Fix Nest Thermostat C-Wire Issues Without Adding a C-Wire 

I recommend venstar's add a wire! It works on all wifi smart thermostats that need a C wire. I've used it on nest and honeywell.

Thank you for this step-by-step guide. I have two thermostats and having the same power issue on both. Will one nest power connector be enough for both thermostats and should I be installing the nest power connector on the same area on the HVAC circuit board? Thanks (again) in advance.

In the official install video it mentions that if you have multiple thermostats you should install it at the Zone Controller. See 2,45 here: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-xlx2Um6D1Z8.html

@@TechWithBrett Thanks. My HVAC is an older model York Diamond 80 and it sits on the attic. I'm not sure where the zone controller is located. Googled it but I couldn't find any sites that describes/locates where the zone controller is. May have to contact local HVAC techs.

It certainly was Intimidating at first and i'm glad I was able to relay what I learned.

This happened to me. Had no idea about the kill switch and almost had a HVAC tech come out to tell me just that… thankfully I caught it and saved myself the fee. New home owner lesson learned.

THIS comment saved me! I had already scheduled a tech to come out tomorrow morning and then I watched the whole video above (LOVE IT! So informative) and the whole time I was thinking, yep I did that and that and that. What I didn't do was put the access panel back on, and I never would have thought of that! THANK YOU!!!!! If Brett sees this, I HIGHLY recommend pinning this to the top!

Great point, I am glad adding a new wire in my home was pretty easy. Glad you got yours figured out! I still have a few wiring mysteries to solve.

Shout out also to those who run the extra C wire in newer built homes, but never utilized. That is a forever grateful moment here. So if you are out there and built our home in 2007 (most likely not, but hey!), and happen to stumble upon this, thank you.

@@Dana-cb7vk You’re welcome

Thanks! 👍

Did you have an error in the app before? It is odd there is no way to detect it is working for you.

@@TechWithBrett Yes 3 days after installing the new Gen 3 to replace the defective Gen 3 I had I got a E195 error. Ordered up the Power Connector and contacted Nest, even they said you can't see like the new 2020 Thermostat a Power Connector section. No biggie I guess but typical Google for you.

@@accordxtc319 ; I'm still undecided if I need one of these or not ... Installed a Nest gen. 3 for my old two wire controlled gas furnace. And the thermostat appears to be fully functional and no error messages on setup without the need of a "C" wire just as Nest advertises it (as with the Nest E). However, on another YT channel at least one HVAC tech claims that while the Nest gen. 3 and Nest E may start out that way appearing to function without a C wire as advertised. He's encountered many instances where this starts causing problems later on. So I don't know, as it's been installed over a month now and still showing no indication of problems.

@@antonioterrell354 For the low cost of the device, you definitely won't go wrong adding it, not adding it well that could be a gamble your not willing to take. I've been happy with adding it. Extra piece of mind I see it as.

@@accordxtc319 ; Ok thanks ... Took your advice and ordered one of these from Amazon and installed it yesterday. I can only "assume" it's working since the Nest 3rd gen. t-stat gives no indication in the settings that the NPC is detected like the 2020 model in the video does. The furnace still works as before without any error messages. I guess the only indication I have is that during installation with power to the furnace turned off I noticed the battery voltage dropped slightly by a few hundredths of a volt. After turning the power back on the battery voltage eventually climbed back up again to where it was before. So I guess that's an indication the thermostat is receiving power through the NPC. But I sure wish Nest had given the 3rd gen. model an indicator in the settings confirming the presence of the NPC's installation.

The one that's with the others. The extra one is for the compressor

@@IkLms11 since you are here, one more question, if I don’t have Cooling system at home, can I use the Y wire for C wire connection? Without the need for buying the Power Connector? Thanks a lot!

You got this!

That is definitely what you should have done

Thanks for sharing this… seems like I have same situation. The y cable connected to terminal on board has an additional red cable connected to it and white cable connected to c terminal.

Yeah some wire ins at furnace level, especially when Central Air is installed, can have all sorts of uniqie scenarios. Pro tip, there. Oirs has all sorts of odd wire ins at furnace level. Grateful for a pre ran, unised blue wire we used for the C wire at a proper splice in point.

Tired this on mine and didn't have any luck. The thermostat kicked on and no air was pumping out the ducts and my AC unit started to build up all kinds of frost.. any idea? I swapped G wire to C at both ends.

You are disabling independent control of your blower motor. Tying your blower fan with AC compressor, but how would it work in winter when furnace is used?

​@@akhtarkh technician here, the furnace circuit board/fan control would turn on the fan when the furnace comes on. The G terminal is there to run the fan independently without the ac or heater running.

So, just to confirm, adding this power connector will allow independent usage of the fan, correct?@@allanbrito13

The colors might be different so you need to verify what color was plugged into what label first and then match the labels.

Between the two: Courrier. Why? Parts. No Idea what the future holds but for HVAC newer stuff is currently what is being made/available (so if you have old stuff double check everything to make sure its ok now folks!), and we have all these worldwide shipping issues. I would avoid Mitsu if at all possible for those two reasons alone: for when something goes wrong. It will, just a matter of when. For us, it was the fan. Luckily, it was before all this nonsense- but not being Mitsu, the part was 10x's easier to find, and if it were to occur now, most likely cheaper due to those availability issues. ;)

@@Dana-cb7vk Thank You for replying Dana 🙂. Your advice, and the "Why", Much Appreciated (◠‿・)-☆ JaneLee 🕊️ - in Suburban Philadelphia 12/8/21

Ok I just bought the nest connector and will try installing tomorrow!

Many older houses only have thermostat wires with 4 conductors. That is the case in my house. I replaced the furnace a few years ago. The new furnace has the C (common) wire connection on the control board. It wasn't used on the old thermostat. It is now needed on the new thermostat. I ran a new wire with 5 conductors.

Did you have a before picture of where the wiring was at? I'd double check the wiring there with what you did.

Yes it will work. It's 30vac max. Not considered high voltage.

Same here … did you figure it out? I’m trying to figure out what I need to do with the additional cable. He said to leave it connected to terminal but wanna make sure that’s right …

It just means you had 2 ran vs an R jumper. It may not be compatible. Depending on furnace level, you may be able to splice both r wires, but atypically the thermostat will have a slot in for both R and Rh.

Same for me!! Did your system detect the power adapter they sent though? Mine didn’t and I’m going on two weeks without air or heat. It look like my blue wire went to the CU, but the pictures I took of my thermostat and furnace were deleted with several other valuable pictures, and I’m lost at what to do. Also live in a building 😢 any advice?

How did you get Google to send you a free one??

@repairdrive not sure if you got your power connector yet but on the initial thermostat set up it asks if you have one. Just select know and it will tell you you need one and gives you a link to order it for free. Took me 3 days to get mine

@@carloshernandez3260 Did some more investigation. Turns out the builders installed an extra unused C wire! I just connected it up top to my Nest and at the furnace to the correct spot and it works flawlessly now 👍🏾

@@repairdrive my access panel shows a C wire but I don’t see it by the thermostat hook up, how do I get it up there?

my unit was heating only, maybe that is why, all set now tho.

@@DanburyConnecticut I noticed… I have similar setup… did you keep the c and other cable going to y? Or do you only have nest power adapter connected to terminal ? Just one cable connected to c and y terminal?

As he said in the video, leave all other connections :)

@@sheldonman14 you sure it’s okay to have two c cables connected to terminal?

@@gerardo.galvan I have had mine configured this way for a few months. No issues

@@sheldonman14 did you also have a red and blue cable connected to y terminal?

Just because the wire is blue doesn't mean that it is connected to the C Wire. Verify on your system at the thermostat and furnace what wiring you have first.

Yes they have all different methods of wiring looks complicated but , am electrician myself so complicated is my last name

@@edgarflores9064 maybe you can help me. I have an old behemoth of a furnace with 3 wire to Stat. Red white blue. Blue not being used, either end. With and old Honeywell turn dial Stat. I replaced wiring with 5 wire. Ran red and white accordingly. Still have blue green and yellow at Stat and furnace. I can use blue for "C" right? I just don't know where to wire it on furnace end because I don't have circuit board. I have a limit switch and th/tr switch. And a transformer, saying load on one terminal. White wire runs from stat to former red wire runs from Stat to limit to th/tr back to former. Please help. I feel like it's a really simple fix . I'm just not qualified. I'm a carpenter not an electrician! Thanks for any help. I'm currently without heat because of this. Thank the lord we had decent weather today.

Sorry I sound to cocky is really hard for me to explain with words plus, I will confuse you even more really sorry

You will need to contact a Nest Pro / AC Tech to help.

Volt meter. Or read the diagram in the panel.

The splice in unit creates a C wire scenario, without the pain of running a completely fresh wire, is the solution. It is indeed a complicated one, but it plagues all smart thermostats when no C wire is pre ran. You can still run that wire, or may even have an unsued ran, wire. That could be utilized as C with splice in usually pre board where the power wire runs up to a splice/wire nut. So when this device is utilized it takes that 24v and runs it into the box and utilizes another wire (usually AC, sometimes fan but that power is not a constant/common, which is what the C wire is) at thermostat level, and then that wire instead of going to AC would go to C at thermostat level. The box reconfigs it to run both C and AC as it still has a splice into AC at/before board (furnace side). The box just acts as a fancy splice in to allow both C as well as AC to properly function. Just be sure not to blow that board fuse as it will surge to the AC unit and moat likely blow open the power unit, causing AC to constantly run. So be sure to follow its instructions to a T. I hope that helps. It is indeed a complicated fix, but it works.

Gotta find where that Y wire from thermostat connects inside the unit and go from there

Just kidding! I put the door back on and it was fine :) thank you for a great video!

That's great to hear! Glad you were able to get it to work.

It is able to help provide more constant power to keep the thermostat working without using the batteries.

Yes, it supports 24V AC.

Thanks so much!@@TechWithBrett

Follow up : It turns out I already have one c wire connected in the hvac system and not on thermostat. I took the red wire out from c wire terminal in hvac and just inserted the c wire from c wire connector . Now my ac works perfect without blowing any fuse

Glad you figured it out.

The Best Connector just pulls power from one of the other lines so it has enough. I heard it's possible to do without this device but I'm no electrician and wouldn't want to try it and break the system.

They should be a place on the furnace panel that is C, do you not have that?\

@@TechWithBrett I don’t. Not that I can tell anyway…nothing is explicitly labeled C

Oh wait, I was using the zone panel not the main furnace panel since I have multiple zones…should I use the furnace panel?

It should go at the Zone Controller. Is this for heating and cooling or just one? storage.googleapis.com/support-kms-prod/WJrWiEaeDuoZnn7t6LNYcRa50GQKt4dECMKH