topic Re: Two C Wires!! in Nest Thermostats

Two C Wires!!

Waku — Fri, 09 Dec 2022 13:04:50 GMT

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!

Re: Two C Wires!!

Houptee — Fri, 09 Dec 2022 15:04:11 GMT

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.

Re: Two C Wires!!

Waku — Fri, 09 Dec 2022 15:44:43 GMT

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?

Re: Two C Wires!!

EdwardT — Tue, 13 Dec 2022 02:44:24 GMT

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

Re: Two C Wires!!

EdwardT — Fri, 16 Dec 2022 03:00:40 GMT

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

Re: Two C Wires!!

Waku — Fri, 16 Dec 2022 03:42:12 GMT

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.

Re: Two C Wires!!

Waku — Fri, 16 Dec 2022 03:43:25 GMT

Re: Two C Wires!!

EdwardT — Fri, 16 Dec 2022 05:19:48 GMT

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

Re: Two C Wires!!

Waku — Fri, 16 Dec 2022 13:43:31 GMT

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.

Re: Two C Wires!!

Houptee — Fri, 16 Dec 2022 16:53:20 GMT

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.

Re: Two C Wires!!

Waku — Fri, 16 Dec 2022 16:59:38 GMT

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.

Re: Two C Wires!!

Waku — Fri, 16 Dec 2022 17:02:21 GMT

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

Re: Two C Wires!!

Waku — Fri, 16 Dec 2022 17:03:48 GMT

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

Re: Two C Wires!!

Houptee — Fri, 16 Dec 2022 17:14:17 GMT

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.

Re: Two C Wires!!

Waku — Fri, 16 Dec 2022 17:20:53 GMT

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

Re: Two C Wires!!

Waku — Fri, 16 Dec 2022 17:21:52 GMT

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

Re: Two C Wires!!

Waku — Fri, 16 Dec 2022 17:25:09 GMT

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

Re: Two C Wires!!

Houptee — Fri, 16 Dec 2022 21:11:37 GMT

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.

Re: Two C Wires!!

EdwardT — Tue, 20 Dec 2022 01:36:46 GMT

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

Re: Two C Wires!!

Waku — Tue, 20 Dec 2022 18:12:15 GMT

Two wires go to transformer. Here is a photo