Before making the jump to a smart thermostat, become familiar with the c wire. The most common problem people have when making the jump to a wifi thermostat is unknowingly lacking what’s known as the c (common) wire. Here’s my neighbors not so pleasant experience:


The great powers of Tony Fadell’s Nest Learning Thermostat seemed so magical. As soon as I convinced my wife it was worth the investment, I went straight to Amazon Prime. After  unboxing the glorious puck thermostat and I proceeded to use my awesome handyman skills to read the directions (skills are not needed, Nest makes it super simple). When it started having trouble connecting it to my wifi, I realized my common wire does not have enough voltage to power the Nest! – My Neighbor

Lesson learned: Most modern thermostats require what is known as a c wire. They really are pretty smart thermostats, so extra power is needed for maintaining a the screen, wireless connection and powering it’s small processor.


What is a C wire?

The c wire provides 24vac power source. The old mercury thermostats did not require any power. Modern thermostats have many extra features that need additional power such as a large screen or wireless connection.

Thermostat wire connects the thermostat to the air handler or furnace allowing for communications between the two. Older wires will have only 2 or 4 strands but the 5 strand will provide that extra wire known as the c wire. When connected inside the air handler, it provides that needed 24vac power for the smart thermostat.

If you’re doing research on buying a smart thermostat, it’s likely you’re also planning on installing it yourself. I’ve put together a quick guide to finding out if you have a c wire already and if not, what are some ways to add a c wire.


Do I have a C wire?

Without disconnecting any wires, remove your current thermostat from the wall. You’ll likely need to remove the wall plate to reveal the wire connections. If you count 5 wires, congrats, you have a c wire! If you’re not so lucky, you’ll need to run a new wire from the air handler or furnace to the thermostat or pick on of the other solutions below.

finding a c wire

The c wire thermostat wire is blue normally. However, the colors of the wires do not necessarily mean anything, it just depends on how the installer initially made the connections. It’s important to label the wires to their connecting terminal before disconnecting.


Is a C wire really required?

Yes. To avoid any future problems, take the time to future proof your home. Nearly all modern thermostats now require a common wire with the exception of the Sensi Wi-Fi Thermostat and ecobee 3 which comes with the Power Extender Kit.


Wifi thermostat but no C wire solutions

Pick a different thermostat

Two smart thermostats that do not require a C Wire: ecobee3 (our favorite) or Sensi Wi-Fi Thermostat

The ecobee3 actually comes with a Power Extender Kit to add a c wire. If you do not have a common wire, the kit will provide the power needed to substitute for not having one. Ecobee is the only company that has included a kit with their thermostat.

The Sensi Wi-Fi Thermostat simply does not require one because it does not have all extra technology that demands for a common wire. It still connects to wifi and has a companion iphone/andriod app like others, but lacks the fancy touchscreen. My grandparents loves that it’s still simple to use like their old White-Rogers. Great for the family so we can make sure they adjust it when they travel.


Run a new wire

Thermostat wire can be found pretty cheap on Amazon. There are two different weights: 18/3 (heat only) and 18/5 (heat and cooling). If you only have heat, you could settle for 18/3, but I would recommend playing it safe with the 18/5.

24vac transformer

A workaround for no c wire can be done using a transformer. Here’s a video that walks through the steps on how to install a smart thermostat without a c wire. I have to admit, it seems a bit hackerish, but it works! Here’s the transformer that I personally like because it has a 20′ cord to reach a far outlet.





Venstar Add-a-Wire adapter

With a little knowledge of wires and the handy step by step directions provide, the Venstar Add-a-Wire is a simple, fast way to add a c wire to your smart thermostat. You’ll need access to the air handler/furnace as well as behind the thermostat. We highly recommend it as a cheap alternative to running a new thermostat wire.




There’s a great step by step install provided by Venstar. Total install time is amount an hour. The only tools needed are phillips screwdriver, drill, electrical tape, and pliers.

Hire a Pro

Hiring a professional is my recommended solution. You spent a couple hundred bucks on an awesome thermostat. Don’t be afraid to spend a little more to get the job done right. They will run a new thermostat wire from the air handler to your stat. If you’re in the Tampa, FL area, we recommend Marlin James. 813-689-0817

Fan wire hack

Do not do this. Yes, this is by far the easiest solution, but you forfeit the ability to manually turn the fan on. Don’t leave any features behind. You paid good money for awesome technology, install a C wire, use an adapter, or hire a pro.

The bottom line is don’t let not having a c wire stop you from upgrading your thermostat. Toss that outdated thermostat (unless it’s a mercury thermostat, which should be discarded appropriately) and start saving time and money with a modern thermostat.