Fix Nest Thermostat Issues by Installing a C Wire 

This process ended up being a lot easier than I thought. Do you have any tips for routing a new cable that is in a more difficult spot?

I have been following you for years, there is no one better than you explaining and giving solutions, thanks for your videos ...

One of your BEST tech videos. Upgrading my thermostat to Alexa enabled, I knew I needed a C wire, but your video showed me how easy it was to work with. I didn't have to pull any cables, and my system had an unused wire so I connected it to the thermostat and the a/c and I was done. Great video. (FYI I don't have a nest)

It should be noted for anyone attempting this, that this can be extremely easy and can also be extremely difficult. If there is even just ONE staple inside the wall holding the original wiring it will cause you nothing but grief. Let’s say there is a staple but it isn’t completely tight to the wire. You’ll think that you are good to pull the new using the old, but it’s not only possible but very likely that once your tape joint connecting the old and new reaches that staple it will snag. Your first urge would be to tug a little harder and then uh oh, it comes apart and now you’ve got two loose ends inside a wall or floor cavity, neither of which is connected to your HVAC system. This can also happen with a hard 90 degree turn, or a hole that was only drilled wide enough for the smaller harness. As soon as that tape joint/splice meets resistance, you’re snagged. The only recourse is to start knocking out drywall to finish the job. Not trying to scare anyone, but I do this for a living, and I’ve seen it happen

Holy crap, that looks way easier then I thought it would be. Thanks for the vid!

I’m so glad I found this video as my Nest was losing Wi-Fi every day when the battery ran down. I went to look at my wiring and there was actually a fifth wire already running from thermostat to hvac that just wasn’t being used. It was super simple to connect it to the C wire in the basement and just plug it into the thermostat. 30min of research, about 3min of work, and my thermostat is back online! Thanks!!

Great job explaining! This is exactly my issue and I’m going to try this tomorrow. Thank you!!!

Haha!! You know I have been trying to figure out how to fish a c wire and 5 seconds before you showed it I realized what you were about to say and got a huge smile on my face! Thx man!!

Thanks. You just made a cold day warm again for my family. Only took an hour including the trip to the store

Dude you rock... I just saved $120 for an hvac tech to come out and install c wire. Thank you so much!

Great tip on “reset”. We had a brief power outage of a few seconds that caused Nest to not recognize the c-wire! I didn’t have to do the full factory reset, just a restart! Thanks

Didn’t even think of using the old cable to fish the new one. Smart!

Two suggestions... 1. Agree with many comments below that thermostat wire may be secured in most instances by staple(s). So before snaking have someone hold low end and someone hold high end, pull back and forth to assess whether there is free play. 2. Use wiring lube to make sure that the "snaked" wire pulls through any holes... its $5 at any supply house or homey.

Hi Brett and thanks for the helpful video! I see you have what appears to be a swamp cooler fan switch on the wall next to the Nest. Have you connected that to the Nest? If so, how does one do that? Thanks!!😀

Glad I saw this video. Wasn't sure how I was going to feed the new wire.

Great vid! Good to know to factory reset after the C is installed. 👍

Thanks Brett. I was wondering why stringing a new wire wouldn't work. Now I feel comfortable buying a Nest. I will be putting the thermostat in a new location. My old wiring is not so easy to get at. Thanks Again

This was VERY, Very helpful. Many thanks!

Thank you very much, I was thinking of installing this since several months, end up installing it yesterday. Rewired as the c wire was missing.Had to cut one hole in the drywall. Didn't removed the old wire, lets see how it runs for few months and then no need to go back.

Thank you for the wiring walk through, i was able to rewire the unit properly and the heat is on!! thanks again Brett.

You were VERY lucky you fished that line through so easily. My luck mine will have been stapled to the studs all the way down. 🤦🏻‍♂️

Great video. Made this mystery less mysterious. Really appreciate it.

Very clear and helpful, thanks

This helped me so much. Thank you!

Much appreciated! I just installed mine today with your instruction.

Sadly, my Nest _appears_ to have a C wire connected, yet for some reason the fan runs randomly even though it's not setup to turn the fan on, which is annoying when it's cold out and it's blowing right on you. But thankfully you made this video which makes it really easy to see what sort of work it would take to set it up again (or at least know what to look at in order to fix it). Thanks!

Doesn’t the new Nest have removable batteries where you don’t need a C-wire? You would only need this for the 3Gen Nest correct?

Another great video... thanks!

Great video, thank you

99 out of 100 times the wire has been run through studs or has a staple that will prevent it from being pulled back. You got LUCKY.

Hi What is E260 error? I don't have a C wire and I get this error which prevents the thermostat from being connected to my phone or wifi.

Hopefully my story will help someone. Last fall we installed a Nest thermostat at my sons house. The replaced thermostat was using 4 wires (no C), so we hooked the Nest with the same configuration. The furnace and Nest worked fine all winter. When my son switched to AC in early summer, there was a consistent clicking sound from a relay on the control board. It clicked nearly once a minute, and was aggravating. Though the AC seemed to run fine and shut off at temp. An HVAC guy came out and said that we had a bad control board. We replaced the board and the problem was still there. After watching this video, I decided to hook up a C wire. My sons existing thermostat cable had 8 conductors. So we hooked the unused blue wire up to C on the back of the Nest and then hooked the blue wire to C on the control board. The AC has run for over 24 hours with no clicking from a relay in control board.

If you have a powered thermostat system would you still have to use batteries with this device. I’m planning on buying one but I don’t wanna have to constantly put batteries in it when an a common/power wire is in place.

Even easier way to do this. Just use the green wire to the city. Most people leave the fan in auto. The green wire is used to energize the fan when the fan is "on". As an hvac tech I have done this many times before. Replacing the state wire is not always as easy as shown. A lot of times the state wire is stapled to the wall studs. When we are working on a new construction job we run a 5 wire.

I’ve tried installing a C-wire adapter on my Trane furnace/ac for my Honeywell thermostat and have been unsuccessful. When I re-wire everything the thermostat has power and air will blow, but it’s not cool air as I want and the outdoor unit doesn’t run… any idea what I might be doing wrong?

Great video! Clear and concise!

Great video didn't expect that.

I have a Question. I am installing a lock case and need to remove the nest thermostat so I can screw in the backplate of the lock case. Do I need to reset the nest to do that? Thanks. Great video btw.

Just wanted to say thank you for your video. I was helping out a neighbor with a new Nest Thermostat and found a blue wire unused. Connected both ends (furnace "C" terminal and the thermostat "C" terminal. It is no longer asking for the power connector.

Hey Brett, love your videos. Can you make an updated smart home tour video for the end of 2020? That would be great.

Happy ThanksGiving Bred

Nice info great tutorial.👍

Is it possible to jump the C and R together and have the red cable connect to R?

I have a combined heating and cooling system. I have an older emerson thermostat. It has an RC and an RH with a jumper between the two. I just purchased the Nest thermostat that you have in this video. It only has an spot for just one R wire. Will this work and operate both my AC and Heat?

Very good my friend for your help I will do that to my one tomorrow

very good information thanks

Photos...especially of the furnace panel, are really important. I thought I fried the motherboard, but I had connected one wire wrong.

Also remember if it’s too hard on one end , you can always try to pull from the board to the thermostat end . One way is always easier than the other

Mine also needed the "C" wire. Without it I could not manually control the fan. But, I did not have to run any new wire. There were several unused wires inside the cable coming out of the wall. I simply connected the unused blue wire at each end. Now, my NEST is 100% functional.

I have only 2 wires. Hooked it up in Summer worked fine till today . Any help appreciated.

Well Done. Thank you

Very good, Thank You!

I don't have a C wire and My Motion sensing works and no battery issues. I have an OB wire so not sure if that mattered.

Question: do you have 2 separate wires in the green hole?

This is just for my gas heater only 2 wires, but it uses a 24 volt system, not sure if i need a C wire?

I tried installing a power adapter c wire but when I have them plugged into, the nest has them grayed out. Any reason why it doesn't recognize it?

Hey I have this thermostat and I have the trim plate I wanna install how to I get the wires out the google nest I don't wanna damage it by pulling them out the wrong way thx

You show a forced air furnace system c wire install only, can you kindly do a video on an instant hot water/boiler radiant system c wire install please?

Appreciate your help explaining patiently and easy to follow 👍

Hi, we have black, red and green wire on our old thermostat where to put in google nest please let me know .Thanks

I first installed the Google Nest Thermostat on my Mr. Heater garage heater, without the C wire. The heater would continuously cycle until I installed the C wire.

Can you show the AC splice? What happened to them

Had mine installed over a year without a c wire and still working great!

Why would the nest control the fan? Would the heater/cooler not turn this on when they are activated (Uk plumber here, we have completely different system)

Awesome job

I have already white wire on my C-Common screwed, can i just add a blue put them together?

I just bought a 2020 google nest thermostat - Charcoal, Appreciate your tips before I install the device.

Thanks for the this, I noticed that there is now a 'Update Wiring' option on the Google Home App -> Thermostat -> Settings > Theromstat -> Wiring -> Update Wiring. So you shouldn't need to factory reset anymore.

Very informative. Thank you so much.I am going to install soon. My motion sensor is not working due to C wire missing in Nest thermostat 2020.

This video is very helpful- just have a quick question- we are basically replacing the old wire with a new one? The new one has the extra for the C wire? Thank you!

Originally when I installed my Nest 3rd Gen thermostat (Y1,G,Rc,W1) I had a problem with the Nest staying charged (no other issues). I realized that it would charge when the furnace fan was turned on, so I created a schedule that would run the furnace fan a few times a day during the season where heating and cooling doesn't come on regularly. During the winter and during the summer the heat/cooling runs enough that it keeps charged. The schedule was only needed during spring/fall. What has confused me is that I turned off the schedule last winter so the fan didn't needlessly come on and I forgot to turn it on for spring time but I realized for some reason now the battery stays charged. This fall it didn't need the fan schedule again either. Can you explain how suddenly the thermostat stays charged without doing any wiring modifications?

My nest worked fine for a year. 4 wire. Motion sensing was fine. It suddenly started with the e298 error. Reverted back to programmable unit.

I wonder if this will solve the issue I’m having with my AC- the Nest turns the entire system on immediately after shutting off. Its like it runs a full cooling cycle and then turns off and back on again. Sounds awful for the compressor.

My blue wire goes to Y on the houses thermostat. Is that correct?

Your lucky no staples! How easy would it be to push another wire into the existing thermostat wiring cover?

If you are lucky, sometimes there will be an unused conductor on the existing wire. You'll need to figure out where to connect the free wire on the furnace controller board C. Sometimes the original installer prefers to just use a battery thermostat rather than connecting to C. I was able to add C wire to my Nest in my first house and never had any issues. I actually did it for my parents as well, without issue. It's generally recommended to always use the C wire anyway, as there could be temperature inaccuracies or triggering delays when the thermostat achieves the set points.

Thanks it worked for me. I've got a question did u change the color of the text on your thermostat cause I have the nest thermostat chalk edition and the test is white on mine so is there a way to change it?

Hello, thank you very much for your videos, they are great! My equipment is only cooling and when I took out the old thermostat it had a C (blue) wire. I connected it according to the instructions but the thermostat never turns on! Please could you help me? Thanks in advance! Daniel

Thanks man

I had an interesting problem with our Nest Thermostat; any load I placed on our two-wire system started the furnace (think; the power it drew to run the Nest). Let me explain further… I started out with a two-wire system, Red and White, Red=Power, White=Call-for-Heat, the legacy thermostat was a standard mercury bi-metal coil. I installed the Nest and I tried placing Red on Rh and the White wire on W1, but the furnace ran constantly. So I ran and added a ‘C’ wire, I kept Red on Rh, the White on W1 (Call for heat), and the C wire, or common to the furnace. The furnace still ran constantly. The Nest thermostat functions by using a small amount of current from the furnace to, 1. Operate, and 2. Keep the batteries charged. I did some testing and ANY current draw over 6 milliamps starts the furnace. Testing with a ‘C’ (Common) wire still drew a small current and fired up the furnace. I found the markings on the furnace to be very clear, Power, Common, and Call for Heat. To be clear, W1 on the Nest is ‘Call for Heat’, the ‘C’ is common and R1 is 24 vac or power. This is not rocket science, if I draw any current from any of the wires it starts the furnace. I’m thinking there must be something screwy with the vintage furnace, vintage 1995. My initial solution was to remote the 24vac power and supply the Nest power from it, however simply placing the ‘Call for Heat’ wire (W1) on the Nest (along with the common wire), the furnace would start. Finally, I remoted the entire mess, I added a small relay and 24vac transformer in a project box near the thermostat and ran five wires up to where the Nest is mounted. Now everything works well… The takeaway here is, not all furnaces are created equal…

Good job

What kind of medieval heating monstrocity do you use over there? In Europe we have high energy efficient gas central heating systems.

Thanks to the front man of Queens of the Stone Age, I now know how to connect the C wire!

WARNING: try this at your own risk! I tried this, but the taped connection was too thick to go through a point (I imagine it being a hole that's just big enough for the wire itself). I had to pull the new cable back out from the thermostat hole, and I broke the taped connection. Now I have the old wire dangling somewhere inside the wall that I can't see and don't know where. What a disaster!

Nice job

Yes, a C-Wire will save your a lot from the hassle of not having one. Without a common wirre the thermostat pulls power from both hearing and cooling. If the AC unit has it's power shutoff for winter or you have dual zone a Common wire is recommended.

Forgot to mention to turn off power before removing wires. Or cutting into wires. Turn power back on last. If wires touch 24 volt will or could burn up your board

Hi Brett. Thanks for sharing this! My question is why do you replace the original cable when it already has a yellow wire? I see that right after you removed the thermostat base. It is around the <a href="#" class="seekto" data-time="135">2:15</a> min into the video.

Why did you leave y and c wires attached I thought they have to go too

I don’t have a common wire, yet it works fine and I still have the motion sensing feature. Does that mean I shouldn’t ever need one?

Nice video brett

You missed showing us how to connect the A/C unit

Can you tell me the tool you used to strip the wires?

After watching your install video, happy to see that you did this video, too. Thanks! So, to confirm, if you have TWO Nest thermostats (upstairs and downstairs), one would need a C wire for both, no?

Adding new wire option is in the app only. No need to reset.

I have a blue wire on the y1 terminal and no wire on c

I only have 2 wires in my existing thermostat does that matter? Do I just cap off the wires that I don’t need in a thermostat wire?

On installing the new nest thermostat all works however my batteries die every 10 days. Clearly there’s a power issue but I can’t identify it any thoughts?

My thermostat wire runs 30 feet under my floor outside to my unit and staples all the way. No chance of this method working on mine.

By your explanation it seems simple enough, however my mothers current thermostat only has a w wire and an r wire and there is a jumper pin in the r & rc. Would I still be able to use the nest after installing a c wire?