What is a C Wire? And Why’s it so Important for Your Smart Thermostat?

If you plan on getting a smart thermostat and don’t have a c wire, please save yourself some headaches and get one. I promise it’s not expensive and just about anyone can do it.

So, how do you do it?


What is the C Wire on a Thermostat?

The c wire is an extra wire that can be used to provide continuous 24V power to any application.  It’s normally used to provide power for the thermostat.

The “c” stands for common. It is often labeled as “c” on thermostat backplates.  Keep in mind that it is not necessarily labeled as c and the wire is not necessarily any particular color. Although there are some best practices, there are no strict standards that wire name and color adhere to.

Although I’ve seen it stated in several places, it’s not correct to say that the c-wire is what powers the thermostat.

Typically, the wires that provide the power (often called the “hot” wires) are labeled Rc (cooling) and Rh (heating). They provide a source of 24V power coming from the HVAC control board.

Sometimes they are separate wires and require separate connections. Sometimes they are the same wire. It will typically be labeled Rh/c.

In order to have a circuit that power can flow through, wires need to connect back to the HVAC control board to complete the loop. That’s what the other wires do.

So, the c wire doesn’t actually provide the power. It provides a return path so that the thermostat can be powered without disrupting the other wires which are used as electrical on/off switches for your equipment.


Why Do I Want a C Wire?

Now that you know what the c wire is, you may be asking yourself, why do I want it?

Today’s wireless thermostats require more power than the simple calculator-screen programmable thermostats of the recent past.

Keeping a connection to a WiFi router or an automation hub is one power requirement that never existed. Trying to maintain a WiFi connection will drain a couple AA batteries within days.

Many smart thermostats come equipped with a large, full-color touch-screen. This is another feature that contributes to the need for a constant power source.

Using a c wire is the best way to provide the constant power that is required.

The colorful, bright screen on the Wiser Air thermostat looks great but uses a lot of energy doing it.
The colorful, bright screen on the Wiser Air thermostat looks great but uses a lot of energy doing it.

Some smart thermostats claim to not need a c wire. One example is the Emerson Sensi WiFi thermostat. This quote from their website makes me think otherwise:

“If you already have a c-wire connection, even though it is not required, we recommend connecting it to Sensi to help improve Wi-Fi connectivity and battery life.”

If connecting a c wire improves WiFi connectivity and battery life, that means it’s less than optimal without it. I don’t know about you, but those are two things that I very much prefer to have at their optimal levels.

If your thermostat loses power completely, your equipment can’t turn on. Do you really want to risk that happening?

Smart thermostats are supposed to make our lives easier. Nobody wants to be replacing batteries once per week or even once every couple months. I want to be able to hook one of these things up to the wall and hardly even think about it for the next five years.

I don’t want to be on vacation worrying about my thermostat batteries or WiFi connection. In fact, I don’t want to be anywhere and have to worry about anything related to my thermostat.


What is a Power Stealing Thermostat?

Some thermostats have tried to get around the no c wire problem by using a technique commonly referred to as power stealing. Power stealing uses the existing circuits to “steal” a small amount of power. The “stolen” power charges a battery, and the battery powers the thermostat.

The existing circuits are designed to act as on/off switches. Their function is not to actually power anything.

There is a certain “threshold” current running through the wire that represents the switch point. Any current above the threshold is “on” and any current below the threshold is “off”.

So, there can be a small amount of current running through the circuit without triggering the “on” condition. That is the current which can be used to power the thermostat via power “stealing”.


How Can Power Stealing Cause Problems?

The problem with power stealing is that the HVAC circuits were not designed for this. It provides a very small amount of power so doing it well is difficult. If you steal too much, you turn your equipment on when you don’t want to. If you steal too little, your battery gets drained and eventually the thermostat will go dead.

More power can be stolen when the HVAC system is on than when it is off. The thermostat can charge its backup battery when the equipment is running but tends to slowly discharge when the equipment is not running. This means that during times of minimal usage, the chance of running your backup batteries really low or even completely dead is a possibility.

To make matters worse, some thermostats like the Nest have a permanent battery. Rechargeable batteries like this will tend to lose their charge capacity over time (as anyone with a smartphone has ever found out). The power stealing with the Nest may work fine at first, but as the battery gets older, you may end up with problems.

For more information about the problems with power stealing, here is a great post on the Ecobee site that explains why they don’t use it.


How Do I Know If I Have A C Wire?

*Before messing with any sort of wiring, it’s always a good idea to shut the power off.

To check if you already have a c wire hooked up, start by taking your current thermostat off the wall to expose the wiring.


Check the labels

You will probably see a few wires connected to some labeled terminals. If you happen to see one connected to the terminal labeled “c”, then you are in luck. You have a c wire!

It's kind of hard to see, but the blue wire on the right is labeled "c". Great! I have a c-wire.
It’s kind of hard to see, but the blue wire on the right is labeled “c”. Great! I have a c-wire.

If there is a terminal labeled “c” and it’s empty, you probably don’t have a c wire. Hang on, though! It might still be there…


Are there any extra loose wires?

Sometimes installers run the c wire from the control board to the thermostat location but don’t hook it up. You may want to take off the thermostat backplate and pull out the wires to see if the extra wire is there.


Check The Control Board

If you still aren’t sure if you have a c wire, go to your HVAC control board. Check to see if there is a wire attached to the “c” terminal.

The blue wire connected to the c terminal corresponds to the one at the thermostat
Ewww. What a mess! However, I can see that the faded blue wire connected to the c terminal corresponds to the one at the thermostat.

If there is no wire, you don’t have a c wire and will probably want to get one. If there is a wire connected, note its color and go back to your thermostat. Whichever wire matches the color, should be your c wire.

Three Ways To Add A C Wire

Don’t get stuck pulling your hair out trying to run a smart thermostat without a c wire. If you’re anything like me, you need to keep all the hair you have!

Don’t worry, it’s not that big of a deal to add a c wire. If you can handle programming a smart thermostat, you can figure out how to add a c wire.

Here’s the three best ways to do it:

1. Install Another Wire

This is the best solution in my opinion. However, it’s not the simplest solution. It requires running a wire from the HVAC control board to your thermostat.

If your home is already finished, this means trying to fish the new wire through the walls without destroying them. Depending on how far from the control board and how finished your walls are, this can range in difficulty from easy to hard.


2. Purchase An “Add-A-Wire” Kit

In situations where it may be too difficult to add an extra wire, the “add-a-wire” kit allows your thermostat to use your existing wires and still get constant power without using power stealing.

Venstar ACC0410 Add-A-Wire Accessory

The kits are installed at the HVAC control board. Installation requires a screwdriver and the ability to read a wiring diagram.

Keep in mind that your control board is probably a fairly expensive piece of equipment. So, if you have any doubt about what you may be doing, it’s probably best to contact a professional.


3. Purchase A 24V Adapter

The 24V adapter has two wires that plug into the thermostat to power it. The other end just plugs into a regular outlet.

This is the easiest solution, but probably my least desirable. It’ll work just fine, but you’ll have an unsightly wire running from the thermostat to the outlet.

You could buy any 24V transformer to do the job, but this adapter is specifically made for smart thermostats. It has the two wire ends already stripped and ready to install in the thermostat. It also has extra long wires in case you don’t have an outlet near your thermostat.


Final Thoughts

Smart thermostats use too much power to be run by batteries. Power stealing can work, but it can be unreliable and is using your wiring in a way it wasn’t designed for.  A c-wire is the best option and makes sure that your smart thermostat will have power as long as your house does.

Any comments or questions? Please leave a comment below and I’ll be happy to start a conversation!

Eric Blank

Eric Blank blogs about smart homes and other connected technology here at thesmartcave.com. He enjoys technology, sports, outdoors, and dabbles in the dark realm of politics. He dreams of someday living in a castle on an island but for now will settle for smalltown, USA.

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Alex - last year Reply

Many thanks for this site. I wish I had found it before I had to search for info about smart thermostat installation. After finding out my HVAC servicers wanted over $500 to just string an extra wire from the HVAC control board in the attic to the thermostat, I decided to to it myself. Seems to work fine.

However, one question – it may be my imagination, but since I did this, it seems like the HVAC fan is louder and air is being pushed out faster, cooling things down faster. Does that make sense to you? Is it a potential problem? Thanks for any info you can give me. Alex

    Eric Blank - last year Reply

    Hi Alex. I’m glad you found some of the content here useful!
    In order to answer your question completely, I would need more information about the fan. Most fans are single speed fans. The thermostat sends a signal to either turn the fan on or turn the fan off. It doesn’t usually have the capability to change the speed of the fan.

Matt Falcon - a couple of months ago Reply

I wish I could find more technical info about how they’re trying to pull this off. Since these are signal wires they’re dealing with, the statement that “you can’t steal as much power when it’s on than when it’s off” is in direct conflict with “draw too much power and it turns on when you don’t want, too little and the battery dies”. See how the two are in conflict? You get more power when the equipment is on – at least, that’s what makes sense. So, when the equipment is OFF, it has to run on battery.

As for the bit about “batteries lose 20% per year”? That’s complete garbage, bollocks, etc. A cell phone battery dies because they’re designed to be constantly cycled and abused, left fully charged to cool off (vs cooling off when mid-charged). A Nest battery is practically never cycled – it stays in the happy middle area, where lithium batteries are virtually invincible – 10+ year life, and they’ll still go strong.

So, while this is the most technical article I found in my search, it falls far short of true technical information. I’d like to know how a Nest can figure out how much current to draw without turning on, how it goes from 24VAC RMS to ~3.8V with low enough losses to “steal” enough power, how the solid-state R-to-function switching works (and steals power in the process), stuff like that…

    Eric Blank - a couple of months ago Reply

    Hey Matt, thanks for the input. I fixed the contradictory paragraph. I originally wrote it the way it is now (because that’s what makes sense logically) but then I talked to someone who seemed to know what they are talking about and he said I had it backwards. But, this makes much more sense. When the system is turned on, the switching threshold is met, you can continue to increase current as much as you need to give the battery a quick charge without any side effects on the system.
    This also more accurately explains why Nest owners were seeing erratic behavior during long periods of system inactivity. Nest would inadvertently trigger system “on” events by sending pulses meant to charge the battery.
    Also, I agree the 20%/year figure in the ecobee article is no longer true. But, to be fair, that article is a couple years old and battery technology moves fast.

Notometosail - a couple of months ago Reply

Eric, thanks for the explanation. very helpful. in adding a C wire I did not see a suggestion to just add a 24v transformer. I installed one of those smart thermostats from Sensi to a gas log stove which came with a 2 wire dumb thermostat. From Home Depot I purchased doorbell 24v transformer for $14. It does require 110v as a source but then I just ran one wire to RC and the other to C and everything is working fine. The original wires are on RH and W.

    Eric Blank - a couple of months ago Reply

    Cool. As long as you have a nearby 110v source, this could be an option.

Nathan - last month Reply

Question: I noticed that there are now 24v c wire products being sold (via amazon is where I found this one) that simply plugs into a wall outlet… thoughts on these? Am I wasting money on this attempt to fix my C wire issue?

    Eric Blank - last month Reply

    Yeah, I noticed that, too. They should work just fine. I added the 24v transformer option to the article

Johnny Waichulis - last month Reply

My control panel is well hidden so I found a wire that has 24V
and connected my C wire to it. Connected the other end to
C on the thermostat and vola my thermostat does not power up.
Help! Johnny

    Eric Blank - last month Reply

    There should be two wires on your 24V transformer. Typically, one is connected to Rh (or Rc) and the other is connected to C.

Gary - last month Reply

Hi Eric, great article, thanks. I’ve read a number of pieces on the c wire and the use of the ecobee and I learned more from your article than any other I’ve read. I have two questions: 1. Where is my HVAC’s control board? (I have the AC unit in the attic and the heating units in the basement. In other words; where is the other end of the wire bundle from my thermostat? Although we have 4 zones, the AC is only controlled from the thermostat that will be replaced with the ecobee4). 2. I notice that the wiring bundle on the thermostat to be replaced has a “spare wire” coiled around the bundle and is not connected—could this be the c wire? How would I check? If it is, and contains 24 volts to it, is it even safe to leave it coiled and disconnected? Thanks in advance and I look forward to hearing your reply. Sincerely, Gary

    Eric Blank - last month Reply

    Typically the controller is part of the furnace. However, I’m not familiar with the specifics of multi-zone systems.
    The c-wire is often blue, but not always. If it is the c-wire, it is a low voltage return wire, so it shouldn’t be a safety hazard.
    Wish I could be more help!

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