How To Fix A Missing C Wire | Nest Thermostat Troubleshooting

It is common when a DIYer goes to install a smart thermostat like a Nest that you might be missing a C wire or even the C and G wire like in my case. I will show you how to get new wire (C wire) from your furnace control board to your thermostat location to ensure your nest or other smart thermostat works properly.

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DISCLAIMER: This video and description contain affiliate links, which means that if you click on one of the product links, I’ll receive a small commission.

[rcisneros]

Tip - Take a picture of the old wires before disconnecting anything.

[spocksvulcanbrain]

The last 15 seconds of the video where you talked about using a wall switch face plate was worth the whole video. Never thought of that yet so simple and easy to do. 👍👍 I used your technique in an apartment I have in another state where I installed a "Sensi" remote thermostat. I can see the inside conditions and control them from 550 miles away with my phone. I've used it for over a year without any problems. I had an issue with the common wire missing which wasn't connected before. But now it is and working fine.

[jjjacer]

I got lucky when i added a smart thermostat, i didnt have a common wire connected, old thermostat used batteries, luckily i had an extra wire that was unused and i was able to find output on the furnace that was always 24v/common (didnt have a terminal for it), this saved me alot of pain as the wire had no play and couldnt tell if it went into the ceiling or if it went into the underside of the home (and being mobile home, both areas are not accessible as i would have to cut a hole in the ceiling or get under the trailer and open up the insulation under the beams.)

[scottkchong]

Thanks for the walk-through! Helped me fix the issue quickly for my Amazon thermostat that required a C-wire.

[Eri]

For EVERY home problem I am looking to solve, you have a video! Thank you SO SO much!

[sailor]

I purchased a plug in, alternate current, 24V converter and ran wires to the smart thermostat. I plugged it in to the closest outlet to the thermostat needing a run of only like 4 ft of wires. It was a lot easier than trying to pick up 24 Volts from the Air Conditioning unit.

[kommoncents]

I used a nest (2nd gen) where I used to live. I had only heat (oil) just a red, & white wire. The nest thermostat worked without a common wire and seemed to get charged for winter once the thermostat had constant activity from using the furnace. I found that it ran out of charge during the summer when there was no activity with the furnace. I then charged the thermostat by the micro USB port on the rear, off the wall, then put it back. The charge lasted quite a while and was OK till winter came. Some time later I added a 24V adapter that became my common wire.

[MrBp]

If your replacement Thermostat is smaller than the old one, a trick you can use is to use a picture frame that is bigger than the old thermostat. You can mount some cork or decorative wood in the picture frame and mount the thermostat to that. Mount he whole thing to the wall.

[pubmeatman]

Must be nice to have that good of access to the control wire.

[Branson]

I live in an apartment and my setup is an older one. Ended up finding the wiring diagram online for the system. Saw that they never hooked up the C wire at either end. Was able to connect it up and it was perfectly.

[akalina]

I just pulled 18/8 wire to my Nest to support 2 stage heating/cooling and to support my AprilAir Humidifier. It was super easy.

[darylreedy]

Thank you very much. Thanks to your video, it occurred to me that the blue wire I had attempted to connect to the thermostat had another end, and who knows if it was connected. When I checked the furnace from your picture (7:04 minutes in), I found that indeed the blue wire had never been connected to the furnace, so no amount of connecting the other end to the thermostat mattered! Thanks tons! I connected it to the furnace and suddenly my thermostat had power and works perfectly!

[rolomaticz]

ALWAYS pull the wire UP, pulling down will get stuck in a small hole with the crap and debris inside the wall. This is very important in older homes with plaster lath walls!

[WizDumbDumb]

I have had this nest learning thermostat for over 3 years and it does save you money but it rarely turned on the second stage heating or cooling. Now this year it seems that my air conditioner compressor will not turn on at all. I am no expert by any means but I have been told that the way this thermostat is designed to work without a C wire and internal jumper for thr RH/ RC can cause major issues like mine to occur

[Icant]

I LOVE YOUR VIDEO.... I have a older home with an old thermostat that required a c wire... before returning the thermostat I wanted to keep/ and possibly update my home in the future.. I would have done this and keep my old therm

[sheleenhudelson]

This is just the video I needed to upgrade my thermostat.

[foureyedchick]

9:08: You achieved magic! The family member going down the steps seemlessly disappeared!

[cxspears]

Thank you so much. This video helped me fix my issue in less than 10 mins. The common wire was run through the wall but wasn’t hooked up at the control unit.

[sbfloyd]

Great video as usual- I suggest talking a photo of original connection on both ends

[eversonl]

Thank you! Your video helped me to change my 4 to 5 wire for a smart thermostat.
Thanks again for your helpful information & tips.