Thermostat C Wire: What It Is And Why You Want One

Here’s the deal. If you plan on getting a smart thermostat and don’t have a c-wire, get one! That’s my advice. You will save yourself a whole bunch of potential headaches.

There is a lot of misinformation out there about the c-wire and how smart thermostats power themselves. There is also a lot of incomplete information. In this article, I have done the research and filtered out the garbage for you.

If all you care about is how to get one and don’t care about the what and why you can skip to here. Otherwise, read on.

What Is A C-Wire?

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.

Heating return terminals will usually be labeled W and cooling terminals will be labeled Y (possibly W1, W2, Y1, Y2 for multiple stages). The terminal for fan control is typically labeled “G”. If you have a common wire, it will most likely be labeled “C”.

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 Need A C-Wire?

Now that you know what the c-wire is, you may be asking yourself, why do I need 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.

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.

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

What Is Power Stealing And What’s Wrong With It?

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”.

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 off than when it is on. The thermostat can charge its backup battery when the equipment is not running and tend to slowly discharge when the equipment is running. This means that during times of heavy 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 Don’t 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 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.

How Do I Get A C-Wire?

So you want a smart thermostat and found out you don’t have a c-wire? Don’t worry, it’s not that big of a deal to get one and the solutions are relatively painless.

There are two ways to get c-wire function:

Install Another Wire

This is the best solution in my opinion. However, 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.

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.

Without getting too technical, what it does is turn one return wire into a switch for two components. These kits will work as long as you never need to turn both components on at the same time. Therefore, the two components that get hooked to the add-a-wire kit will be the furnace and the air conditioner. Don’t worry about figuring out which wires go where. Each kit will come with its own detailed instructions.

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.

Here is a list of add-a-wire kits (also sometimes called wire extender kits):

TheSmartCave Official Opinion

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.

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!

 

Any comments or concerns? 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.