Thermostat Wiring Explained: An Easy-To-Read Guide for Homeowners

When central AC first came onto the market the thermostat was very simple. All they did was turn the system on and off and keep track of the temperature of the room. They didn't need their own dedicated power supply.

But, in this new age of heightened technology, smart homes, and fancy touchscreen displays your thermostat needs a constant source of power.

How does it get this power? The thermostat C wire, or common wire, enables power to continuously flow power to the thermostat.

How do you know if your system has a C wire? If you're considering upgrading your thermostat is that something you can do yourself? Here we tell you everything you need to know about the thermostat C wire and the basics of thermostat wiring so you'll no longer be in the dark.

What is a Thermostat C Wire?

The C wire is an extra wire that is used to create a continuous 24V power loop between your thermostat and the rest of the HVAC system. It allows the power to be used for any application and allows more advanced thermostats to function to their highest potential.

C stands for common and it's usually labeled as C. It can be any color. It won't always be labeled. While there are standard practices they aren't required and so they don't apply to every installation.

While some people think the C wire is what powers the thermostat, this isn't entirely true. Typically, the "hot" wires, or the wires that provide power to the unit, are the heating and cooling wires, labeled Rc and Rh respectively.

They provide the source of 24V power that comes into the main HVAC control board itself. The other wires on the panel connect the circuit so the loop carries power back and forth to the board.

So, the C wire doesn't power the thermometer rather, it provides the return path that completes the loop and avoids power disruptions.

Ok, But, Why Do I Want a C Wire Again?

You may be thinking all of that sounded really complicated and unnecessary but, the fact of the matter is, thermostats have come a long way from the mercury blob moving a needle of yesteryear.

Today's systems incorporate their own WiFi signals to be able to program the device from anywhere. The thermostat becomes the main hub of your smart home configuration and runs all the operations. This takes a lot of power.

If the thermostat was depending on batteries you'd need to replace them every other day to keep up with the power demand.

In addition to the WiFi signal, most modern thermostats have a beautiful large and full-color touch screen. This is an additional power draw that needs constant power to maintain efficiency and functionality.

At the end of the day, your thermostat is not something you want to run the risk of powering down unexpectedly when it is vital to the performance of your entire system.

No thermostat equals no AC or heat. Do you want to risk that happening while you're away from the house? It's best to get the correct equipment installed even if it costs a few extra bucks.

Switching the Thermostat Is a Piece of Cake If You Already Have a Thermostat C Wire to Connect To

Maybe you're considering switching your current thermostat over to a smart device. It's a simple enough switch and install. You simply need to complete the circuit between the R wire (the red wire) and the C wire to start the flow of 24/7 energy.

But, if your system has no C wire setup already things can get a little more complicated. It's a pretty simple process to check if you already have a C wire at your disposal. With a few tools and some elbow grease, you'll know exactly what you're dealing with.

Let's go over how to check for a C wire.

1. Explore the Backstage of Your Thermostat

Go ahead and detach the thermostat from the wall. Don't be worried about hurting it, sometimes it takes some doing to get the faceplate off.

Once the faceplate is detached take a look at the wire configuration of your unit. If there is a wire connected to the terminal labeled as "C" then it's a good assumption you're all set.

If you don't see one, all is not lost. It could just have been stuffed into the wall for later use and not labeled. Take a flashlight and check it out to see if you see any unused wires in there. If you don't see anything take a look at your furnace.

If your furnace has a C wire then the other end is definitely shoved into the wall behind your thermostat. Here's how to check your furnace for a C wire.

2. Check Out the Wiring of Your Furnace

Before you start messing around with anything electrical, it's a good idea to shut off the power to the unit before you begin.

Once you've cut the power, go ahead and locate the cover and pop it off to expose the wiring. Sometimes this is easy and sometimes it's harder depending on the model of the unit.

Once you have it opened up look for a line of screws with wires coming out from under them. They should have labels like R, C, W, W2, G, and Y/Y2.

If there is a wire coming out from the "C" screw than you're all good. If there is no C wire then you'll need to have the new wiring installed.

Remember, if at any point this becomes overwhelming or is starting to worry you, it's a good idea to call in a professional to do it for you.

Can You Have a Smart System Without a C Wire? Do I Really Need One?

If you're going to upgrade your thermostat it's a good idea to have a C wire installed. Newer homes include them automatically in the build to be up to code.

While it's technically possible to run a system without one it's highly inefficient. You'll be going through a fortune in batteries or wearing out the HVAC system itself by borrowing power.

Almost every modern thermostat will need a C wire to function correctly. Better to install it now because technology isn't taking a step back anytime in the near future.

I've Heard a Nest Works Without a C Wire, Can't I Just Use That?

Nest thermostats have become popular and claim to be able to work without a C wire. However, the power still needs to come from somewhere. Instead of pulling from the central power of a C wire the Nest uses your system itself for power, as long as it remains running.

If the system isn't running the Nest will pull its power from your heater in what's referred to as pulses. These pulses draw little bits of power from the heat wire. However, for some heaters, this is a major issue because it's like it is being turned on and then off multiple times per day.

There's wears out your heater and isn't efficient for your power bill.

The Fan Wire Trick Makes Your Home More Inconvenient for You and Future Residents

Yes. You can use the fan wire as it's own C wire. But, this means you'll eliminate your ability to manually turn the fan on and off.

Don't take these kinds of shortcuts to lower the value of your home. If you're going to buy a fancy new thermostat include the installation of a C wire in the budget for the transfer.

If You're Investing in a Smart Home System Invest in a C Wire Installation Too

We hope this article has explained what a thermostat C wire is, why it's important, and why you need one for the modern smart thermostats.

A C wire is your best bet for a constant power source that doesn't draw from the rest of your system or require constant battery changing to maintain functionality.

Our professional HVAC specialists can handle installing your C wire and your whole new thermostat setup. Whenever you're dealing with something electrical and an important aspect of your home like the HVAC system it's best to trust the pros with the job.

Contact us today to schedule your inspection and see what we can do for you. Here at Mathison Air Conditioning, your satisfaction is our top priority.

Having won the best of Baltrop County two years running we know what we are doing and take care of our community. We look forward to meeting you soon!

Give our team a call for the most dependable HVAC repairs in your area.

Let us keep your system humming throughout the seasons! Call the number below to schedule a service with one of our experienced HVAC technicians. If you don't have time to call us now, and would like to send us a message, fill in the contact form here.

(512) 284-3412

Call now!