Thermostat C-Wire: Everything You Need To Know

In an older days thermostats were pretty basic and they did not need their own power supply. Today smart thermostats connects to WiFi network, have back-lit display and several sensors. They use more power than old thermostats and need to have their own power supply.

Thermostat C-Wire is that extra cable providing extra power to the smart thermostat. This wire also known as a common wire.

Smart Thermostat Wiring

Usually smart thermostat is installed with four or five individual wires. Each of these wires has a name featuring a single letter.

thermostat wiring infographic - c wire
  • Red/R* – power wire for your heating and cooling system.
  • Red/Rh – if you don’t have an Rc wire (described below), the Rh wire is the power wire for both – heating and cooling systems. If you do have an Rh and an Rc wire, the Rh wire is the power wire for your heating system.
  • Red/Rc – if you don’t have an Rh wire (described above), the Rc wire is the power wire for both – heating and cooling systems. If you do have an Rc and an Rh wire, the Rc wire is the power wire for your cooling system.
  • Blue or Black/C – a C-Wire or a common wire. It connects your heating and cooling system to the common ground and provides power to your thermostat.
  • Green/G – controls your fan. The fan pushes cool or hot air through vents into your home.
  • White/W – controls your furnace. W1 wire controls 1st stage hearing and W2 controls 2nd stage heating.
  • Yellow/Y – usually controls cooling system. Y1 wire controls 1st stage cooling and W2 controls 2nd stage cooling.

* – wire color / wire name

As you can see technicians selected the names for the wires mainly based on the insulation colors. Wiring letters and colors help them to choose the right wire to connect to the correct terminal on the thermostat.

Sometimes wire color might not match that scheme described above. I recommend always to make note to which terminal each wire connects on the old thermostat before removing it. Sticking label to each wire works best.

What Is The C-Wire?

C-Wire is an power cable that provides current to smart thermostat devices.

If you have C-Wire install it simplifies the smart thermostat installation process. Given that you thermostat needs one.

First, you connect wires to the thermostat. Then you go through the step-by-step setup process and your thermostat is ready to use.

Some of the smart thermostats can be installed without a dedicated C-Wire. It can leach electricity off of the four wires you have installed already. Thermostat will turn on and off furnace or air conditioner to draw some power and keep it alive.

This practice works with the Honeywell Lyric and the Nest Learning Thermostat. Yet, it does have a few problems.

Power stealing, as Nest’s engineers usually refer to it, can wear out your furnace or air conditioner. The short on and off cycles are not very healthy to those machines.

Other devices like the Emerson Sensi and ecobee3 use a different approach involving the existing wires. They might aim for low power consumption or use a power extender kit (otherwise known as PEK).

Do I Have A C-Wire?

You may already have a C-Wire installed in your existing thermostat. Look for a blue or a black wire – this is the common colors used for a C-Wire.

Most of HVAC professionals are labeling the wires they put into homeowner’s walls. They do it either with small tags or with a little diagram explaining the color codes.

If you’re having some difficulty telling the wires apart without any sort of diagram, then you’ll want to consider the colors of wire you already have installed.

Usually any red wire is bringing standard power to the thermostat. If you have two separate red wires, then one (Rh) is for the furnace. The other most likely is for the air conditioner (Rc).

White wires usually carry the relay for a heater while yellow ones do the same for the air conditioner.

You might have a brown wire if you have a heat pump. This wire takes care of emergency heat functions. Green wires are generally for the fan relay. Orange and blue wires do various types of changeover work. By process of elimination, you should be able to figure out at least if you have a C-Wire already installed.

What If I Don’t Have C-Wire?

Some people decide to add C-Wire themselves. While others decide to instead have a professional do so. There are also several kits available that can make the task easier. The easiest option is to select a thermostat that doesn’t need a C-Wire.

Using Smart Thermostats Without a C-Wire

A few versions of the ecobee, including the ecobee3 and ecobee4, can work with a power extender kit (PEK). These kits can make an ecobee thermostat compatible with any configuration. You might want to look into this kit before you start running extra wires inside your walls.

The Nest Learning thermostat can charge their batteries using the regular HVAC wires. This means you may not ever have to worry about the C-Wire in a majority of cases. Nest owners will need to have a common wire when an HVAC system has intermittent power cycling issues or uses a zone relay panel.

The engineers at Emerson designed the Sensi with low power consumption in mind. It means that it works with a majority of systems out of the box even if they don’t have a C-Wire.

Each Emerson Sensi thermostat comes with a pair of regular AA batteries. They provide backup power for several months. Some users find that they don’t even need to change the batteries. It again eliminates the need for the C-Wire.

Venstar Add-A-Wire Adapter

Venstar makes their own version of a power extender kit. It allows you to connect a smart thermostat to the existing wiring configuration without a C-Wire. The thermostat will think it has a C-Wire connected even though it doesn’t. The device is relatively inexpensive, and it’s easy to install.

While it doesn’t solve every problem, the system can simply split one wire into two. Then reassign one of the remaining wires as a C-Wire.

Each furnace panel is different from ones used by other manufacturers. Luckily Venstar adapter has wiring diagrams available for a majority of HVAC systems.

In the video below you can check what does it take to install Venstar Add-A-Wire adapter:

Emerson Thermostat Common Wire Kit

Emerson offers a thermostat common power extender kit, like the ecobee PEK. It makes possible to run a C-Wire when any sort of extra wiring isn’t an option. This adapter seems to be variation of Add-A-Wire kit mentioned above.

In the video below you can see how to install Emerson Common Wire Kit:

Power Stealing

Power stealing is a method when you allow your thermostat to continue to cycle the HVAC system on and off to get some power during those cycles.

The idea is that you could power a smart thermostat off the circuit that control the HVAC system’s power network. Stealing too much power may make the system behave in a strange way.

Technically, the thermostat only needs to charge up a battery so many homeowners don’t really see an issue with power stealing. Users convince themselves that they won’t be taking much power and configure it to consume only a light amount of power. What happens is that this way the thermostat battery doesn’t get charged up all the time.

On top of this, most smart thermostats have batteries that deplete their life by around 20 percent of capacity annually.

The Infamous Fan Wire Hack

Some people prefer to make their own custom hack. They flat out turn the fan wire into a C-Wire without adapters or any other devices. Fan wire hack is permanently putting the fan on automatic and using the G-Wire as a power cable for smart thermostat.

This hack is popular, but completely not supported by most of thermostat manufacturers. It’s also an “excellent” way to disable the manual setting on your house fan, so no one should really give it a try.

Hire HVAC Professional

Arguably, the best way to add a C-Wire to any HVAC system is to hire a professional technician. Ask the technician if a quote would cost anything, and then have them offer an estimate first.

Shopping around could save you some money. It might get you a more knowledgeable person than going with the first one.

While it might cost more this way up front, you won’t have to worry about making any costly mistakes. You probably won’t have to worry about any lost functionality as a result either.

Before hiring a contractor, make sure to ask them if they’ve ever installed a C-Wire before. While it’s always good to give someone a chance, it might not be the best idea to let an inexperienced individual install a new wire in your home.

Ask about their licenses and make sure they give you an itemized estimate. This will make comparing different contractor proposals much easier.

Don’t be afraid to ask if there are any special offers or promotions. You shouldn’t feel ashamed because you save a little money. As long as you’re working with a reputable contractor who you’re sure can configure a C-Wire for you.

Running A New Wire Yourself

Note: I strongly recommend hiring professional electrician to work with your home wiring installation. Working with wires and electricity without proper knowledge can be dangerous. It may result in serious injury or even death.

You may have a background in the field of residential wiring or have enough knowledge to work with wiring safely. In that case, you might consider adding a C-Wire by yourself.

The biggest decision you’ll need to make is what kind of wire to use. Make sure to avoid small gauge wire that won’t hold up to the kind of wattage that a WiFi thermostat needs. If you only have a furnace without an air conditioner, then you’ll probably want to install 18/3 cable.

Homeowners with both heat and AC units will need a full 18/5 cable. You might want to consider installing an 18/5 cable even if you don’t have an air conditioner. This is because you’d have to replace all this wire if you were going to in the future anyway.

You’ll also want to keep four wire nuts on hand. One of these wire nuts can be used for each of the conductors that you’re going to be using.

Fire block sealant and electrical tape is also a good idea to keep on hand if you’re going to be adding a C-Wire yourself. Needle nose pliers and some way to strip wire are really all you might not have in the garage right now.

Off course a couple of screwdrivers are more than enough to attach the wires together. You also may want to have a caulking gun on hand to seal any new cracks you might be making on the walls.

External Transformer Method

You can configure an external transformer to provide power to a smart thermostat. In the video below you can see the step-by-step instructions how to do it:

The idea is to get a standard type 24VAC transformer. These boxes aren’t something that’s specific to the HVAC industry. You will be able to find them in any electrical parts store.

As long as you feel comfortable with it, you can put a hole in the wall near the thermostat. Then you need to make a second one near the baseboard. Then you feed a wire to the thermostat and attached it to both the transmitter and the thermostat box.

Everything else would get plugged in from there. Making the holes and matching the right adapter to your device is the hard part.

You might not have much difficulty with it even if you have no prior electrical experience.


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