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Two C Wires!!

Waku
Community Member

I’ve had Nest 1st generation installed by electrician, and he didn’t know much about nest. Anyways my old thermostat had two spots to ‘C’ and two cables were connected. When electrician installed nest he used one of those cable to connect to C. But my nest keeps saying no C wire is detected. After he left I tried one of other wire but it says same no C wire connected. How can I make C wire works on my nest?! Please help! 

28 REPLIES 28

Houptee
Silver Product Expert
Silver Product Expert

You have to go to the furnace and see if he connected the C wire to the C screw on the furnace.

Post pictures of the nest wires and the furnace wires.


Houptee -- NJ Master HVAC Licensed Contractor

Waku
Community Member

Thanks! It just our heating system(no AC) is two wire system. There are no C wires. So what we have it transformer that feed 110v and step down to 24v and those two wires are connected to nest. Is it possible to pigtails them and put it into C wire spot on nest? 

EdwardT
Community Specialist
Community Specialist

Hi folks,

 

@Houptee, thanks for the help!

 

@Waku, thanks for reaching out. Could you share a photo of how your Nest Thermostat is wired so we can take a closer look?

 

Thanks,

Edward

EdwardT
Community Specialist
Community Specialist

Hi Waku,

 

It's me again. I wanted to check back in to see if you have other questions and concerns. Feel free to let me know if you do. 

 

Thanks,

Edward

Waku
Community Member

Hi! Thank you for your reply! 
Here I attached pics of my nest. So Green and Black used to be connected to C. There were two C spots ( not like C and C1, there were literally two Cs) I talked to the technical support team on the phone. She said voltage is showing lowering than it should, although when an electrician detected 24v when then touch green and black wires. I guess we use only one of them it shows low? But we don’t know where to connect other C to. fYI Red wire is not doing anything. 

Waku
Community Member

5AE4DD44-E9BB-4FCE-900D-1A7BD371E533.jpeg

F876DA6D-78C9-461E-B48D-B125A3337E63.jpeg

EdwardT
Community Specialist
Community Specialist

Hi Waku,

 

Thanks for sharing the photos of your Nest Thermostat. To make sure that the black wire is a C wire, please check the control board of your HVAC system and check if the other end of the black wire is connected to a terminal with a C label.

 

Thanks,

Edward

Waku
Community Member

Hi, yes so I see these two wires to our transformer that is fed to 110v. Our panel does not have C terminal (confirmed with brand) so what the electrician did back then was to feed 110v to transfer to down to 24v divided into two wires that are black and green. With these two yes we have 24v but only one is fed to C to our nest right now. 

Houptee
Silver Product Expert
Silver Product Expert

One wire on transformer is +24 volts that goes to R on the nest.

The other wire will have to go to C on the nest. it's like negative -24 volts.

White goes to W on nest and to the heating system to make it turn on.


Houptee -- NJ Master HVAC Licensed Contractor

Waku
Community Member

Thank you! That's what I was thinking too! But then, where this wire currently connect to R go? This was connect to R with old thermostat too. 

Waku
Community Member

I also don't have multi meter, is there way to know which one is +24 volt and -24? Thank you!

Waku
Community Member

I tried to put two wire together but they are too thick together not going in..any tips?

Houptee
Silver Product Expert
Silver Product Expert

No do not put 2 wires together!

One wire comes from your transformer and goes to R.

The other wire from transformer goes to C.

If you touched them together it's a short circuit and can blow the transformer damaging it.


Houptee -- NJ Master HVAC Licensed Contractor

Waku
Community Member

Ok! So do I take out the wire that is currently (and previously) connected to R? (see my picture above it's blue)

Waku
Community Member

FYI Green and Black are two wires that come from transformer. 

Waku
Community Member

So when I asked you if I put two wires together, I meant either black or green wire AND blue wire.

EdwardT
Community Specialist
Community Specialist

Hi folks,

 

@Houptee, thanks for the help!

 

@Waku, I wanted to follow up and see if you are still in need of any help. Please let me know if you are still having any trouble from here, as I would be happy to take a closer look and assist you further.

 

Thanks,

Edward

Houptee
Silver Product Expert
Silver Product Expert

Without seeing pics of the furnace to see what the electrician did we cannot help you figure it out.

Go to the transformer and take pics of the 24v wires and where do they go after they leave the transformer?

Where do the wires connect on the furnace?

You have different color wires spliced in the box behind the Nest and I cannot determine where the wires go after they are taped in the box to different color wires.


Houptee -- NJ Master HVAC Licensed Contractor

Waku
Community Member

Two wires go to transformer. Here is a photo 

Waku
Community Member

3BAD7025-87C7-48E0-B8A3-89D924B409BC.jpeg

Waku
Community Member

It might be hard to see but red wire is cut off 

871A0373-328C-4475-A963-C57E9D5E3B77.jpeg

Waku
Community Member

The. White is connected to white and red is connected to blackish wire 

A3A0B628-088F-4D6B-BCEF-7F14CDDE33C1.jpeg

Jhonleanmel
Community Specialist
Community Specialist

Hi everyone,

 

Waku, thanks for the photos. I suggest reaching out to a Nest Pro technician to properly wire your system with your Nest Thermostat.

 

Best,

Mel

Waku
Community Member

These white and red wires are coming from here

7E56285D-0DB2-42B1-A366-CDF22C424A0B.jpeg

EdwardT
Community Specialist
Community Specialist

Hi folks,

 

@Houptee, thanks for the help!

 

@Waku, I wanted to follow up and see if you are still in need of any help. Please let me know if you are still having any trouble from here, as I would be happy to take a closer look and assist you further.

 

Thanks,

Edward

Houptee
Silver Product Expert
Silver Product Expert

Follow the Red wire back to the transformer and mark the screw on transformer R then the other screw on the transformer will be your C common.

Do you have extra wires not used in the cable going to the old thermostat? I see wires wrapped around the one cable in the pic is that the cable that goes to your old thermostat?

If not and you only have 2 wires at the old thermostat you can buy 5 conductor thermostat wire at Lowes or Home depot they sell it in a plastic bag 50 ft or 25 ft rolls.  Then if possible run the new wire to the Nest and connect Red to R white to W and blue to C, then at the furnace hook the red white up same as the old wires but also connect blue to the transformer C screw (once you figure out which screw that is).


Houptee -- NJ Master HVAC Licensed Contractor

Waku
Community Member

All went well! Thank you for your help!! 

Markjosephp
Community Specialist
Community Specialist

Hello Waku,

 

Awesome! I'm glad to hear that it's been sorted out. We can probably consider this one finished, so I'll close the thread in 24 hours unless we can help with anything else.

 

Thanks,

Mark