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Do You Need an Electrician to Install Your New Thermostat?

Where to look for your C-wire and what to do if you can’t find it.

Do You Need an Electrician to Install Your New Thermostat?If you are even moderately handy, you might scoff at the idea of hiring an electrician to help install a new thermostat. And you would be right—in most cases. However, if you are planning to install a new “smart” thermostat requiring a continuous power supply, and you do not already have a C-wire, you could very well need professional help to install your new device correctly.

What is a C-Wire?

In older thermostats that didn’t have all kinds of electronic components like WiFi connectivity, backlit screens, etc., only one wire was needed to deliver power to the system. But with modern thermostats, you need a second wire to complete the circuit so that power can flow continuously to the unit. This wire is known as the C-wire or common wire.

How to Tell If You Have a C-Wire

The easiest way to see if you already have a C-wire installed is to simply open up your thermostat and check out the wiring. You should see a whole bunch of terminals, including one labeled “C.” If there is a wire connected to the terminal, you’re all set.

Of course, if you’ve never had a smart thermostat before, it is likely that the C-wire will not be attached. But don’t give up just yet—the wire may have been run to the thermostat but simply not connected.

If you don’t see the C-wire in the thermostat, head to your furnace. Open up the access panel and check the terminals. If a C-wire was run, it should be connected here (even if it is not connected at the thermostat). If you find a C-wire here, it probably means that the other end is stuffed into the wall just behind the thermostat. You can simply pull the wire out and hook it to the C terminal in your new smart thermostat and you’ll be all set.

No C-Wire?

If you do not find a C-wire, you may have several options, not all of them good.

First of all, you may find some DIY articles that advise using the fan wire (G terminal) as your C-wire. While this technically will work, you’ll lose the ability to turn your fan on and off manually, which of course is less than ideal.

Another possibility is to try to find a smart thermostat that doesn’t require a C-wire. There are a few models out there that claim to fit the bill, but be careful—some models compensate for lack of a C-wire by drawing power directly from the furnace or air conditioner. This is fine when the system is running, but when it is not running the thermostat will essentially have to turn it on in order to draw power. Depending on the equipment, this may cause excessive wear and tear.

A better option is to just install a C-wire. This may cost you a little extra now, but it will ensure your home is compatible with current and future thermostat technology. To get a C-wire to your thermostat, you can either have a new wire run between the furnace and your thermostat, or have a device such as an “Add-a-Wire” installed to convert a 4-wire system to a 5-wire one. In either case, it is advisable to hire a professional electrician for the job just to be sure this important upgrade gets done right.

Posted on August 27, 2015

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