Wi-Fi Thermostat Has No C-Wire? (Here's What You Should Do!)

Benjamin Panico
by Benjamin Panico

Wi-Fi thermostats connect to your wireless internet so you can adjust them remotely. This super handy, smart device often requires a c wire to have the proper hook-up. The c wire connects power directly from your thermostat to the furnace. However, if you do not have a c wire, there is no need to worry.

There are a few solutions available to folks who want a Wi-Fi connected thermostat but do not have a c wire available. You may decide to rewire your house, get an adapter for the thermostat, or get a thermostat that runs on battery power. There are also thermostats designed for line voltage systems, so you have a few different options to choose from.

We will go over all these options and the best thermostats available below. But first, you are probably wondering how to find out what kind of wiring you have. This is an important first step because your current set-up will determine which options below are available and make sense for you.

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How to Find Out if You Have a C Wire

First things first, you need to know if you have a c wire. If you live in an older house, the answer is quite possibly no. Luckily, it is easy to check for yourself in order to make sure you know what set-up you are starting with.

First, go to your circuit breaker and turn off the power to this area. While the power draw to a thermostat is very low, it is best to be safe when dealing with wires. Next, you just need to take your thermostat’s face off the wall and look at the wires connected to it. You should both count the wires and check the terminal labels that are connected.

Below is a chart showing the common colors for wires. This applies only in cases where you have a five-wire set-up. If you do not have this set-up, the other possible options are discussed below this.

As you can see in this five-wire set-up, the c wire is usually the blue one. But you should know that the colors are more like mere suggestions. This is not set in stone, so to be sure, find the label “C” for “common,” and check that there is a wire attached here.





Counting Your Wires: If You Have a Three or Four Wire Set-up

As noted above, this chart shows the specific scenario in which you have five wires to hook up to your thermostat. The other best-case scenario would be if you have a sixth, likely black wire. In either of these cases, you would have what you need to be compatible with a smart thermostat system. Unfortunately, if you are here, it means you probably do not have this set-up.

If you count only three or four wires, then it could be that the fifth, c wire, is hiding. It is very common for five wires to be installed but the fifth not utilized with older thermostats that do not need it. Check in the wall to see if the blue c wire is there and just not being used right now. A system with three or four wires is running on low voltage and maybe your current thermostat just didn’t need the fifth one. You may very well have that blue wire if all the other ones are there.

Counting Your Wires: If You Have a Two Wire Set-up

If you pull off the thermostat and see only two thicker wires, this is a line voltage thermostat set-up. These wires are most likely red and white. Unfortunately, these wires are also not compatible with most smart thermostat systems.

That said, there is no need to fret! There are adapters available to turn a line voltage system into a five-wire system that will be compatible with your new Wi-Fi enabled thermostat. There are also a couple of brands of smart thermostats that actually work with this older set-up. Therefore, you do have some options available.

Option One: Re-wire Your Home

So what do you do once you have looked behind your thermostat and determined that there is not a c wire present? There are a few ways to handle this set-up in order to make your system compatible with a smart thermostat. There are also a few different Wi-Fi connected thermostats available for basically every set-up. We will cover all the options for upgrading your home, as well as the smart thermostats available.

The most difficult option is also the way to best future-proof your home for newer devices down the road. Rewiring your thermostat to furnace system may seem daunting, but it is the best investment in your home. Having the proper wiring now will mean that your system is always compatible with any new technologies that come out later.

To do this, you simply need to pull out the existing wiring and put new wires in. You will want to get at least five wires, but having a sixth would not hurt. In fact, some electricians may recommend eight conductors, but that isn’t as common. This eighteen-gauge, six-conductor wire (labeled “18/6”) from Amazon is very affordable. A break down of the costs for some sample lengths of wiring is below.





Consider Hiring a Professional for This

When you get the new wiring, you need to connect it from your furnace to the thermostat. Depending on how far away the two are, this may be a very quick or intimidatingly big project.

A final note on this option is that, due to the difficulty of this project, you might choose to hire a professional. In this case, the cost will be quite a bit higher. Depending again on the length of the wiring, it may take a few hours. You can have a professional provide a quote for the service and also provide advice if you choose to do it as a DIY project.

Professional Contractor

Hourly Rate

Electrician$50 – $150

Option Two: Get an Add-a-Wire Adapter

For Four-Wire Set-ups

A slightly easier way to get that fifth wire for your smart thermostat is to use an “add-a-wire” adapter. This product was invented specifically for this purpose. However, you need to have four wires for it to work. If you have counted your wiring and just need one more, this might be a great option.

The Venstar Add-a-Wire is very affordable and easy for a DIY enthusiast to install. It costs around $30, and you should be able to get everything set up within an hour. Once you do, you will be able to choose any smart thermostat on the market, and it will work with your set-up.

For Line Voltage Set-ups or Three-Wire Low Voltage Set-ups

There are also adapters available for those two-wire line voltage set-ups. This adapter by Aube Technologies also works for a three-wire set-up, which is great since the Venstar does not. The adapter boasts easy installation, and it only costs $40.

The adapter will turn any line voltage system into a low voltage system. Once you have a low voltage system, your set-up will be compatible with most smart thermostats available. These adapters are definitely easier options than rewiring your home. Even easier are the thermostats below that come with adapters built-in.

Option Three: Get a Thermostat with a Built-in Adapter

Ecobee is a brand of smart thermostats that always come with an adapter, no matter what model you get. The adapter allows you to easily install this product in the case of a missing c wire. The pricing for these products is below. Keep in mind that prices may differ slightly based on the number of additional room “smart sensors” you decide to include.

Both products claim to be installable in 45 minutes or less, so if you go this route, it will be a breeze! The costs of these thermostats are obviously a little bit higher than other DIY solutions. However, it is very comparable with other smart thermostat systems that include adapters or other power solutions.

Option Four: Get a Battery-Powered Smart Thermostat

Yes, these do exist! If you found yourself without a c wire, and do not want to deal with an adapter, you are not out of luck. There are in fact smart thermostats that are compatible with this difficult set-up. The answer for you is a thermostat that runs on double-A lithium batteries.

At around $100, the LUX Geo smart thermostat is an affordable option with a lot of great features. It actually can even be powered by a micro-USB cable if you do not like changing batteries.

LUX Geo Smart Thermostat


In white$99
In black$115

You will not feel like you are settling for less with this smart thermostat. It pairs with a mobile app for remote usage, and it has seven-day programming, so you can set it up and never think about it again. The LUX thermostat also can connect to an Alexa device, though it is not built in like with Ecobee’s model. It also has an air filter monitor and can be positioned vertically or horizontally.

Another cool feature that this smart thermostat offers is radial geofencing. Essentially, it uses your phone’s GPS to trigger the heat or A/C whenever you are close to home. This is a great way to save on energy usage, but also make sure you don’t come home to an uncomfortable temperature.

Option Five: Get a Smart Thermostat Designed for a Line Voltage System

Again, yes, these do exist! You may be surprised since line voltage systems are not very common these days. Most smart thermostats do not bother to try to accommodate them for the few homes that have these set-ups. Luckily, a few smart thermostats do.

If you go this route, you will not be future-proofed for a future thermostat you might like that requires a c wire. If that is not a big deal to you, these thermostats are a very simple (and very affordable!) solution to your problem.

One great option is Casa Energy’s Caleo Wi-Fi line voltage thermostat specifically for electric baseboard heaters. This device is around $90, and two additional models (for fan-forced heating and electric radiant floor heating) to be released soon.

The Z-Wave Plus by Stelpro is another option for a Wi-Fi connected thermostat, as long as you have a baseboard heater or convector. At $80, this smart device is quite comparable to the Caleo.

One thing to keep in mind with this is that this device requires a controller for programming. You cannot program it directly through the device. While this may be a little frustrating, it is still an easier solution than any of the wiring fixes noted above.

Other Great Smart Thermostat Options (Most Require C Wire)

Google Nest

Finally, if you do not have any of the restrictions noted above, or if you re-wire your home, you can choose any smart thermostat you want. In this case, the Nest by Google is a very popular option. Once you have your five wires set-up, the Nest makes installation very simple.

If you see that the Nest does not require a c wire, you should be wary. The Nest works by using lithium batteries that recharge themselves off your home’s power when the heating or cooling is on. This works great if you have a c wire dedicated for this purpose.

However, if you do not have enough wires, the Nest will have to draw power from the wires that control the heating or cooling. Sometimes, this accidentally turns on the system when the Nest is just trying to recharge its batteries.

You do not want a thermostat that is turning on by itself. This is a great way to waste a lot of money on utilities. It can also cause the thermostat to die altogether since it will not be able to recharge at all. Therefore, the Nest is really only recommended for set-ups with a c wire.

Google Nest Smart Thermostat

Price Range

Thermostat E$139
Learning Thermostat (self-programming)$234 – $249, depending on the color

Emerson Sensi

The Emerson Sensi smart thermostats are very budget-friendly options. If you are not looking to spend too much money, but still want some smart features, this is a good option. Their thermostats have full Wi-Fi connectivity and a mobile app to access the easy programming controls.

Depending on your heating and cooling system, their model with buttons may not require a c wire. (The touchscreen model does require a c wire.) You do not need a c wire in the case where you have a conventional heating and cooling system. This would be if you have a gas or electric furnace, air conditioner, and boiler. However, you do need a c wire if you have cooling only, heating only, or if you have a heat pump.

Emerson Sensi


Smart Thermostat (with buttons)$78
Smart Thermostat (with a touchscreen)$127

Honeywell Products

Finally, Honeywell is a very reputable brand that you will probably recognize. They have been around for quite a while. Despite keeping the old-fashioned appearance, their thermostats now do in fact have smart features.

Most of their models do require a c wire, though. If you are going for a Honeywell thermostat, make sure to get an Add-a-Wire adapter. Otherwise, you can check out the T5+ that comes with a power adapter already.

The Benefits of a Having C Wire Connection

There are a lot of great work around options if you do not have a c wire. However, you may decide that you would actually prefer that your thermostat have one. Essentially, the point of the common wire is to provide continuous power to the thermostat.

This allows it to constantly monitor the room temperature and provide the most accurate reading. These thermostats provide the most comfortable heating and cooling, and you would not experience any temperature swings with this. Additionally, constant monitoring also allows these thermostats to be more energy-efficient since the readings are more precise.

As discussed above with the Nest, any battery-powered thermostat will lose this functionality. This is something you should keep in mind before choosing the Nest or LUX products. The thermostat will not be on and monitoring your room’s temperature when the batteries are recharging.

You ultimately may decide that this is okay. Battery power is definitely an easy solution to not having a c wire. However, adapters are not that expensive and easy to install. Plus, the thermostats that come with adapters (like all Ecobee products and Honeywell’s T5+) are reasonably priced considering this added feature.

Do You Need Your Thermostat Repaired or Reprogrammed?

Get free, zero-commitment quotes from pro contractors near you.

Related Questions

Where is the best place to put a thermostat in your home?

As long as the thermostat is not exposed to high temperatures (like next to a fireplace) or direct light, anywhere works. Placement in the sun can cause it to misread temperatures and therefore function improperly. You also want to make sure it will not get bumped easily, which could turn on or off the heating and cooling system unnecessarily. If you need to move your thermostat, it is a relatively simple DIY project.

How long can you expect your thermostat to last?

A thermostat that is not damaged or placed in a bad area can last up to ten years. Thermostats are easy to maintain and typically reliable for as long as the connected wiring is. Most of the companies mentioned here have shorter warranties, which is to be expected. Nest has a one-year limited warranty with a three-year extension for certain conditions. Ecobee, Casa Energy, and LUX all have three-year limited warranties.

What can happen if you install a thermostat incorrectly?

Installing a thermostat wrong can cause a lot of problems. The best-case scenario would be that you blow a circuit breaker. The worst-case would be that the electric shock starts a fire. Thermostats typically use very low voltage to run, so a shock from this will probably not kill you. That said, it is a good idea to turn off the power to that area before beginning.

Additionally, you could send a shock back to your boiler or furnace and damage your heating or cooling system. Depending on your DIY experience, you might prefer to have a professional do it quickly and correctly. If you do take on this installation project, just make sure you match up the wires correctly!

Benjamin Panico
Benjamin Panico

Benjamin is a freelance writer and graphic designer. He is passionate about DIY projects and finding creative ways to upcycle things headed for the landfill. Based in Oakland, CA, Benjamin enjoys playing guitar and gardening.

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