No C-wire? Install a Nest thermostat at your own risk

Nest says you don’t need a C-wire – until you do.

Read enough Nest reviews and you’ll eventually spot a common theme: Nests sometimes fail in systems without a C-wire.

We were curious about these seemingly “random” Nest failures, so we called a few local HVAC companies and asked them for their off-the-cuff opinion on Nest thermostats. All of the professionals we talked to were critical, telling us to avoid Nest unless we had at least four wires (including a C-wire). One HVAC repairman we spoke claimed dead Nests constituted 90% of his “no heat” calls last winter. (We have no way to tell if that’s an exaggeration, but he wasn’t the only one who told us that dead Nests were a real problem.)

So what’s the deal? Why do Nest thermostats fail in systems without common wires, and how can you tell if it’ll happen to you?

Here’s what we’ve found.

The Nest’s “pulsing” problem

The Nest thermostat contains a rechargeable lithium ion battery. This battery runs the programming and keeps the thermostat connected to WiFi, but WiFi connectivity is draining – the battery doesn’t last very long on its own. So, to keep itself going, the Nest recharges itself from your HVAC system’s wiring.

In a system with a C-wire, the Nest charges itself from the C-wire’s current and all is fine.

In a system without a C-wire, though, Nest recharges its battery when the heating or cooling runs. It diverts a little bit of the power to itself and charges its own battery. This isn’t a new concept – illuminated doorbells and lighted switches use this same technique to take just enough power to light the little bulb inside without triggering the doorbell chime or opening your garage door.

In most systems this, too, works fine – assuming you’re running your heating or cooling frequently enough.

But let’s say you aren’t. Let’s say the weather’s mild and you’re just not running the heating or cooling that much right now.

In this case, Nest will very briefly attempt to draw power from your HVAC system on its own. Some users call this “power stealing”. The Nest only “power steals” when the HVAC system is off, and it stops when the system is on. This call for power is meant to be brief and undetected, a tiny “pulse”, but some systems are very sensitive and see the Nest’s small power draw as a call to turn on. Of course, once the system is on then the Nest stops its attempt to charge itself and the furnace or AC turns off again, but then the Nest is still low battery so starts to draw power again. The end result is a furnace or A/C in a loop of rapidly turning on and off.

Many users, across many review sites and forums, report various ways this behavior manifests: 

• Strange noises from your furnace or boiler: banging, chattering, clanging (Marco at marco.org shares a sound recording of his boiler being turned on/off by the Nest)
• Furnace is rapidly being turned on/off
• Air conditioning compressor is rapidly being turned on/off
• Fan gets stuck on/off
• Heat pump gets short-cycled (ie: running for 30 seconds then turned off while the internal air handler is still going)
• The Nest works all winter, then fails in the summer. Or, it works all summer and then fails in the winter.

Reports on the Nest pulsing issue from around the web:

• Solution to pulsing sounds and other odd behaviors (u/maldata on reddit.com)
• Nest incompatibility without C-wire (marco.org)
• Use a common wire with a Nest thermostat or suffer the consequences (electronicdesign.com)
• The Nest Thermostat Rant Page (greyghost.mooo.com)

When doing your own research on the Nest pulsing problem, look for terms like: pulsing, cycling, short cycle, power leeching, no c-wire, heat-only and cooling-only.

Note that “pulsing” is a term the Nest community has come up with for the problem, and Nest themselves do not seem to name this problem in any specific way.

Will it happen in your system?

Every system is different and there is no reliable way to know ahead of time how your system will behave. Some people theorize that newer, fully-electronic HVAC systems are more likely to experience problems from the Nest “pulsing”, but we’ve also seen reports of this problem from very simple, “old-fashioned” systems that have no electronic components.

Here’s what we know:

• Nest’s compatibility checker might give you the green light, but that’s no guarantee you won’t eventually need a C-wire
• Nest’s own stance can be summarized as “try without, then add a C-wire if you experience this list of problems” (nest.com)
• The problem can take anywhere from less than a day to several months to well over a year to manifest
• The problem seems to be associated with mild weather or periods of not running any of your HVAC components
• Installing and using a common (C) wire or a wire-replacement kit (such as the Venstar Add-a-Wire) seems to prevent the problem entirely

Hack-y fixes for the Nest “pulsing” problem

There are a couple of “clever” fixes for the pulsing problem, including:

• Connect the heat wire (W) to the C terminal during the summer and connect the cooling wire (Y) to the C terminal in the winter. This means changing your wiring with every switch from heat to cooling and back again! It also requires both heating and cooling equipment. We think this is a poor solution.
• Connect the fan wire (G) to the C terminal. This means you can’t use your fan independently. It’ll just run when the system runs. We think this solution sucks, too.

Either of these “solutions” means you’re either changing wiring twice a year or not getting full control of your fan. We don’t recommend these workarounds – upgrading to a smart thermostat should make it so your life is easier, not harder.

The real fix: get a C-wire or install an adapter if you want a Nest

Basically, you have two options if you want to ensure your Nest installation is solid:

• Run new wires between your furnace and your thermostat (hire a pro or DIY with a new spool of thermostat wire)
• Grab a Venstar Add-a-Wire, which adds a 5th wire to an existing 4-wire setup

One more alternative: get a smart thermostat that’s designed to work with systems lacking a C-wire. Check out the ecobee line of thermostats – all ecobees ship with an adapter for C-wireless systems. The adapter was mentioned by most HVAC pros we talked with, and they all spoke highly of the little “motherboard” you can install to make up for a missing C-wire. Another good choice: the Emerson Sensi. In most systems, the Sensi doesn’t need a C-wire at all.