‘C’ is for Smart Thermostat Compatibility (and Common Wire) - firstenergyhome | Blog
89 views | May 12, 2020

‘C’ is for Smart Thermostat Compatibility (and Common Wire)

Attached are the basics of what you should know about C-wires: what it is, why you need one and how to resolve the issue if you don’t have what you need.

‘C’ is for Smart Thermostat Compatibility (and Common Wire)


Does it have 4 wires or 5? This question can be important when it comes to upgrading your home HVAC system to run on a smart thermostat. Many older systems use 4 wires, but you’ll probably need a fifth wire, known as a common wire (or C-wire), to successfully install your new device.

Below are the basics of what you should know about C-wires1: what it is, why you need one and how to resolve the issue if you don’t have what you need.

Continuous power

Today’s smart thermostat needs a continual source of 24V power. That’s because the device must be in constant communication with your home’s Wi-Fi network. Your device might also have a LED color screen that stays backlit. Again, this takes continuous power--and it runs through this wire.

Does your thermostat setup come with this important wire? Let’s find out.

‘C’ is for common

Some newer conventional thermostats come with this wire, which might or might not be in current use. To find out, remove your existing thermostat from the wall and look at the wire terminals attached in the back. If there’s a terminal labeled ‘C,’ it means the thermostat accommodates a common wire. If there’s a wire attached to this terminal, you should be in business.

If you have the ‘C’ terminal and there’s no wire attached, it could be that your installer didn’t connect that wire because there was no need for it at the time. But it might be stuffed up into the wall.

Furnace search

If you have a common wire terminal on your thermostat, but no C-wire, you can check out the situation on the other end of the connection--in your furnace. Make sure you turn off power to your furnace before removing the cover and exploring.

You should see a row of screws with or without wires running from them. These screws are likely labeled: R, C, W, Ws, G and Y/Y2. If there’s a wire running from the ‘C’ terminal, that’s likely to be what you’re looking for. It could mean that the C-wire runs from the furnace toward your thermostat, but it’s not wired into your thermostat because your installer saw no need at the time. All you need to do is find that wire and connect it to your smart thermostat.

Color coding clues

Another way you may identify the common wire is by color, but this is hardly foolproof. The “official” color for the common wire is blue or black. Wires that are red, yellow, green or white, may indicate those wires serve other purposes--if the installer followed accepted color standards. But there’s no guarantee that those standards were followed.

If you have no C-wire

If you do not have oneyou can add a C-wire to the wire bundle in the cable running between furnace and thermostat. This might be a job worthy of a licensed electrician.

You could buy an adapter kit to get that needed wire. This would be an add-on purchase or you could consider going with a smart thermostat that includes the adapter in the kit. Most adapter kits are promoted as being inexpensive and easy DIY projects. And finally, some devices do not need a C-wire. Instead, they tap power from the furnace.

To conclude, you are certainly better off if your current thermostat comes with common wiring. But it might be just a few more added steps—with or without a professional—if you do not have a C-wire. We invite you to link to more information on this and related subjects at the FirstEnergy Home Learning Library

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