What is C-Wire? Everything you need to know!

Millions of people around the world use smart thermostats. If you are planning on replacing your old thermostat with a smart thermostat and you lack a C-wire, please get one. Having a C-wire is advantageous, and you’ll be more comfortable that way.

Definition of the C-wire

A C-wire (Common wire) is the wire that is used to give a continuous supply of 24-volt power to a thermostat In simple terms. The ‘C’ means common.

On most thermostat backplates it’s marked as ‘C.’ You should, however, note that it is not necessarily marked as ‘C.’ In quite some thermostat wiring, the C-wire is usually the blue-colored wire. This is not the case for all wiring systems.

How It Works

Though it is a common notion, it may be incorrect to state that the C-wire powers the thermostat. Typically, the wires that give the power supply (usually termed as the ‘hot’ wires) are marked Rc (for cooling) and Rh (for heating).

They give a 24-volt power supply from the HVAC system’s control panel.

At times, they’re separate cables and need different attachments. Other times they may be the same cable. This cable is usually marked as Rh/c.

To have a circuit that can allow the transfer of power, wires must connect back to the control panel of the heating and cooling system to finish the loop. That is the function of the other cables.

Normally, the heating return ports will be marked ‘W,’ and the cooling ports will be marked ‘Y’ (for multiple stages it may be ‘W1’, ‘W2’, ‘Y1’, ‘Y2’). The port for the fan control is usually marked as ‘G.’ If your system has a common wire, it will be marked ‘C’ at most times.

As you can see, the C-wire does not provide power to the thermostat. It gives a return path such that the thermostat is powered without disrupting the other cables. Which act as on/off switches for the appliances.

Importance of a C-wire

Contemporary wireless thermostats need a lot more power compared to traditional thermostats. Maintaining a wireless connection to an automation hub or a router is something that the traditional thermostats never needed.

Trying to keep a wireless network connection will drain AA or AAA batteries in a short time.

A large percentage of smart thermostats possess a big and brightly colored touch screen. This is yet another specification that adds to the necessity of a constant power supply. A C-wire is the best mode to get the constant supply of power that is needed.

C-wire / Non C-wire
Some thermostat manufacturers claim that their smart thermostats do not require a C-wire. This may not be the case for all. A C-wire is recommended to assist in improving the wireless network connectivity and prolong the battery life.

Smart thermostats are meant to make life easier. Replacing batteries weekly or even monthly is not something that anyone would like to do. Most people want to install their smart thermostats and not have to think about them all the time.

No one wants to be on holiday or anywhere else worrying about the wireless network connection or the batteries.

There’s no need to risk your thermostat losing power completely, and your system cannot switch on. Just get a C-wire to be on the safe side.

Power Stealing

A couple of smart thermostats have avoided the C-wire challenge by employing a method popularly known as ‘power stealing.’ This method utilizes the current circuits to ‘steal’ some quantity of power.

The ‘stolen’ power is used to charge a battery, and in turn, the battery provides power to the thermostat.

The current circuits are meant to be on/off switches. They are not designed to power any appliance. There is a particular threshold current flowing through the cable that stands for the switch point.

Current that is more than the threshold is ‘On,’ and current which is less than the threshold is ‘Off.’

As a result, there may be some amount of current flowing through the circuit that doesn’t trigger the ‘On’ state. This is the current which is used to power the thermostat through power stealing.

HVAC System

circuits were not designed for power stealing. This technique provides very small quantities of power, therefore, doing it effectively is very hard. If you steal excessive power, you turn your system on when you did not intend to.

If you are still very small amounts, the battery is drained, and the thermostat eventually stops.

Power stealing is more effective when the HVAC system is switched off than when it’s on. The thermostat will be able to charge its extra battery when the system is off and slowly discharge the power when the HVAC system is on.

This shows that at times of heavy usage, the probability of draining the extra batteries is very high.

Furthermore, a couple of smart thermostats such as the Nest thermostat, possess a rechargeable battery. All types of rechargeable batteries (e.g., for smartphones) eventually lose their charge capacity as time goes by.

Power stealing with the Nest Thermostat may work well at the beginning, but over time, you will start to experience issues.

Read: How improper installation of C-Wire can damage your HVAC System”


How to Check if You have a C-wire

  1. The first thing you need to do is to switch the power off. Go to the circuit breaker of your HVAC system and turn it to the ‘Off’ position.
  2. Remove the existing thermostat from the wall to expose the wiring.
  3. Read the labels: It is likely that you will come across several wires attached to some labeled ports. If you see one that’s attached to the port labeled’, that means you have a C-wire. This is pretty good!

If you happen to see a port labeled ‘C,’ but it is empty, check for any spare wires. At times, installers may run the common wire from the HVAC system’s control panel to the thermostat’s position, but fail to connect it.

Uninstall the thermostat’s backplate and pull up the cables to see if there’s a spare wire.

If you’re still not successful in finding a C-wire, go to the control panel of your HVAC system. Be on the lookout for a wire that is connected to the ‘C’port.

If there’s a wire attached, master its color and return to your thermostat. The wire whose color matches is the C-wire. If you can’t see any cable, then you don’t have a C-wire, and you should probably acquire one.

How to Get a C-wire

So you just found out you don’t have a C-wire and you want to connect a smart thermostat? Getting a C-wire is not challenging, and there are plenty of solutions. Let us take a look at some methods you can use:

  1. You can Install Another Wire

This is probably the best step to take. Here, you will have to open up your HVAC system. You will have to run a wire from the control panel of the HVAC system to your thermostat. Try as much as possible to fish the new cable through your walls without ruining them.

Depending on the distance between your control panel and the thermostat, and how your walls are finished, the work may be easy or difficult.

  1. You can Buy an Add-a-Wire Kit

In circumstances where it’s too hard to install a new wire, the ‘Add-a-wire’ kit lets your thermostat use the current wires and still receive a constant supply of power without employing ‘power stealing.’

The Add-a-Wire Kit transforms one return wire into a switch for two appliances. The kit will function for as long as you don’t need to switch on both appliances at the same time.

As a result, the two devices that get attached to the Add-a-Wire Kit are the air conditioner and the furnace. Every kit is shipped with its instruction manual; this will help you in connecting the wires properly.

The kit is installed at the HVAC system’s control panel. You need a screwdriver and the know-how of reading a wiring diagram. An HVAC system is quite costly. If you have any uncertainties about what you’re doing, you should probably call a certified electrician to help you.

Some examples of Add-a-Wire Kits are:

  • Ecobee3 smart thermostat is shipped with its Add-a-Wire Kit. Which is commonly known as a Power Extender Kit (PEK).
  • There’s the Venstar ACC0410 Add-a-Wire Kit that you can purchase at Amazon.
  • The Wiser Air Thermostat has an Add-a-Wire Kit included, and you can also get one at Amazon.
  • Honeywell provides the Honeywell THP9045A1023 Wiresaver wiring module for their Wi-Fi thermostats.
  • There’s also a wire extender for the Schneider electric wiser thermostat.

Smart thermostats utilize way too much power to operate on batteries. Power stealing may be a useful technique, but it is not very reliable. You will also be using your HVAC system in a way it wasn’t meant for.

The End

A C-wire is the best choice. It ensures that your thermostat is powered continuously as long as there’s no black-out. Don’t stress yourself by trying to run a smart thermostat without a C-wire!

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