Categories
Knowledgebase Nest Thermostats

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.

 

19 replies on “No C-wire? Install a Nest thermostat at your own risk”

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?

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.

I have an older boiler system with two wires running from the aquastat to the “old” thermostat. I’m going to install the Nest E thermostat, but want to add a third wire to serve as a C wire. So, I’m going to buy a new spook of wire, and I only want to use three of the wires (I’m assuming the harness comes with more than three wires). The problem is, my aquastat only has two posts for the 2 existing thermostat wires. Where in the aquastat can I hook up the third wire to serve as a C wire? The Aquastat Relay is a Honeywell L8148E. I know I could just purchase an adapter that supplies a C wire, but running one from my furnace to the new thermostat seems more direct and eliminates any other source of power going awry.

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?

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)

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!

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.

So, the C wire is the common…essentially a ground.
When the equipment is not running, there will be 24v on the R wire. (Check it to a ground) There is never half of 24v anywhere, if you know how to use a voltmeter..(Remember..ALWAYS check to a ground. You’ll live longer) ..The thermostat is basically a switch, exactly like a light switch, but opposed to you flipping a switch, the stat does it automatically with temp rise or fall, When the stat calls for cooling, the switch closes between the R and Y terminals, sending 24v to the Y terminal, starting the cooling. If you remove the stat and jump these wires out, the cooling will turn on. The C wires’ only function is to allow power through the circuit to recharge the battery in the stat, and keep your stats digital display up and running, without a drain on its battery… Add the C wire. Best and surest way to have cool summer nights, and warm winter nights. That’s not something you want to worry about every time it drops below 30° F.

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?

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.

Mine finally went after 4 years – so it will happen, it’s just a matter of time. I had two of them and they went at nearly the exact same moment.

I and my wife are in our 70’s. In October 2018 we had a large service company install a nest thermostat. Ran good until the summer heat came this summer where temperatues have reached high 90’s to 100, here in Utah. In the last 7 days they replaced the nest with a new nest. Unfortunately the first one trained the system and replacing the 2 did no help what so ever. The air-conditioned around 4:00 each afternoon would stop working and it the fan would come on for 30 seconds then die, with the same code I think it was 103E or E103 error. It is Labor day tomorrow but I will contact them to remove the nest and install a standard programmable hard wired thermostat that was in the home when we bought I. The home was built 6 years ago.
Google should stop selling tbese nest thermastats. Tears ago if this happened to a manufacturer, a class action lawsuit would immediately be activated against the manufacturer. I believe Google owns and controls Nest, large companies like Google are too big and thus they just come out with remedies to work around it rather then totally rewiring.
Sometimes technologies move too fast coming up with quick remedies rather than just taking the product off the market, and fix it. Typical software comanis remedy trying to fix a hardware problem usually will fail. Any way, that’s my opinion.

Huh? My home was built in the NW in 1989. Forced air furnace only. I believe it only used the red and white wire to thermostat. I got a nest about 18 months ago and has been flawless. Open app and have turned it completely off for 2 Summers now. Just turned it back on about a week ago.
¯\_(ツ)_/¯

Great reply there Justin. Very informative.
My trusty Gen1 Nest has been serving me well for many years. About 6 months back my ancient heating only HVAC died, requiring replacement. New one is also a gas heating only ducted unit. Original unit worked forever with Rh, w1, C and G.
No matter what I did with the new unit, when I installed it 6 months back, the Nest would not sense the w1 wire as being connected even though I can measure 24v between the two terminals. So I resorted to the 220-ohm 5w resistor between C & W and then the Nest correctly sense the w1 being connected as well. This config with just Rh and W1 connected worked for about four months until a few weeks back. The HVAC started powering down and “pause for x minutes” on the screen when it should be heating. Even with battery at 3.8v. This continuous cycling eventually lead to the HVAC controller locking out. Annoying things is the Nest is getting plenty of power when the HVAC is Off with the resistor between C & W. 3.82v Voc 58.95v Vin 58.95v lin 40mA (i). Once the HVAC is ON though I see 3.82v Voc 58.95v Vin 7.48v lin 0mA (a) leading to the unit shutting down. (With HVACs digital controller connected no issues occur)
Given that a) I cannot get the Nest to sense W1 without the resistor trick and b) my Nest Wont recognise the “C” wire being connected in lieu of the resistor, I am in a bind.
What would happen if I apply both methods simultaneously? So resistor between C and W to allow the nest to charge when off and in my case sense the W1 is actually connected AND also connect a “C” wire to the Nest as well.
Alternatively does factory resetting the 7 year old Nest 1 allow it to do a better job of sensing connected wires.

Sure could use some advice and hope this site is still active.

I have a boiler for infloor radiant heat. I set the temp to 68 and leave it alone. My Nest e has become erratic in the past week. Comes on and runs 15 minutes and then off and seems to come on at any temperature occasionally.
After reading the article discovered I have no C wire. On the terminal block of the Taco pump are six slots for wires. The red/white wires from the Nest are in the first two respectively. The 3rd and fourth slots have wires to the boiler. The fifth slot is empty. The sixth slot has the green fan wire from (or to) the Nest. I have no fan. According to the Taco manual the fifth and sixth slots are 24 volts. Can I move the green wire in the thermostat to the C wire terminal?

I read the fix above about moving the G wire wire to the C terminal and having fan issues but then again, I have no fan. There are 5 or 6 wires and i could use the blue one instead of the green.

Although I don’t vary the temperature, I have the Nest so I can check it when I’m on vacation to monitor things.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.