C-Wire Thermostat - Smart thermostat wiring guide (2023)

In the past, thermostats were pretty basic and didn't need their own power source. Today, smart thermostats connect to the Wi-Fi network, have a backlit screen and various sensors. They consume more power than older thermostats and need to have their own power supply.

The C-Wire thermostat is that extra wire that gives your smart thermostat extra power. This thread is also known as common thread.

smart thermostat wiring

Typically, the smart thermostat is installed with four or five individual wires. Each of these wires has a single letter name.

C-Wire Thermostat - Smart thermostat wiring guide (1)
  • Red/R*– power cord for your heating and cooling system.
  • red/right- and youDoes not havean Rc wire (described below), the Rh wire is the power wire for heating and cooling systems. If youtimeone Rh and one Rc wire, the Rh wire is the power wire for your heating system.
  • red/rc- and youDoes not havean Rh wire (described above), the Rc wire is the power wire for heating and cooling systems. If youtimean Rc and an Rh wire, the Rc wire is the power wire for your refrigeration system.
  • Blue or Black/C– a C-Wire or an ordinary cable. It connects your heating and cooling system to common ground and supplies power to your thermostat.
  • Verde/G– control your fan. The fan pushes cool or warm air through the vents in your home.
  • Blanco/W- control your oven. The W1 wire controls the first stage listen and the W2 wire controls the second stage heat.
  • Yellow/Y– usually controls the cooling system. Wire Y1 controls first stage cooling and W2 controls second stage cooling.

* – thread color/thread name

As you can see, the technicians selected the wire names primarily based on the colors of the insulation. The letters and colors of the wiring help them choose the correct wire to connect to the correct terminal on the thermostat.

Sometimes the color of the thread may not match the scheme described above. I always recommend looking at which terminal each wire connects to on the old thermostat before removing it. Sticking a label on each wire works best.

What is C Wire?

C-Wire is a power cable that supplies power to smart thermostat devices.

If you installed C-Wire, it simplifies the smart thermostat installation process. Since your thermostat needs one.

First, connect the 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 from all fourfiosyou already installed it. The thermostat turns the furnace or air conditioner on and off to extract some energy to keep you alive.

This practice works with Honeywell Lyric andNest Learning Thermostat. However, it has some problems.

power theft, as Nest engineers like to call it, can wear down your heating or air conditioning. Short on and off cycles are not very healthy for these machines.

Other devices such as Emerson Sensi andecobee3use a different approach involving existing cables. They can aim for low energy consumption or use apower extender kit(also 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 black wire - these are the common colors used for a C-Wire.

Most HVAC professionals are labeling the wires that run through the homeowners' walls. They do this with little labels or a little diagram that explains whatcolor codes.

If you have a hard time distinguishing the wires without any kind of diagram, consider the colors of the wires you have already installed.

Typically any red wire carries standard power to the thermostat. If you have two separate red wires, one (Rh) is for the oven. The other most likely is for the air conditioning (Rc).

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

You may have a brown wire if you have a heat pump. This cable handles emergency heating functions. The green wires are usually for the fan relay. The orange and blue wires perform various types of transition work. Through the removal process, you should at least be able to find out if you already have C-Wire installed.

What if I don't have C-Wire?

Some people decide to add C-Wire itself. While others decide to have it done by a professional. There are also several kits available that can make the task easier. The easiest option is to select a thermostat that does not require C-Wire.

Using Smart Thermostats Without a C Wire

Some versions of ecobee, includingecobee3miecobee4, you can work withpower extender kit(PEK). These kits can make an ecobee thermostat compatible with any setup. You may want to review this kit before you start running additional cables inside your walls.

OTermostato Nest Learningthey can charge their batteries using normal HVAC cables. This means that you may never have to worry about C-Wire in most cases.cases. Nest owners will need to have a common wire when an HVAC system is having intermittent power cycling issues or using a zone relay panel.

Emerson engineers designed Sensi with low power consumption in mind. That means it works with most systems out of the box, even if they don't have a C-Wire.

Each Emerson Sensi thermostat comes with a pair of standard AA batteries. They provide backup power for several months. Some users feel that they don't even need to change the batteries. Again eliminates the need for C-Wire.

Venstar Add-A-Wire Adapter

Venstar makes its own version of apower extender kit. Allows you to connect a smart thermostat to your existing wiring setup 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 easy to install.

While it doesn't solve every problem, the system can simply split one cable in two. Then reassign one of the remaining wires as a C wire.

Each oven panel is different from those used by other manufacturers. Fortunately, the Venstar adapter has wiring diagrams available for most HVAC systems.

In the video below you can see what is needed to installVenstar Add-A-Wire Adapter:

Emerson Thermostat Common Wiring Kit

Emerson offers a common thermostatpower extender kit, such as ecobee PEK. It makes it possible to run a C-Wire when any type of additional wiring is not an option. This adapter appears to be a variation of the Add-A-Wire kit mentioned above.

In the following video you can see how to install the Emerson common wiring kit:

power theft

power theftit is a method when you allow your thermostat to continue turning the HVAC system on and off during these cycles.

The idea is that it can power a smart thermostat from the circuit that controls the mains of the HVAC system. Stealing too much power can cause the system to behave strangely.

Technically, the thermostat only needs to charge one battery, so many homeowners don't see a problem with power theft. Users convince themselves that it won't use much power and set it to use only a small amount of power. What happens is that the battery of the thermostat is not charged all the time.

Also, most smart thermostats have batteries that deplete their life by about 20% of their capacity annually.

The infamous fan cord trick

Some people prefer to make their own custom hack. They convert the fan cable to a C-Wire without adapters or any other devices. The fan cable trick is to put the fan on auto mode permanently and use the G-Wire as the power cable for the smart thermostat.

This trick is popular but not fully supported by most thermostat manufacturers. It's also a "great" way to disable your home's manual fan setting, so no one should try that.

Hire an 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, then ask him to offer an estimate first.

Shopping can save you some money. This can make you a more experienced person than going with the former.

While it may cost more up front, you don't have to worry about making costly mistakes. You probably won't have to worry about any lost functionality either.

Before hiring a contractor, ask if they have installed a C-Wire before. While it's always good to give someone a chance, it may not be the best idea to let an inexperienced person install new wiring in your home.

Ask about their licenses and make sure they give you a detailed quote. This will make it much easier to compare different offers from contractors.

Don't be afraid to ask if there are any special offers or promotions. You shouldn't feel embarrassed because you save quite a bit of money. As long as you work with a reputable contractor that you trust they can set up a C-Wire for you.

Run a new thread yourself

Observation:I highly recommend hiring a professional electrician to work with your home's electrical wiring. Working with cables and electricity without proper knowledge can be dangerous. This could cause serious injury or even death.

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

The most important decision you will need to make is what type of thread to use. Be sure to avoid small-gauge wires that can't handle the kind of wattage a WiFi thermostat needs. If you only have a furnace without air conditioning, you'll probably want to install 18/3cabo.

Homeowners with heating and air conditioning units will need a full 18/5 wire. You may want to consider installing 18/5 cable even if you don't have air conditioning. That's because you'd have to replace all that cable if it was going to be in the future anyway.

You will also want to have four wire nuts on hand. One of these wire nuts can be used for each of the conductors you will be using.

Fire block sealant and electrical tape are also a good idea to have on hand if you are adding a C-Wire yourself. Needle nose pliers and some form of wire stripping are really all you might not have in your garage right now.

Of course, a few screwdrivers are more than enough to join the wires together. You may also want to keep a caulking gun handy to seal any new cracks you may be making in your walls.

External transformer method

You can set up aexternal transformerto power a smart thermostat. In the following video you can see step by step how to do it:

The idea is to get a standard 24VAC type transformer. These boxes are not specific to the HVAC industry. You can find them at any electrical parts store.

As long as you're comfortable with that, you can drill a hole in the wall near the thermostat. Then you need to make a second one near the socket. Then, you feed a wire into the thermostat and connect it to the transmitter and the thermostat housing.

Everything else would be connected from there. Drilling the holes and matching the correct adapter to your device is the hard part.

You may not have too much trouble with this, even if you have no prior electrical experience.


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