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Nest Learning Thermostat: Installation, battery issues, and the importance of the "C" wire August 30, 2012

My furnace's control board. The "C" terminal has no connection to the thermostat in this picture. (The white wire on the C terminal goes to the A/C.) I connected the unused blue wire (bottom center) to the C terminal.

The Nest now confirms the active "C" wire.

I recently bought and installed a Nest Learning Thermostat to replace my old non-networked thermostat. I show the installation, demonstrate control from mobile devices, and provide a general review in the above video.

It's been about a month since I installed the device, and I found one important issue yesterday. My Nest dropped off the network for 7 hours, and upon investigation I discovered that the battery was low and it turned off the Wi-Fi radio to save power. Many other people have reported problems with the battery, which is scary because your thermostat is one device that you absolutely want to work 24/7 -- you don't want your pipes freezing when you leave town and the Nest decides to run out of juice!

It turns out that my thermostat wiring, like in many homes, does not provide a "C" wire (common 24VAC) for completing a circuit that provides constant power to the unit. This sort of wiring worked great for old-fashioned mercury thermostats -- it provides a red 24VAC power wire, and "call" wires for turning on the fan, heat, and air conditioning. When the thermostat needs to turn on one of those appliances, it simply closes the circuit between the red wire and the relevant call wire. Smart thermostats rely on batteries to power their smartness when no circuit is closed. When an appliance is running (i.e. one of those three circuits is closed), it can perform "power stealing" to sap power from the closed circuit for its operation and recharging the battery. For simple programmable thermostats, power stealing is probably sufficient. However, for a power-hungry device like the Nest that needs to operate a Wi-Fi radio, this mode of operation can be problematic for several reasons:

  1. If you live in a nice place like Colorado where you can open the windows and go days without using the heater or air conditioner, the control circuits are never closed and the Nest's battery doesn't have an opportunity to recharge.
  2. Power stealing is an imperfect backwards compatibility hack, and can't necessarily provide enough current to recharge the battery even when the appliances are operating. This is because the current may be limited by resistance in your furnace's control board.
  3. When the HVAC appliances are not running and the battery needs to be charged, the Nest performs an even worse hack than power stealing: it pulses the heater call circuit on and off very quickly to steal some power, and hopes that the pulses are short enough to keep the furnace from activating. I haven't noticed any problem with this, but at least one person has found that this wrecks havoc on their heater.
  4. The Nest uses a "Power Saving Mode" of Wi-Fi to reduce the power consumption of the radio and prolong the battery life. (And hopefully require less overall power than it can steal from the call circuits.) Nest indicates that some non-conformant wireless access points may not fully support this mode, thus causing the Nest to consume more power. (Perhaps more quickly than it can be replenished.)

I was lucky that my thermostat wiring contained an extra, unused (blue) wire, and my furnace's control board provided a 24VAC common terminal for a "C" wire. After hooking up the blue wire at the furnace and the Nest's base, I now seem to have successfully provided a 24VAC "C" wire to the Nest, and hopefully my battery issues are behind me.

I do think that Nest is perhaps overly optimistic about their power stealing and circuit pulsing being able to provide adequate power to the device. There's certainly no warning about this potential issue when you provide your wiring information to their online compatibility tool.



U mentioned that you connected your extra blue wire to the "C" terminal. When you were done does that mean you had the white ac wire and the blue wire connected to the "c" terminal on the board connected to furnace? Thanks for your help in advance!

Posted by Todd on October 08, 2012 at 06:32 PM MDT #

Todd -- Sorry for the late reply. Yes, I now have both the white wire and the blue wire connected to the "c" terminal. The other day, I pointed this out to a furnace repairman who was looking at my furnace concerning an unrelated issue, and he confirmed that was exactly the right thing to do.

Posted by David Simmons on November 21, 2012 at 02:28 PM MST #

Heating system have color a code c = black, it would help some one in the future working on your system to mark both wires in question black/c.

Posted by Bill Macfarlane on November 25, 2012 at 06:39 PM MST #

This post was great info, David. I was experiencing the exact issue you described battery on the nest was too low to connect to WiFi because it wasn't charging enough on heating cycles. I have connected a full-time 24 volt supply as you described and all seems fine now. I will ping back if I have problems. I wish Nest had better info on this...

Posted by David Mickelson on January 06, 2013 at 08:21 PM MST #

Hi David, Could you post the wiring connections done under the nest itself if its not too much trouble? Also will keeping the fan ON rather than AUTO also work? Thanks in Advance

Posted by John on January 21, 2013 at 04:56 PM MST #

You may want to indicate which version of the Nest stat you're running. In the fall of 2012, Nest released its second generation thermostat which resolves battery issues and more.

Posted by Nest User on January 24, 2013 at 09:57 PM MST #

Thanks so much for taking the time to post this. I recently bought a SECOND GENERATION Nest and ran into the same battery problems you did. Luckily, my thermostat had an extraneous/unused blue wire that started feeding power to my Nest when I connected it to the 'C' terminal (on the Nest) and the 24Vac terminal (on my thermostat). Thanks again!

Posted by Brady T on February 06, 2013 at 10:17 AM MST #

Thank you for this! I just installed the Nest in my house, and Have NOT had any issue, however while researching about the device, I came across your page, and realized I too had no 'C' wire, but had a surplus blue wire. I just connected the blue wire to the C terminal of my furnace, and at the Nest. Thanks to your page, I have completely avoided the issue the rest of you have experienced! Props!

Posted by Winter on February 16, 2013 at 11:03 PM MST #

I just installed a first gen Nest and have been having battery issues. Thanks to this post I understand why that may be. This is the one time of year in Arizona when the AC and Heat don't need to run. I can easily go days without turning either on. I too have an unused blue wire at my thermostat. I think I'll test it and see if their is any power running to it.

Posted by Jason on March 29, 2013 at 10:11 AM MDT #

I have a different issue. This is the second 1st Gen Nest I had installed. The initial issue with first Nest was this: Upon installation, I found that the Rh (red wire) when placed into the Nest will continuously run my outside compressor unit, however only when the air conditioning is turned on. I had an AC guy come out recently, he tested the solenoid in the Compressor unit and a few other things, then told me it was my Nest base unit, maybe a short in the base. I spoke to Nest support, they sent me another 1st Gen unit. I installed that one as well. Good news, well for only a few days anyway. The unit worked great. No compressor continuously running outside when I turned the AC on. However, once I installed it onto the Wifi and started to program the unit using my Iphone. The thing started running the compressor again...always on when the AC is turned on. I've done a reset and everything. Anyone run into this yet? Nest seems to be of no help.

Posted by David on April 15, 2013 at 08:03 PM MDT #

I'm doing a bit of a weird thing with my Nest 2 and am running into problems. Nest support are very helpful but don't seem to quite get it. Can someone advise? I'm connecting Rh and C to a 24VAC 1A power brick to keep it running. Then, via wifi, it talks to my Vera 3 home Automation system which has software capable of polling the Nest. For example the Nt can choose to heat, cool or turning the fan. None of these actually exist, but the home automation system can see what Nest is trying to do and then issue IR commands to my AC unit to copy what Nest wants. The reason is that my AC unit cannot be connected to a standard 24VAC thermostat let alone a Nest. The issue I have is that the battery drains after about a day. I taught that with Rh and C connected it could charge itself? Sometimes it comes up with an error saying Rh is not connected, but it is and it does have power. Any ideas? Many thanks.

Posted by Rob on May 10, 2013 at 01:46 AM MDT #

Just connected C wire up.Nest is no longer glitching.It now comes up as I approach.So far no low bwttery crap.Thanks for the info

Posted by Reg Beckett on June 02, 2013 at 03:20 PM MDT #

Thank God I found this page. I live in California and during the summer I am fortunate to almost never use my AC. I have have my Nest turned off (Meaning no Heat or Cool status) for about 2 months. Today it got fairly hot. So, I got on my iphone to turn it on to Cool setting and no connection with a message saying it had not been connected to the network for about 20 hours. Checked the Nest unit directly and it showed a low battery message. Guess if finally ran the battery to low to talk to my wifi router. Anyway, I am going to try connecting the extra blue wire as mentioned on this page. Makes a lot of sense to have a dedicated wire for recharging the battery. In my case I haven't had a close circuit in about 2 months so I'm guessing the battery has been in sleep mode for as long as it can. Thanks again for shedding light on how the unit charges by "power stealing". I had no idea. The folks at Nest support should know this as well. Cheers!!! Seph

Posted by Sephryan on June 27, 2013 at 03:38 PM MDT #

Hi, I installed a Nest 2nd gen 2 days ago in my house. The previous thermostat was only on a 2 wire system, however I have run a third wire which seems to contain 24v. This still isn't helping. I have the C wire connected to the C terminal on the nest, where should the other 2 wires be? Currently, they are in the R and W terminals, however when I connect the C wire the unit won't work at all. When I disconnect the C wire it works, however it's power stealing trick is running my hydronic circulating pump constantly. I have sent 2 online support requests to Nest and they don't respond, even though they state they'll reply within a business day. Any ideas??

Posted by Damien on July 13, 2013 at 03:14 AM MDT #

I have exact same "C wire" problem. Decided to connect blue wire to C terminal on both sides. I still don't get 24v power to charge Nest battery yet. Also, I am confused with first and second post which says connect blue and white to same terminal (white) on furnace. Am i correct in understanding Todd and David post?

Posted by Sundar on August 26, 2013 at 04:00 PM MDT #

I just added a NEST tommy home and only had 4 wires. I set up the thermostat to turn on the fan for at least 15 min each hour and that seems to help keep the battery charged and move air around the house. Thanks for the heads up and if I have problems with a low battery in the future, I may have to fish through a new 5 wire cable,

Posted by Ryan on September 09, 2013 at 11:55 PM MDT #

I come from a background in AC and so I already ran the Common wire down to my thermostat (basically like you show) so I was ready for the Nest, but a horrible time with my Nest, the Nest company and Reliant Energy. I was told the Nest would work by Reliant, but after weeks of waiting and 15+ hours of my time on the phone with Nest while they had me try all types of troubleshooting steps and while I had 3 $200 Nests's in my home with a credit card hold on each.... They finally got it working. During that time Reliant then told me that I was stuck with the thermostat since I had signed up for service with them and there was nothing they could do. My Nest has been working for 3 weeks now and Nest tech support does not even know it and they have not tried to contact me. Horrible experience !!!!!

Posted by Nickso on October 02, 2013 at 01:30 PM MDT #

I had the same battery low problem (Gen 2). I only had a four wire cable from my furnace to the thermostat. I ended up moving the green "G" wire that controls the independent operation of the fan to the "C" position in the furnace and then connected the same green wire to the "C" position in the thermostat. That solved my battery low problem. I can no longer independently use the fan, but I rarely used the fan only option anyway.

Posted by Kurt Weymouth on October 14, 2013 at 01:03 PM MDT #

Hi all! If you are sick and tired of being on the phone with nest support for hours and waiting two days here, two days there for a new base or a whole new unit replacement, please do this. Run a 24v common wire and all your problems will be gone. Nest will not tell you this because they want to sell as many units as they can and will tell you "do not need a common wire". I was about to throw mine up on ebay and try to recoup some of my money until I ran the C wire. Problems I had: First Gen, Nest kicked on Ac in freezing temps, fried the ac start capacitor. Whole Nest unit was replaced by Nest. Returned that one to Lowe's and got the Second gen. Second Gen, Nest would think its satisfied at temp and blow out cold air non stop when it was below freezing temps outside. (Talk about waking up at 3am to change out to the old thermostat because it was about 50 degrees in the house sucked!)... Blower would continue to run until I killed breaker. Nest blamed this on the fuse in the base was blowing. After many bases and whole unit replacements, I finally did this. IT WORKS...I hope someone reads this and it helps them before they give up on the nest and in my opinion, it really is an awesome thermostat!

Posted by Jim on November 21, 2013 at 07:08 PM MST #

Well this post totally saved me. I have a gen 2 nest that was constantly disconnecting from my wifi and telling me the battery was low. I connected the C wire from the nest to the furnace and instantly I could see the battery voltage begin to climb. within an hour I was back online and haven't had an issue since. I never had to call nest but this instruction should absolutely be on their support website, and they should obviously be instructing people to connect that wire from first installation. Thanks David!

Posted by Julian on December 05, 2013 at 10:10 AM MST #

This is going to become more of an issue with the Nest 4.0 firmware that seems to draw substantially more power than previous versions. I just had a $420 bill to diagnose a heating issue that was caused by the Nest drawing too much power since the firmware update. The initial diagnosis looked like a fault with combustion intake / exhaust, but it ended up being that the only thing that solved it was bypassing the Nest. I'm going to have a hard time recommending the Nest to anyone at this point... unless they make things right over that $420.

Posted by Kris on December 06, 2013 at 11:25 AM MST #

I had the issue with low power on my nest, I hooked up the blue commen wire and I have had no issue since then.

Posted by Brad on December 10, 2013 at 05:21 PM MST #

David, I can't thank you enough for posting this. I was having intermittent cycling of our heat and it would never continuously heat to the target temp. The unit kept disconnecting from wifi just as you described and the battery was run down. It actually happened on Christmas Eve, about a week after I installed the unit and it was around 4 degrees Fahrenheit outside. I connected the common and it has been solid ever since. Thanks again!

Posted by GOT on December 26, 2013 at 06:48 AM MST #

You rock! My second new Nest experienced power problems. I sent a message to nest but have yet to hear from them. My second furnace would start and then shut down due to the power save feature. Now it is working fine. Thank you a lot! Derek

Posted by Derek Lawrence on December 29, 2013 at 10:36 PM MST #

I have a 2nd Gen Nest and have had many of the problems listed here, but cannot seem to solve them. I had the common wire installed, and it seemed to work fine. Then all of a sudden, (maybe from firmware update??) my air conditioner would be working at same time as my heating. It was determined that there was a short on the wiring plate and was told to disconnect my C wire, which contradicts the suggestions I have read on this website. As a result, I have encountered the "low battery" frequently. So what do I do to fix this? How is everyone connecting their C wire without shorting out the plate and kicking on the air conditioning unit in the winter??

Posted by Jason on January 02, 2014 at 07:52 PM MST #

Thanks for posting this! I have a second gen nest Display-2.6 Software: 4.0.1 ( was 4.0 ) Prior to putting the C wire in place, the battery was around 3.4-3.5 volts. Now that the C wire has been in place for about 6 hours its showing around 3.9 volts. Assuming its a lipo battery, that is fully charged ( for a 3.7 lipo and some aging ) as best I can tell. Curious, can someone with the fix also post power/voltage information. There are a few values on the technical info screen of the nest which are not visible on the web ui. I'm curious what others' values are. Activity, Long 0 ALS vis. 679 ALS infra 58 Battery: 3.906V VOC: 38.95V VIN: 38.95V lin: 100mA (i) Software version: 4.0.1 Model: Display-2.6 Base Software Version: 4.0.16 / 2.1 I think the main values to look at are Battery, VOC, VIN and lin. I think lin is the ampre supply of the battery, EG its charge level. Size-wise, that sounds about like what would fit in the nest. is an example of the battery which would match the 100mA. Thanks!

Posted by bill on January 11, 2014 at 07:53 PM MST #

I've just started having this issue though my heater has been going regularly so I assume the batter is starting to die. I won't be getting another nest though for a number of reasons. The newer Honeywell that actually gives you more info than just the set temp while still looking good is a likely replacement. My circuit board on my central air heater failed in less than a year with Nest which seemed strange as no other townhouse, all built at the same time with the same hardware, had an issue. Then after reading the message boards about the way it steals power and furnace failures others were having it became pretty clear that the way the board failed was highly likely to have been caused by the Nest. So it cost me over $300 in repairs for something they don't tell you about.

Posted by PaulB on January 29, 2014 at 01:46 PM MST #

I can confirm that as of 4.0.2, a gen 2 Nest still has this problem. If regular heating cycles during the sub-zero temperatures in Minnesota are not enough to keep the Nest charged, I'm not sure what would.

My theory is that some furnaces do not put out enough current for the intermittent power to do the job, whereas other furnaces do just fine.

I also connected the blue wire to the C terminal on the furnace board (which also has the white AC wire), and my problem was solved.

To those thinking that a move to Honeywell will solve the problem: think again. The Honeywell won't run at all without the C wire. They say so right in their documentation. To me, that means they draw even more current than the Nest, not less.

Posted by MartinD on February 01, 2014 at 12:30 PM MST #

I have a 2 1st gen Nest's and one of them has this problem. It is much farther away from my WiFi router, so I think that has something to do with it. Basically, my solution is, I take the nest off the wall whenever the battery gets low and I plug it into a wall USB charger for a few hours. This charges the battery and the nest is good again for a few months. It's a little inconvenient but not really that big of a deal.

Posted by Ryan on February 28, 2014 at 09:44 AM MST #

This is exactly what i was looking for and you saved me a lot of time. It works for bot first and second generation nests. I tried both in two different homes and works perfect and nest detects c wire. Thank you so much. :)

Posted by Burak on March 26, 2014 at 02:38 PM MDT #

Mine has said "software updating" for a week now. I cannot turn AC on, or change temp, or anything....?

Posted by jkap on April 12, 2014 at 08:45 PM MDT #

i mean it says "updating system"

Posted by jkap on April 12, 2014 at 08:51 PM MDT #

I also recently installed a nest thermostat and experienced the wifi issue. Around midnight it was a little warn so I grabbed my phone to adjust the temp. It would not connect. So I started looking and found this. I have no experience with AC work. But I had installed the thermostat myself in about 20 min. I did not hook up the blue C wire because it was not hooked up to my old thermostat. However as the rest of you had said that would solve the issue. So at midnight I got out the tool bag hooked up the blue wire and now all seems to be working great. Thank you all for your help George

Posted by George on May 22, 2014 at 05:24 AM MDT #

Installed a 1st Generation Nest thermostat in Aug 2012 and it worked "OK", until recently, although many instances of dropping off the network. Came home after being out several hours yesterday to find low battery indicator flashing and A/C running on with inside temp down into low 60's. Very luckily ran across your post & responses to it when I was just starting to troubleshoot the issue. I tried your suggestion of connecting full-time 24volt supply to "C" terminal on Nest and it immediately corrected all my issues related to low battery, to include the dropping of Network, lost control of A/C, screen not coming on when approaching Nest, and others. Battery voltage is now well up from where it was prior to adding dedicated 24V. Thank you very much, David, for taking the time to post your finding, as it saved me a lot of time, potential frustration with dealing with Nest or HVAC Service, & very probably some $. Appreciate your consideration of others with this issue and sharing the fix.

Posted by Dennis on July 27, 2014 at 03:43 PM MDT #

Hi, I did a "full-featured NEST in EU" hack, see here

Posted by Marek on January 05, 2015 at 05:05 AM MST #

Im finding on my nest its showing a n1 fault

Posted by Nelson on January 08, 2015 at 12:47 PM MST #


Posted by Marek on January 08, 2015 at 01:00 PM MST #

UTFG :) error code list

Posted by Marek on January 08, 2015 at 01:02 PM MST #

I have a spare blue wire that I want to use as a common but on my furnace there is a red wire already connected to my "c" conductor. It is to be noted there are two red wires one going to the "r" conductor and the other going to the "c" conductor. I also have two white wires, one going to the "w" conductor and the other is going to the "y" conductor along with the yellow wire. Would it hurt if I placed the blue wire on the "c" conductor with the red wire attached?

Posted by Todd on August 23, 2015 at 04:39 PM MDT #

Could you post the wiring connections done under the nest itself ?

Posted by karma on December 11, 2015 at 02:20 AM MST #

Thanks for this helpful info. We are having a problem with controlling an older oil fired FHW system using a Nest, diagnosed by Nest as a charging problem so we put in a third wire, but it hasn't corrected the issue. When the Nest is calling for heat, it toggles the water circulating pump on and off on a 5-6 minute cycle until it is satisfied. The face of the Nest goes orange and dark alternately so the Nest knows what it is doing but the result is only about half the output of the system. We are headed for replacement of the 30 year old relay/controller, which seems incompatible with the Nest. Any advice appreciated. If we get it fixed, I'll report back.

Posted by Thomas E Moore on December 20, 2016 at 07:17 AM MST #

Thanks for this helpful post. I have an older house, and the thermostat cable only had 4 wires, with the intended C wire actually controlling the AC. I was alarmed when I woke up to my Nest shut down due to low battery, after a week of good weather that required almost no heating. Your post was very helpful in diagnosing the issue. I am not surprised that Nest does not advise about this in their compatibility check, as it might discourage buyers. I bought more thermostat cable than I will ever need for $25, and pulled the new wire. Everything works great now and I will not have to worry about this on/off pulsing. Thanks for all the help

Posted by WKG on March 24, 2017 at 02:31 PM MDT #

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