How to Connect an Unused C Wire (Common Wire)

How to connect an unused c wire in your furnace compartment. Firstly make sure you have a C wire. In order to see if you have one, refer to this video: • How to Tell if Your Th...
Once you follow the steps in this guide and have a C wire that is connected you can now install your choice of smart thermostat! Some thermostats even allow the use of without a c wire, by using an adapter or wire extender kit.
My Top Smart Thermostats:

[@yamileemigliori4]

Wow, you're a life saver! You're the only one that I found that explained how to connect the C-wire when you do not have the letter panel on your inside unit. I was about to go buy the cheap thermostat again and forget the smart thermostat. THANK YOU THANK YOU for making this video.

[@karenbaggs7731]

Rob! You are awesome! I am a 58 yo nurse and was able to get the C wire connected for our new Nest Learning Thermostat. I scoured the internet and KZbin and finally found exactly the information needed thanks to you. Thank you so much for your help!

[@mannycaz9085]

This was exactly situation. An old A/C unit without a control board. This video helped me so much. Luckily there was an unused C-Wire, no Ecobee PEK for me.

[@jimmyengstrom1068]

Like so many others have said, THANK YOU for this video!! In my situation I had an older "non-smart" thermostat downstairs that controlled both a steam boiler for heating, as well as a central air unit that I wanted to upgrade as my local utility was basically giving away the Emerson Sensi smart-thermostat. The thermostat wire going up to the Air Handler in the attic did have an "unused" blue Common wire but when I went upstairs in the attic I saw that it was not connected to anything. I saw that the wire that goes out to the compressor outside for the AC had two cables that were pigtailed into others outside the AC Handler. The White wire was connected to a wire that was then connected to the Yellow "Y" cable going to my thermostat so watching your video I was then able to figure out that the other wire (the red one that was connected to a Brown wire that went to the control board inside the AC handler) was the 'common wire' in that circuit so I simply did pretty much what you did in the video, turned off the power and then pigtailed the blue common wire to to the same junction as the red/brown wire and voila! It worked!! the only difference was that in my case I had to do an extra pigtail connection at the thermostat wire to 'extend' the C wire from the thermostat to the junction where the red/brown wire was connected.

[@randalljabeth4396]

Thank You! I tried to install a new WiFi thermostat and my C-wire wasn't working. I was ready to return it to Home Depot until I saw your video. The C-wire was disconnected at the furnace. I looked at the wiring diagram on the furnace and found where I needed to connect the T-wire. Hooked the new Thermostat up and it worked. Love the video. Thanks again

[@JorgeGonzalez-yt8sy]

after many videos and forums, this is the vid that helped the most. we have an old a/c from the 80's. whoever installed it used two bundles of wires, one for the thermostat and the other for the condenser (i.e. instead of low voltage two wire, they used another thermostat wire; i guess that's what they had on-hand). then they connected the yellow wire from the thermostat to the red wire of the condenser, i guess to make it more confusing. so following what you said at about the 4:35 mark, i connected the unconnected blue C wire of the thermostat bundle to the opposite of the yellow wire which for me was a white wire connected to a terminal block with a label of "3" (which i haven't seen in any videos or forums). this vid combined with the wiring diagram made me confident that the "3" was the common. thank you very much !

[@mark2213]

Thanks for making this so clear. All the other videos talk about C terminals and additional adaptors. Mine has no circuit board to find a C terminal at. So finding the right ground wire was the trick just like you have shown. Thanks again!

[@chayden3667]

Of all the videos I watched, this was the one I needed. Best knowledge/insight on where to connect the blue wire at the furnace. Thanks.

[@davidwferrar]

Great little video. Just as you described. Found blue wire tucked away behind old thermostat and went straight to the same blue wire within my air con unit. Took me all of 2 minutes to identify and connect up. Now all working fine with wifi connected. Thank you. Keep up the good work

[@ram6796]

You're videos have bee the most helpful I've found regarding the C wire. Thank you.

[@JDSileo]

This was amazingly helpful. I moved to a condo that didnt have the C wire used. The explanations of the wires going to the compressor helped me trace where the C wire ought to go. and Boom! I have a smart thermostat again.

[@chilton225]

Great video! So you are essentially connecting the circuit I got it and smart showing you used gloves thanks for all the tips! Struggling to get the cover off the furnace panel now.

[@lewisstephens4807]

Thank you, my setup was almost identical. I was able to get my new thermostat up and running thanks to your video.

[@donjuan8867]

Great video Rob!!! Simple and easy explanation. And no ads to interrupt thank you

[@rocxylemmon8535]

Thanks buddy! My mother is not ac savey and she needs her kids to help her after covid killed our dad so after they got a new ac unit installed, they never installed that c wire so after foing some homework i suspected the blue wire after looking over the schematics but your video just reassured me, after doing your work, the new thermastat came on after flipping the braker back on, now we kids can help our mom with out going over there just for a adjustment. BRO THANK YOU SO MUCH AND GOD BLESS YOU! ❤

[@vivalageegee]

Thank you!!!!! Straight to the point and explained in plain English! You deserve an award.

[@joshanderson1598]

Your video saved my bacon. I have a 4 wire to my furnace/Thermostat, without a "C" common wire. I needed a 24V wire to run my new WIFI Thermostat. The "24V HUMM" screw on my furnace board was not a 24V hot. But this was and it works great. Thanks man.

[@lovetoexplorewithjamieandm477]

This is the exact setup I have in our new condo and this worked perfectly for us. Thank you for the video!

[@dawnlovejoy4935]

Great video and explanation! I had no trouble with the thermostat side but found the nastiest dust filled furnace. Couldn't find the blue wire because they had cut it so short. Couldn't read any connectors because the dirt and dust. Poor installation of the unit in a small closet and being on a ladder to access. It was all horrible. But your video matched the exact unit and wiring. Got it running in just a few minutes! Thank you so much!

[@Marlin365]

Awesome video, I had 5 phone calls with the Honeywell T9 techs, and no joy. My air handler has no control board. Within 20 minutes of watching your video, I was able to get it working, since you showed which wire to connect for the Common!

[@schpitz101]

Extremely helpful video, thank you! I had been having power issues with my Emerson Sensi for years, but kinda ignored it because the batteries did the job. However, when I wired in my Ecobee 4 it wouldn't turn on. The multimeter showed 0V on the C wire. I opened the side of the air handler and found the C wire, but no control board...just bundles of wires. This is where this video came in! It was so helpful to have somebody explain so clearly the wiring shortcut. After a bit of "jiggery-pokery" I had power to my C-Wire and everything is working great now. Thank you so much!

[@theresarice5947]

Thank you! I have the same issue , your description and video was very helpful! Made my installation of my new thermostat a success!

[@markgoodson7720]

Thanks man. Good talk through and straight to the point. Making it easy for those of us that don't know HVAC.🤘

[@RobSomeKnowledge]

No problem I’m glad I could help you out man!!

[@applepotamous1376]

Thanks for the video! It saved the day! Kept my family warm tonight

[@howdag711]

Thanks for the great Video!!! I was just a minute away from calling a HVAC company. I came across your video and was able to get my smart thermostat working in 2 minutes!!! Saved a bundle. Thank you!!!

[@RobSomeKnowledge]

Anytime my friend! That’s why I post these videos! I’m glad it could help you out!

[@johnathanharris9387]

Worked like a charm! Excellent video your video was the only one that got the job done. Thanks for posting

[@Michael-gg4nq]

This video helped me fix the issue with my thermostat installation. Thank you.

[@CountryBarbie15]

I don't even need a new thermostat, but I still watched 🤣 always great info!

[@juanjoseestrada6089]

Thank you for your instructional, it was easy to follow an the troubleshot to the point, today i have my new thermostat installed with no previous electric wiring knowledge.

[@billsheridan8196]

Great video, save my day. Thankfully you had an older unit similar to mine. I just had wires nutted together and no C terminal. Keep up the good work!

[@RobSomeKnowledge]

I’m glad I could help! That’s why I post these videos!!!

[@Ricardinho101985]

You da man, sir!!!!! Well explained , straight to the point, just what I needed to still my new thermostat. THANK YOU!!

[@adno2015]

This video helped me so much! I can't tell you enough how thankful I am!

[@audi_kid]

2022 just got the eco bee 🐝 and had this issue !!! The instructions don’t tell you when you don’t have a panel .. your video was a life saver on that C wire I was like wtf lol but you saved my butt specially trying to do this last minute in TEXAS 85 degrees lol great video !!!!

[@crystalhenderson8267]

Omg ty! I was getting ready to return my thermostat because I couldn't find a c wire port in the hvac but ur instructions worked!

[@cheekss86]

This video helped me install my thermostat correctly. Thanks for your video

[@jackstyron7619]

Great job with your help I found the c wire in my air handeler...my ecobee works great again thanks again yours good teacher

[@Macgyverjrofficial]

Thank you sooo much !! My system was setup like this with just wires and no labeling. I used the blue wire like in the video and connected to the ground for my AC compressor !!

[@1Brain4Wheels]

Very good video, I’m going to try and tackle this project next weekend - this is extremely helpful! :)

[@ArtPolania87]

I've been meaning to access the unit because I have this issue, eating up batteries every 2 weeks. And the c wire wasn't connected to the unit. Thank you very much!👍👍👍💯.

[@cdiaem1338]

Man your KNOWLEDGE is PRICELESS, THANK YOU, Thank You, thank you You earned a subscriber

[@gerryjohnson7357]

Thanks for posting this video! Going to upgrade to a new smart thermostat, and this is just what i was looking for.

[@bhagyeshmohite3123]

This was excellent, plain simple and no runarounds with unnecessary talk. Straight to the point. Thanks a ton and God bless you good man. Keep up the good work.

[@zoomzoomboomboom]

And no music! 👍🏻

[@zhaoyun1230]

This saved my day. Thank you! I thought I had to extend the C wire and connect directly to the board.

[@RobSomeKnowledge]

It was quite frustrating for me at first too until I did a bunch of research. I’m glad I was able to help!!!

[@ddog1218]

@@RobSomeKnowledge I tried to wire directly to the c terminal, it did not work, there is a make delay timer hooked to my c terminal and the air compressor, can not get it to work

[@andrewsharp712]

Thank you, the wires were not colored correctly on my furnace, but I was able to figure it out by identifying the ac compressor wires as you suggested!

[@ragelikecage4012]

Thanks! Just fixed my "non-working" c wire issue for my upstairs thermo. We got new thermos for both up and down, but for w/e reason the downstairs one already had a c wire connected to the furnace while the upstairs one was wrapped on the wire still. It took longer to get into the attic than it was to just simply attach the c wire where it needed to go. Both thermos are working perfectly, nice to finally have wifi and smart features with them.

[@RobSomeKnowledge]

No problem. I’m glad it could help you out!!

[@Miguel-pv4ue]

Thank you!!!! This definitely helped me. Lady at homedepot was saying you going to need an electrician. I proved her wrong with your help lol!

[@RobSomeKnowledge]

Haha that’s awesome!

[@samm9017]

Thanks a bunch bro! You saved me $500 as I was able to install it myself. 👍👍

[@RobSomeKnowledge]

Glad to help! Love to help people save some money!

[@jagtheripper5297]

We appreciate people like you doing a selfless act to help others 👍🏻

[@RobSomeKnowledge]

That’s the whole purpose of my channel to help others out and save people some money vs calling someone! I’m glad it helped you out, thanks for the comment, that helps out the channel!!

[@rocxylemmon8535]

I agree!

[@rocxylemmon8535]

​@RobSomeKnowledge im the same way

[@Bill-tb3kx]

Excellent video! It was perfect and solved my problem exactly - Thank you!

[@lancepickle8466]

EXACTLY the video I needed to find! Thanks!

[@prashanthshanbhag69]

Thank you, I installed Amazon Thermostat and it was not turning on. Voila C-wire was not connected. This helped!

[@brycyncampbell7899]

You saved me a technician fee! Thank you

[@T.Ruba_]

3 years old and still gets the job done thank you!

[@RobSomeKnowledge]

Love to hear it! I’m glad it helped you

[@Vluhd]

Thank you! I know nothing about this and you helped me get my new thermostat working!

[@RobSomeKnowledge]

That’s awesome!! Glad I could help ya out

[@austinuhri]

The video was very nice but it would have been nice at the end of the video to see you verify the wire voltage, for the blue wire, at the thermostat with a volt meter.

[@rocxylemmon8535]

Yeah! I tried at first to get a reading but none was on it, i assure you it's there though lol! Has to do with being a seiers circuit.

[@dens3096]

AC guys do it for purpose so they can charge you extra cash when you want to install WiFi thermostat 😂

[@rocxylemmon8535]

Thought so! I thought this too but i was like nope! I can do this!

[@amadorsoto1438]

You made me look like a wizard for my wife. Thank you for your knowledge good sir!

[@RobSomeKnowledge]

Hahah 🧙‍♂️ 👍🏼. Anytime my dude!

[@getahungebre3291]

My New Google Nest Thermostat is working fine because of this video. You are awesome. Thank you

[@judevecoli865]

Thanks for this. I'm still trying to find the control wires inside my HVAC unit. But now I know what to do once I get in.

[@mikelove5119]

This helped me out a lot man appreciate the video

[@chilton225]

It worked!!! Thanks best video I watched on this most helpful thank you.

[@1Fine69c]

DUDE thank you. i was dealing with this same problem today. Had to sub to the channel for this one.

[@TKreeps7635]

Thank you very much! Exactly what I needed, great explanation

[@agredo]

THIS video REALLY help me out !!! thanks man !!!!!!!

[@saldesigns257]

Great video! it was helpful in my situation. Keep up the great work!

[@1kennyroberts]

Because this video was so helpful not only did I liked it... I subscribed to the channel

[@RobSomeKnowledge]

Thank you, that means the world to me!!!

[@shenghegu]

Great! Thank you so much for your sharing!

[@tristansteiner6727]

Incredible channel. You're legend

[@ef2744]

Excellent explanation. Thank you.

[@samdang5246]

Thanks. This video helps me create a C line.

[@blanejohnson6294]

Perfect Explanation! Thanks!

[@Turrence]

Thank you! This video was very helpful.

[@drumbyte]

Great job explaining it!

[@SharkeyBrawler]

Video saved me. Thank you soo much!

[@JourneyontheTrail]

Very helpful...thank you!

[@iankz4144]

Thanks man. You saved me a lot of money🤩🤩

[@mohammadsaber420]

This is exactly what I have. Thanks

[@danielpedroza5277]

This helped so much. Thanks!

[@crotopoc]

Another excellent video!

[@binyacom]

You rock. Thank you for saving me the cost of having to pay someone. I knew about the common wire but had no idea how to hook it up at my new apartment’s intimidating looking ceiling accessed furnace. Thermostat and A/C low voltage wires were just as you described. My ecobee is reinstalled and back online. 😅

[@AdelaCarvajalRealtor]

Thank you. We did it. Great explanation 🙏🏼🙏🏼🙏🏼🎉

[@darvinmcclellan5070]

you made it look "easy peasy"...I hope. I opened up my furnace/ac and thought, "oh my gosh, where do I go from here!"

[@sand.590]

Thank you brother 👍

[@cjxgraphics]

Now, I’m wondering why they didn’t just connect the wire in the first place instead of tucking it off to the side and wrapping it around another wire. Thanks for the video!

[@RobSomeKnowledge]

To prevent any accidental shorts from the C wire, shorting onto a power wire, or similar situation.

[@AndresGomez-ey6lh]

This video saved me money 👍

[@ShirleyAerial]

This worked for me and my Amazon Smart Thermostat. My system was older and it just appeared the C wire from AC unit went to a ground. But that ground was all I needed to make it work in that junction the AC wire nut connected to.

[@michelgarciag]

I have exactly the same system, however How would you do if your new thermostat comes with an adapter for non c wire system such as ours ? I got the wyze thermostat. Thank you for your help.

[@sbolwing]

Good Job!! Thank you

[@Sh-mh9bf]

Thank you for the post! I tried the same thing and still the thermostat says power on common wire is not detected. Any suggestions would be great!

[@hectorreyes2167]

Thanks! Your video really helped me.

[@RobSomeKnowledge]

You’re welcome I’m glad it helps!!

[@Jaslaon]

You are the man!!

[@RobSomeKnowledge]

Thanks!!! Glad it helped!

[@njboesman]

this is great and probably works in 90% of applications, but here in the northeast, we have gas boilers and baseboard heat which means no control board at the boiler(instead we have an Aquastat). In my application, the smart thermostat will be controlling both the heat and the AC (R1&C used). I have extra wires (3) coming to the thermostat and a 24v transformer near by boiler, the question is where to connect the 24v to the device?

[@brianmorris7498]

Thank you so much ! !

[@nereus333]

Thank you!

[@user-ni9ug1cl5z]

Thank you!!

[@gersongarcia8057]

Thank you 🎉

[@GreenSculptures-Abhi]

Thank you , thank you, thank you❤❤

[@robertharowitz707]

Great walk through. Thank you so much! My unit had similar wiring. It worked! Thanks again.

[@ImmanuelWoodworking]

Life saver!

[@ror3999]

love the name of channel

[@RobSomeKnowledge]

hahah thanks!!