What's a C-wire, and why should you care?

The C, or "common" wire can make -- or break -- your thermostat installation. Here's what you need to know.

Megan Wollerton Former Senior Writer/Editor
4 min read
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Installing the Honeywell Lyric thermostat.

Colin West McDonald/CNET

In a New York Times article published yesterday, writer Nick Bilton discusses a software glitch that caused his Nest Learning Thermostats to stop working.

As Bilton describes it, "Although I had set the thermostat to 70 degrees overnight, my wife and I were woken by a crying baby at 4 a.m. The thermometer in his room read 64 degrees, and the Nest was off." Additional customers noted similar problems through Nest's online forum.

The Google-owned company claims to have addressed the issue by pushing out a firmware update to "all Nest Thermostats that were affected," but Bilton (and likely others) had already sworn off their Wi-Fi thermostats by then.

Software bugs certainly aren't new to the Internet of Things. Every gizmo that's "smart" and "connected" can be exposed to non-hardware-related hiccups and hacks.

The Ring Video Doorbell recently updated its own software to account for a flaw that made it relatively easy for hackers to access proprietary Wi-Fi info; Insecam, a website that posts a bunch of live feeds from private home security cameras, calls out folks (in a very public way) who forget to update their passwords from the default to something more complex. Our smart home team has even experimented with jamming a wireless security system.

Can a 'common' wire help?

Regarding thermostats, one thing you do have direct control over relates to a single wire called the C, or "common" wire.

Contrary to its name, the C-wire isn't actually all that common. The furnaces and air conditioning systems in many homes, especially older ones, tend to have four wires or fewer, and often exclude the C-wire altogether. My previous house had a four-wire configuration -- an R wire (for power), a G wire (for the fan), a Y wire (for AC) and a W wire (for heat). There are dozens of possible configurations and at least as many different types of HVAC systems, too.

Because I had an old-school thermostat with a very simple display, it was able to draw enough power from the included battery to work consistently.

Today's connected models, though, with power-hungry Wi-Fi connections and full-color screens, often need more than a simple AA or AAA battery -- unless you're OK with changing the battery all the time. Thermostats like the Honeywell Wi-Fi Smart, the Honeywell Wi-Fi Smart with Voice Control and the Ecobee3 enlist help from the C-wire, a fifth or extra wire that provides the negative charge needed to supply enough power for those newer features.

A note on 'power stealing'

Other models, like the Nest Learning Thermostat and the Honeywell Lyric can use a C-wire, but they don't require it. If your HVAC system doesn't have a C-wire, these thermostats and others instead draw power directly from those other wires when the AC or heat is on. When neither system is running, those thermostats will cycle the power on your HVAC system briefly in order to supply that extra dose of power. Many HVAC professionals, as well as Nest and Honeywell Lyric competitors, tend to refer to this practice as "power stealing."

While power stealing can work -- and did work well in my old condo for years after I upgraded to a second-gen Nest from my older thermostat -- it has some inherent risks. For instance, turning on your heat or AC for no other reason than to power other parts of the thermostat can actually damage your HVAC system. Burst of voltage streaming through the system at unexpected times can lead to short circuiting.

Take a tour of Nest's third-gen thermostat

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When in doubt, talk to an HVAC specialist about your options. For testing purposes, CNET Technical Editor Steve Conaway installed a C-wire in my condo and it took him roughly 5-10 minutes. Of course, I benefited from a free C-wire install since it was for testing review units (now we have a nifty CNET Smart Home where we test out all of our smart home products), but it is a very straightforward install for any seasoned professional -- one that shouldn't take too long or cost too much.

Because a C-wire can supply power more reliably, installing one is a safe bet for *most* homes (check with an HVAC specialist in your area to confirm that a C-wire is compatible with your system). Using one won't necessarily help prevent one-off software glitches like the one Nest customers -- including Bilton -- dealt with recently. Users in Nest's online discussion board reported experiencing the glitch despite using a C-wire connection. Still, it can provide a bit more peace of mind if you're worried about the overall health of your HVAC system.

I reached out to Nest to comment on this piece and haven't received a response. As always, I'll be sure to add in any updates as I get them.