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No C-wire? Install a Nest thermostat at your own risk – Smart Thermostat Guide

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

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:

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
Photo of the Venstar Add-a-Wire adapter and wires

The Venstar Add-a-Wire is an inexpensive adapter that turns a four-wire system into a five-wire system.

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.



10 Responses

  1. Dan says:

    Hi ,
    Maybe you can help?
    I have Nest 3 that has HVAC and furnace heat wired to it . I have the old C wire problem. My HVAC is 20 years old, very old but works and only has 3 wires coming out the unit. I cannot get access to the board on the thermostat.
    so basically

    I have 3 wires for my HVAC yellow, green and Rc
    and 2 wires for my heating White and Rh

    I see the blue Common and an extra wire that are not wired into my HVAC system just hanging next to it and go to my thermostat and are behind my thermostat plate.so I could use those wires if needed.

    I could buy a 24v plug in transformer and hook it up to my c wire and the other wire to the spare wire that goes to my thermostat back plate. If I already have 2 dedicated red wires where would this 2nd wire go? if i double up to Rc would that cause problems?

    • John says:

      Dan I literally have the same problem. Contacted numerous HVAC companies about this and they said it might not work with Nest. This really sucks.

  2. Wood says:

    Save yourself a headache install the C wire.

  3. Eli says:

    I have questions that I hope someone can address. I have separate heating and cooling systems. I should be able to hook up both systems to the same thermostat, right?

    I think I only need one “C” wire that supplies 24v to power the thermostat, right? Right now, there are two “C” wires, one from each system. I wouldn’t think that I would hook wires up to the thermostat, but I don’t know which is why I am asking.

    I also have two R/Rc wires, again one from each system. Can I connect both together to the jumpered R/Rc terminal on the thermostat?

    • Joe says:

      No u cannot connect the two R wires. You would be supplying power from two different transformers (one from heating equipment and one from cooling equipment)

  4. Ross says:

    Thank you so much for the article! Because of it I realized that I must have a blown fuse – since I had the C wire, but my NEST still didn’t work. Surely enough I found the blown sucker, replaced it and all is good now!

  5. Justin says:

    How are you so completely wrong about the so called “Pulsing” problem.

    Tes systems can “Pulse” without a “C” wire, however the Nest is not “Pulsing” the equipment on and off to get power, the equipment is doing the “Pulsing” all on its own.

    The Nest can ONLY power steal when the HVAC equipment is not running(cooling in a heat/cool system)(heat in a heat only system). When the equipment is not running the “Rc/Rh” wire supplies 1/2 of the 24vac, and the “Y” supplies the other 1/2 of the 24vac…go ahead and measure it with a multimeter.
    when you activate the A/C the thermostat connects “R”&”Y” sending the voltage on “R” down the “Y” wire to furnace control board and turning on the A/C. That’s where the issue begins…if you take your multimeter and measure between “R”&”Y” while the A/C is running you will not get 24vac because you only have 1/2 of the power while the system is running.

    What causes the “Pulsing” is that when the Nest is power stealing when the equipment is off it draws a small amount of power, many heating equipment control boards are WAY too sensitive and see that small trickle of power the nest is drawing when power stealing as the Nest requesting the furnace to turn on the A/C which it does. As soon as the furnace turns on the A/C the 24v at the nest disappears and the nest stops power stealing, then the furnace stops seeing the power draw and turns off and the full 24vac returns to the nest and it starts power stealing again and the furnace turns on again…..thus happens over and over thus causing the “Pulsing”

    There are 2 ways to stop the “Pulsing”

    1)Connect a “C” wire
    2)Connect a 220-ohm 5w resistor on the furnace between “C”&”Y”(or”W” if you have a heat only system)

    Using a “C” wire is the best fix, it gives the nest a dedicated wire for 1/2 of the power and has access to full power at all times system running or not, and it doesnt have to power steal.

    Using a 220-ohm 5w resistor between “C”&”Y” gives some of the power a way to bypass the hvac controller, just enough to let the Nest power steal without the overly sensitive hvac controller thinking the Nest wants it to turn on.

    The “C” wire is always the best fix because the nest will still. Even with the resistor fix, the nest will not get any power when the equipment is running.
    If the equipment runs long enough non stop the battery in the Nest will be depleted and thebnest will have to shut down the system to try to recharge.

    So always use a “C” wire if you can

    So, yes you are correct that using a “C” stops the pulsing but you are quite far from the reason why the pulsing happens in the first place.

    I am not sure where the nest pulsing the equipment story came from, but it’s not the case, its the equipment causing the pulsing and its easy to fix.

  6. Steven Paglusch says:

    We have a Nest and No C wire. I know the C is the best fix, but haven’t had a chance to get to getting it done yet. When our Nest first died, I found this article and tried the idea of switching the Heat to the C. So I switched it on my nest and on my furnace, but no charge was coming through. Sometimes it would last an hour or so and others it wouldn’t read the wire at all or it would just be using the fan and not actually cool anything. Suggestions?

  7. Dave says:

    I’ve heard about this problem so came to this page to learn more about it. Nice article. I moved into this home nearly three years ago. I have two heating/cooling zones so have two furnaces, two A/C units, two humidifiers and two thermostats. When moving in I had a difficult time figuring out how to use the “simple” thermostats and if they were programmable, I couldn’t figure them out. Had an A/C-Heating guy out to deal with another problem and I asked them how to work the thermostats, he couldn’t figure them out either. That’s when I decided to go with Nest. Installed them easily and had no problem. This was right before winter and I decided when I bought the place I would get new more efficient furnaces and A/C units to replace the nearly 19 year old units that came with the house. I was going to wait until the heating season was over. Well the blower on the furnace heating the largest area of the house went out. Decision was $500 to fix the blower and then throw it out with the new furnaces coming in next spring, or do it all now and save the $500 and start heating/cooling my house more efficiently sooner. So I decided to replace the HVAC then but went through a few chilly nights as it took a week before they could install. I got Bryant Furnaces and A/C units (high efficiency) if it matters.

    So coming up on three years with the Nest thermostats with no problems. None whatsoever. And I have no “C” wire. Not sure if this eventually happens with all Nest thermostats or most or if it matters what kind of a system one has. But after nearly 3 years of running two of them on two furnaces and two A/C Bryant units, not a single problem. Just sharing my experience.

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