Thermostat C-Wire - Complete Guide to Smart Thermostat Wiring

Thermostat C-Wire: Guide to Smart Thermostat Wiring

Last updated: July 25, 2018

In an older days thermostats were pretty basic and they did not need their own power supply.

Today smart thermostats are usually constantly connected to Wi-Fi network, have backlit display and several sensors so they use more power than traditional ones do. Therefore, most of the thermostat need to have their own power supply.

A common wire, better known as the thermostat C-Wire, is that extra cable that provides additional power so that these newer smart thermostat devices can get the amount of negative charge they need.

Smart Thermostat Wiring

Typical thermostat installations use a configuration that comes with four or five individual wires. Each of these wires is given a name featuring a single letter that helps technicians remember which each one does.

Names were more than likely originally chosen for insulation colors, but they were expanded over the years for a number of different reasons. The wires generally feature the following assignments:

Thermostat c-wire

While these are part of a more or less established common use of wire colors in the HVAC world, you might have dozens of variations as there is no official standard for thermostat wire colors. Some feature a second-stage heating or cooling system. Other units come with additional furnace power terminals. You'll generally be able to tell them apart because these lines are color coded, but then again some installers might have neglected to use the proper colors when configuring a thermostat. 

There might be that some manufacturers did not follow the commonly used coloring scheme and introduced their own wire color schemes instead. This tends to make thermostat wiring a bit complex, unless you will make note to which terminal each wire was connected on the old thermostat (e.g. sticking label to each wire will work best).

Some of the smart thermostats can be connected without a dedicated C-Wire. It can leach electricity off of the four wires you have inside of your wall. Your furnace or air conditioner will occasionally go on and off very briefly in order for your thermostat to draw a sufficient amount of power from it and keep itself alive. This practice actually works with the Honeywell Lyric and the Nest Learning Thermostat, but it does have a few problems.

Power stealing, as Nest's engineers usually refer to it, can eventually wear out whatever machine gets cycled to feed the thermostat's power hungry appetite. Other devices like the Emerson Sensi and some versions of the ecobee3 use a slightly different approach involving the existing wires. They might aim for low power consumption or simply use a power extender kit (otherwise known as PEK). That being said, you'll have a much easier and far less involved time if you have a thermostat C-Wire instead.

What is the C-Wire?

C-Wire or Common Wire, is an power cable that provides current to smart thermostat devices so they can get the amount of negative charge that they need.

Since the C-Wire does nothing but provides the negative charge, you won't have to worry about power extenders or HVAC cycles. You simply wire the thermostat up and get going. Not only does this technology simply the installation process, but it has gotten so reliable that one could actually be added to an existing older unit in order to make configuring your smart thermostat that much easier.

Just to get the idea of the installation process here is the video where Emerson Sensi thermostat is being installed and wired:

Do I Have a C-Wire?

You may already have a C-Wire installed in your existing thermostat. This would make installing a smart thermostat that much simpler. The luckiest homeowners will find that they not only have a C-Wire but also have a label affixed to it. Otherwise, look for a blue or black wire. Those colors are commonly used for the C-Wire.

An increasingly large number of HVAC professionals are labeling the wires they put into homeowner's walls either with small tags or with a little diagram explaining the color codes stuck behind the existing thermostat. This should also help you find all the other wires you might have back there.

Transformers featuring regular 24 VAC power systems probably use a black or blue insulated wire for the C-Wire if it was already installed. Should you have an extra wire in the installation that doesn't match any of the other color codes, then more than likely this is a C-Wire.


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. More than likely, any red wire is bringing standard power to the thermostat.

If you have two separate red wires, then one is probably for the furnace while the other is more than likely for the air conditioner. A black or blue wire is usually the C-Wire. 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?

You have several options if you don't have a C-Wire. Some people decide to add one while others decide to instead have a professional do so. There are also several kits available that can make the task easier. Perhaps the easiest option is to simply select a thermostat that doesn't require a C-Wire, to begin with.

Using Smart Thermostats Without 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 additional wires inside your walls. Unlike some of the other options, the PEK is fully supported by the manufacturer.

 If you're concerned that the PEK will draw additional wattage and therefore consume power, then you might want to see if you can save a little extra power on your overall HVAC settings once your new thermostat is set up so you can make up for it.

All three generations of the Nest Learning thermostat can charge their batteries using the regular HVAC wires. This means you may not ever have to worry about whether or not you have a C-Wire in your installation in a majority of cases. You shouldn't have too much trouble with power stealing when using these thermostats either, which makes the whole C-Wire question somewhat moot.

The only time that Nest owners will absolutely have to have one is when an HVAC system has intermittent power cycling issues or uses a zone relay panel. Homeowners usually don't have zone relays, which further reduces any risks involved. This basically leaves a few uncommon scenarios, such as those who find that their gas valve vibrates when their Nest calls for the furnace to switch on.

The engineers at Emerson designed the Sensi with low power consumption in mind, which means that it works with a majority of systems out of the box even if they don't have a C-Wire. Each Sensi unit comes with a pair of regular AA batteries that provide backup power for several months in most cases. Some users find that they don't even need to change the batteries that often, which again eliminates the need for the C-Wire. 

Venstar Add-a-Wire Adapter

Venstar makes the famous Add-a-Wire adapter, which allows you to connect a smart thermostat directly to the existing wiring configuration as though it had a C-Wire even though it doesn't. The device is relatively inexpensive, and it's easy enough to install. While it doesn't solve every problem, the system can split one wire into two without too much trouble.

Each furnace panel is ever so slightly different from ones used by other manufacturers. This means you might have to do a bit of troubleshooting if you go this route. Nevertheless, you should find that Venstar already has wiring diagrams available for a majority of HVAC systems already in use. Basically, you might be able to split off one of the other wires in your thermostat configuration into two of the control lines then reassign one of the remaining wires as a C-Wire.

Emerson Thermostat Common Wire Kit

Much like the ecobee PEK, Emerson offers a thermostat common wire kit that makes it possible to run a C-Wire when any sort of additional wiring isn't really an option. You might consider this small box if you plan to work with an Emerson thermostat and you have a complete 24V HVAC system. In the future, these units might work with other brands of thermostats but they're locked into their specific hardware vendor for the time being. It might eventually transition into something more like the Venstar over time.

Running a New Wire Yourself

If you have a background in the field of residential wiring or at least know something about the way electricity works, then you might consider adding a C-Wire by yourself. You can run a new power cable directly from the rest of the electrical exchange. 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 more than likely 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 because you'd have to replace all of this wire if you were going to in the future anyway.

You'll probably 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.

More than likely you have all the tools that you need on hand already. Needle nose pliers and some way to strip wire are really all you might not have in the garage right now. A couple of screwdrivers are more than enough to attach the wires together. If you have to go through any interior material, then you'll want to have a caulking gun on hand to seal any new cracks you might be making. Fortunately, you'll probably be able to work with the holes that are already cut for the existing wires provided that the insulation is good on the new cable you're running.

Hire HVAC Professional

Arguably, the best way to add a C-Wire to any HVAC system is to simply hire a technician. Ask the technician if a quote would cost anything, and then have them offer an estimate first. Shopping around could save you a few dollars and might get you a more knowledgeable person than simply going with the first one in your neighborhood. While it might cost more this way up front, you won't have to worry about making any costly mistakes and 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 either. You shouldn't feel ashamed just because you save a little money as long as you're already working with a reputable contractor who you're sure can configure a C-Wire for you.

Stealing Power

While Nest's engineers have a tendency to talk about power stealing the most, this is actually a problem that occurs with thermostats that don't feature any sort of low voltage options. Power stealing is when you allow your thermostat to continue to cycle the HVAC system on and off. More than likely, you'll see it defined as some other way, but in this modern era of smart thermostat equipment that's exactly what it translates into. Since various companies have developed other methods of handling operation without a C-Wire in place, you don't want to ever resort to power stealing any longer.

The idea is that you could theoretically power a display screen off the circuit that control the HVAC system's power network. Stealing too much power makes the system behave in a very strange fashion. Technically, the thermostat only needs to charge up a battery so many homeowners don't really see an issue with power stealing. They convince themselves that they won't be taking much power and therefore they configure it to consume only a light amount of power. What happens eventually is that the thermostat's battery just doesn't get charged up. On top of this, most smart thermostats have batteries that deplete battery life by around 20 percent of capacity annually.

One big paradigm to keep in mind is the fact that the circuitry there is a switch. It wasn't meant to power things. Using it this way is an unsupported hack. Nevertheless, it's not the only hack you'll come into contact with. ​

The Infamous Fan Wire Hack

When using an Add-a-Wire device, you could combine the Y-Wire and the G-Wire into a single wire then use the freed up one as a C-Wire. Some people prefer to make their own custom hack where they actually flat out turn the fan wire into a C-Wire without one of these devices.

As well as popular, this hack is completely not supported and few manufacturers have ever approved it. It's also an "excellent" way to disable the manual setting on your house fan, so no one should really give it a try. If you're curious because you know a technician who keeps talking about it, then what they're referring to is literally permanently putting the fan on automatic and using the manual relay as a power cable. Once the system gets restored, you'll never be able to use the house fan the way you used to. The irony in all of this is that you could install a smart thermostat with dozens of bells and whistles while also sacrificing a feature so basic that it predates the 20th century.

External Transformer Method

If you feel a bit more comfortable with residential wiring, then there's one other method you might consider. Keep in mind that this isn't for everyone, but it's certainly a good bit better than the fan wire hack you might have considered briefly. You can actually configure an external transformer to work with your existing lines in order to provide power to a WiFi thermostat. It should be noted that this is an involved process, but it should work for anyone who has experience wiring a 24VAC transformer box.

The idea is you purchase a 24VAC transformer of a standard type. These boxes aren't something that's specific to the HVAC industry, so you might be able to find them in any list of electrical parts. You'll need the right wire type to match the type that the internal electronics uses. These pieces of equipment are the same exact type that you're probably familiar with for use as wall-wart battery adapters. They've long been used as chargers.

As long as you feel comfortable with it, you can actually put a hole in the wall near the thermostat along with a second one near the baseboard. You would then feed a wire to the thermostat and attached it to both the transmitter and the thermostat box itself. Everything else would just get plugged in from there. Making the holes and matching the right adapter to your device is the hard part.

Some manufacturers actually make this process extremely simple. If you have a 24VAC HVAC system without a C-Wire, then you might be interested in the adapter that comes with ecobee thermostats. This installs right at your furnace to make up for the missing C-Wire. You might not have much difficulty with it even if you have no prior electrical experience.

Perhaps most importantly, this keeps manual control over your house fan completely intact. You might want to run your house fan independent of your AC unit during the warmer months. Blower fans use far less electricity than the AC unit does as a whole, which helps to reduce energy usage even beyond what your smart thermostat can do by itself.

Resources & Further Reading

Thermostat Wiring

How to Wire Thermostat?

Thermostat Wiring

Thermostat Wiring Colors

Thermostat Signals & Wiring Colors

Identifying Thermostat Wires

Thermostat Wiring Color Code Guide

Thermostat Wiring Colors

Thermostat C-Wire

What's a C-Wire

The C-Wire Issue

Thermostat Common Wire Overview (Video)

When Nest Needs a C-Wire

Do I Need a C-Wire? (PDF)

Installing Thermostat

How to Install a Wi-Fi Thermostat

How To Install And Configure A Smart Thermostat

Installing Emerson Sensi Thermostat

Installing a WIFI Thermostat with no C Wire? No Problem! (Video)

C-Wire Alternatives

Ecobee Power Extender Kit

Installing ecobee4 with a PEK (Video)

Venstar Add-a-Wire (PDF)

Venstar Add-a-Wire Manual (PDF)

How to Power the Thermostat Via an Additional Transformer (Video)

The Problem with Power Stealing

Nest Thermostat Problem

Substitute G-Wire for C-Wire (Video)

Adding C-Wire

How Can I Add a C-Wire to My Thermostat?

How to Add a C-Wire to Thermostat

Hiring HVAC Professional

​10 Tips for Hiring a Heating and Cooling Contractor