as Switched OFF.

As a result if the thermostat takes small amount of the current from the existing circuit, then it will not turn ON the circuit and hence that power/current which is taken from the circuit can be used to power the thermostat and considered as stealing.


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