Thermostat wiring, explained

Welcome to! This website explains the purpose of your HVAC system’s c-wire and options to consider if you don’t have a c-wire and want to have a WiFi enabled smart thermostat. This website is intended for North American readers.

Thermostat wire, explained

Thermostat wire is a bundle of 18 or 20 gauge copper conductors. Each individual conductor is sheathed in a protective, flexible coating of a different color. The exact number of colored wires varies, as does the gauge of the wires. 18 gauge wires are slightly thicker than 20 gauge wires.


When shopping for thermostat wire, it’s common to see a number like “18/5” on the packaging. The first number refers to the gauge, the second number to the quantity of individual copper conductors inside the bundle.

At your thermostat, thermostat wires emerge from the wall and are stripped at the ends to reveal the copper inside. The copper ends are hooked to terminals on the thermostat’s base.thermostat_wires_wall

Over in your furnace and other accessories, the other ends of these wires are hooked to similarly labeled terminals.

» See our thermostat wire color guide for more detail on wire colors

What is the c-wire?

In a low voltage HVAC system, the c-wire, also called the “common wire”, is one of several copper wires that run between your furnace and your thermostat. Not all HVAC systems have this wire.

If present, the c wire enables the steady flow of 24 volt power to the thermostat. This constant flow of power is necessary for most WiFi enabled smart thermostats and many digital thermostats.

Your system might have any combination of wires and colors. You might have as few as two wires (likely red and white) or as many as six (or more) in a multitude of colors.

Typical low voltage 18/5 thermostat wire includes five colors: blue, yellow, green, white, and red. Keep in mind that any color wire can be used for any purpose, and your thermostat’s installer might’ve been “creative”.

Common HVAC wiring setups

Heat-only 2-wire system

In the most basic of HVAC systems (such as an older forced air system) you might find just two wires and a simple mercury thermostat at the other end. The terminals inside the thermostat are probably marked R and W.

If this describes your system, check out the Nest Learning Thermostat ( link). The Nest should work with your system with no additional wires needed.nest_r_w_2_wire_system

Best options for systems without a c-wire

Add an adapter


We recommend the Venstar Add-a-Wire adapter, which can make a 4-wire system act like a 5-wire system by combining the signals of two wires onto one.

It sounds complicated but it’s easy to install (easier than running totally new wires, anyway) and is the most frequently recommended and most popular solution to the missing c-wire dilemma. The Venstar Add a Wire unit installs inside (or near) your furnace.

This 5 minute YouTube installation guide gives you a good idea of what to expect, and the diagram below from illustrates the final assembly.


Get an ecobee thermostat + power extender kit


Ecobee’s WiFi smart thermostat, the ecobee3, is one of the best-reviewed smart thermostats of the last two years. See positive ecobee3 reviews here, here, here, and here. One of the reasons it’s so well-liked is because it comes with a power extender kit that gets hooked up inside your furnace to add power-giving goodness to your existing 3-wire or 4-wire setup (either one works).

ecobee3’s included power extender kit eliminates the need to run new wires in a 4-wire or 5-wire system

No c-wire needed, and you get a great WiFi enabled thermostat with all the bells and whistles in the process. See Step 3 of ecobee’s official installation instructions to get a better understanding of the PEK setup.


… or an Emerson Sensi smart thermostat


The Emerson Sensi WiFi programmable thermostat is also a top choice because it’s just like the programmable thermostats everyone already knows and loves (digital screen, low power consumption) but with all the goodness of a modern WiFi thermostat. The Emerson’s companion app is great for setting up schedules, overriding existing programming, and monitoring usage.

I have a C-wire, does that change your recommendation?

Not really, having a C-wire just expands your range of options. In our opinion, the best thermostats on the market today coincidentally happen to be the ones that don’t require a C-wire.

Here are our top 5 wifi thermostat picks for spring 2016:

  1. ecobee3 *Best seller among CWireGuide visitors*
  2. Emerson Sensi *Best budget smart thermostat*
  3. Schneider Electric Wiser 
  4. Honeywell WiFi Smart Thermostat
  5. Nest Learning Thermostat (3rd generation)