What Is a C-Wire On a Thermostat?

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If you’re asking this question chances are you’re looking at getting a smart thermostat. While some thermostats, particularly older models, don’t require a c-wire, it’s more often the case that any modern thermostat will need one, and most smart thermostats will note this requirement.

Most wall mounted thermostats, smart or otherwise, are known as ‘low voltage’, and the c-wire discussion only applies to these types of heating and cooling systems. Low voltage refers to the fact that the control system uses a 24V power supply in contrast to a line voltage system which runs the full 120V right to the thermostat.

What is a C-wire?

All low voltage thermostats require some source of 24V power in order to do their thing, unless they are old, purely manual switch type models. Simpler types can get enough power from the wiring the controls the heating and cooling functions, but anything with more internal electronics will need a dedicated power supply so as not to interfere with the switching functions of the HVAC system but drawing too much voltage through the control wires.

The C in c-wire stands for ‘common’, and acts in the same way as the neutral wire in high voltage applications. It provides a dedicated return path for the electrical load needed by the thermostat to power it’s own features. In the case of smart thermostats this is particularly necessary because of the presence of communication radios (WiFi, Z-Wave, or some other protocol), touch screens and possibly additional sensors.

4 wire and 5 wire thermostats

Thermostats can have various wire terminals to support different HVAC features, such as two stage heating, heat pumps, auxiliary fans and so forth. Ignoring these extra features, basic thermostat functions need four wires, with a c-wire making up a five wire system. It’s important to note that the colors of the wires are NOT standard, and can vary between manufacturers and installers. Use the labels on the terminals to identify the correct wires in your system.

A 4 wire system will have:

  • G - Fan control

  • R - 24 VAC input

  • Y - Compressor (cooling) control

  • W - Heat control

A 5 wire system will include an addition wire, the C-wire, or Common wire:

  • G - Fan control

  • R - 24 VAC input

  • Y - Compressor (cooling) control

  • W - Heat control

  • C - Common 24 VAC

How To Find Your C-wire?

The easiest way to determine if you have a c-wire is to check your existing thermostat. It’s a good idea to turn off the breaker to your HVAC first to avoid damaging your equipment if wires get inadvertently pulled. Once done, you should be able to pop the cover off your thermostat and see the wiring terminals.

Most thermostats are designed to make this fairly accessible, but some may be on the back. In that case you’ll need to pop the thermostat itself off the back plate and gently pull the wires out of the wall a little so you can see the terminals on the rear.

Check if you have a low voltage thermostat

First we need to ensure you have a low voltage system. This will be obvious if there are four or more multi-colored wires that are thinner than typical electrical wiring. If there are only two or four thick red and black wires joined with wire nuts, then you have a line voltage system.

Your only option in this case is to get a line voltage smart thermostat. The options in this space are a lot more limited, but there are a few products that may suit with some possible limitations on supported features. See our list of buyer’s guides further down for some options.

Look for the C-wire

Assuming we have a low voltage thermostat we can proceed to check for the c-wire itself. If you have a c-wire it should be connected to the C terminal (naturally) on the thermostat. The wire is most often blue, but there is no actual standard to guarantee that is the case, so checking the terminal itself is best.

If there is no wire connected to the C terminal, you may still have one. Thermostat wiring is generally supplied in a bundle, and the c-wire may be simply tucked into the wall. You can pull a little more out and see if it’s there or not. You could also do this as the other end to see if there is an unused c-wire or even one connected to the control board of your HVAC.

Remember, because the wire color doesn’t actually matter, if there is any unused wire in your wall that goes back to the HVAC system then you can use that by connecting the other end to the C terminal on the control board.

Thermostat wiring with a c-wire

What If I Don’t Have a C-wire?

If you find that you don’t have a c-wire (and you do have a low voltage thermostat setup), there are some options available to resolve the situation.

Get a thermostat that doesn’t need a c-wire

On the face of it, this is the easiest option, but it may not be the best. There are certainly smart thermostat models that don’t need a c-wire, although most of these still have the option to use on. Usually the c-wire requirement is avoided by using batteries to power the thermostat instead. Seems perfectly fine, right? In my experience, you’ll likely find it’s a bit of a hassle.

Not only will you have to deal with replacing batteries every few months (or even weeks) depending on various factors like usage, placement, model, and signal strength, you’ll likely find that the performance of the device is not as good without a permanent power source. It’s a common complaint with battery models that they are less reliable in terms of WiFi connection as well as climate control performance. Of course you’ll also run the risk of the battery dying while you’re away and the thermostat not working at all.

Use a c-wire adapter

In some cases you will be able to use a c-wire adapter. These are small devices that can take the inputs from the HVAC control board and give you the required outputs as well as a c-wire. This works by re-purposing one of the existing wires between your thermostat and HVAC system as the c-wire and then using some electronics to replace the functions of two of the other in-use terminals, usually the G and Y.

There are a few models of smart thermostat that come with such an adapter in the box, such as smart thermostats from ecobee and Wyze. Other brands can use an add-on unit like the Emerson Common Wire Kit or Venstar Add-a-wire accessory.

Use a 24VAC transformer

Another alternative to the c-wire adapter could be a 24VAC power brick. This would need to be plugged into a conventional wall outlet near your thermostat, so it’s the least desirable option because you’ll have the power cable hanging out there for all to see. If you’re really in a pinch, though, this may be a viable option.

These transformers are actually the same power supplies that wired doorbells use, so they’re fairly easy to get hold of, and have plenty of wire length to reach a power outlet. One example is this highly rated model from Hotop which comes with a good length cable and a whole bunch of cable clips to secure it to the wall.

Below is an example of how a similar transformer would be wired to provide continuous power to the thermostat. The AC shown is a simplistic representation, but you can see how simple the connections can be for the power.

Thermostat c-wire 24VAC transformer diagram

Install a c-wire

Ultimately the best solution is to just install a c-wire and be done with it. This involves pulling out your existing thermostat cable and replacing it with a 5-wire version.

You can pick up rolls of thermostat 18 gauge cable from Amazon in various lengths relatively cheaply, and if you go for an 8 wire cable like the one pictured you’ll have some future proofing as well.

You’ll need to be careful to photograph and label all your existing wires at both the thermostat base plate end, and the furnace or HVAC control board, and be sure to do any wiring changes with the power off to prevent damage to your expensive equipment.

As with anything regarding your thermostat, if you’re not comfortable pulling cable through your walls it’s best to get a professional to do it properly.

18/8 thermostat cable

Thermostat Buying Options

So now we know more about what the c-wire is for, how to find it, and what to do if it’s not there, we can look to smart thermostat models that we can use in each situation. We have a few buyers guides for different thermostat requirements:

Thermostats that work without a C-wire

Our picks for the best smart thermostat options that are specifically designed to work without a c-wire at all.

Thermostats for Amazon Alexa

See the best Alexa compatible smart thermostat models that work with or without a c-wire. We also include some good budget options.

Thermostats for Z-Wave

See the best Z-Wave compatible models that work with a c-wire, run on batteries, or run on line voltage systems, also with budget alternatives.

Summary

Assuming you don’t have a line voltage heating system, a c-wire is the best way to provide power to your thermostat, especially in the case of smart thermostats where you have higher power requirements for fancy touch screens and communication radios. Some thermostats can use battery power, but it’s a hassle and you’re better off getting a c-wire in there somehow.

There are a few ways to do this depending on your situation, budget, and the effort you’re prepared to spend. Be it using a cheap c-wire adapter, plugging in a 24VAC transformers, or just getting a proper c-wire installed, this is most often a solvable problem.