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Please Help -- Wiring for Nest Thermostat (add Common Wire?)

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  • Oct 10th, 2019 9:44 am
[OP]
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Aug 22, 2009
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Guelph, ON

Please Help -- Wiring for Nest Thermostat (add Common Wire?)

Hi Everyone!

I'm looking to replace my current thermostat with a Nest. Unfortunately, it seems that I don't have a "Common" wire at my thermostat (and I would really like a Common wire feed so that I don't have to rely on the Nest's internal battery). My thermostat is wired with four wires like this (RC/RH, G, Y, W):

Thermostat Wiring -- Current v1.png

I thought I would have to get a VenStar Add-A-Wire, but when looking at the wires outside my furnace, I'm presented with this:

IMG_4873.jpg

To make it easier to interpret, I drew out a wiring diagram for the mess of wires outside my furnace:

Furnace Wiring -- Current v1.png

I found it interesting that it appears that an orange-coloured wire is presumably running to my thermostat, but is cut off outside my furnace. Is there any opportunity for me to use that orange wire to add a "Common Wire" to the thermostat? I did try to pull out the wire at my thermostat to see if there was an orange one tucked in the wall, but something was preventing me from pulling the wire out of the wall further than in the photo below. I was hoping to pull the wire out to the point that I would see the brown wire casing and hopefully an unconnected orange wire. If you think that I likely have an orange wire and that it can be used as a "Common Wire", then I could drill a larger hole behind my thermostat to try and find it.

IMG_4877.jpg

Thanks for any help!
Last edited by VifferFun on Oct 5th, 2019 2:56 pm, edited 1 time in total.
21 replies
Deal Addict
Jan 21, 2018
4669 posts
4720 upvotes
Vancouver
The "C" in C wire stands for Common, not Control. It just provides a connection to the common terminal of your 24vac transformer to let the thermostat draw power directly.

You appear to have 4 wires at the thermostat, and presumably you can tell what they are connected to by the legend that's too small to read in your photo. That would be a typical 4-wire connection. You mention a separate orange wire at the furnace end that's cut off, but from what you say it seems that it isn't part of the 4-wire cable running to the thermostat.

If that's the case, you don't have a C-wire at the thermostat, but you do have a 4-wire cable running to the thermostat, so you should be able to dual-purpose one of the wires using the 4-to-5-wire adapter provided by Nest or purchased separately from Venstar.
[OP]
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Aug 22, 2009
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Guelph, ON
Hi Scote64,

Thanks for your reply!

Sorry, I definitely used the word "control wire" when I meant to use "common wire" so I've edited my post to correct this.

I do have four wires at my thermostat, but I suspect that a fifth orange-coloured wire is tucked into the wall. The diagram below illustrates it the best. On the right is the wire going to my thermostat. I believe that it's a 5-wire strand, but that the orange is simply cut off in my basement. Is there any way that I can connect the orange wire in my basement to something to turn the orange wire into a "common wire" to feed power to the Nest thermostat? If yes, then I'll do a more invasive dig into my thermostat wall to see if I can find the other end of the orange-coloured wire.

Furnace Wiring -- Current v1.png

Thanks!
[OP]
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Aug 22, 2009
573 posts
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Guelph, ON
I just pulled the panels off my furnace so that I can see the ends of the wires that are connected to the furnace:

Furnace Connectors v1.jpg

Everything is connected in the furnace as I thought it would, and it confirms that the black wire coming from the furnace is indeed the Common Wire (24V). In light of this, I have redrawn my wiring diagram below:

Furnace Wiring -- Current v2.png

Could I get a common wire to my thermostat by simply:

1.) Buying some wire and connecting (1) to (2) as shown by the green dashed line (so that the orange wire presumably in my thermostat wall becomes a common wire), and

2.) Digging around in my thermostat wall to see if I can find the other end of the orange-coloured wire?

Thanks!
Member
Jun 23, 2019
363 posts
208 upvotes
If you don't care about being able to manually control the blower fan you can repurpose the "G" wire to the "C" wire. I did this at my dads house because he only has 4 wires, and he doesn't care to have control of the fan.
Deal Addict
Aug 16, 2005
2658 posts
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North York
You could get something like this which you wire up inside your furnace and uses your existing 4 wire setup.

https://www.amazon.ca/Venstar-ACC0410-W ... 0876&psc=1

I had to use something similar when I had my Honeywell Lyric T5 and my new ecobee came with a 4 to 5 wire kit too. This will give you full functionality and control over your furnace.
[OP]
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Aug 22, 2009
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Guelph, ON
Thanks for the replies :)

I do know that I can simly steal the blower fan wire, but I want to retain the full functionality of my HVAC.

I'm also aware of the Venstar device (and I've actually bought one already), but I'm just thinking that it may not be required. From the diagram in my post above, can't I simply add a common wire to my thermostat by making the connection shown in the green dotted line? Or would that not work since it appears that the Common Wire from the furnace is running to the A/C or something?

Thanks!
Deal Addict
Apr 18, 2005
2556 posts
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Mississauga
I got the ecobee 4 as well.. and since I'm relocating my thermostat location I pulled 5 wire.. now need to figure out how to hook the furnace end as well.

Will take pics of I figure it out.
Deal Addict
Aug 16, 2005
2658 posts
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North York
TLSRULZ wrote: I got the ecobee 4 as well.. and since I'm relocating my thermostat location I pulled 5 wire.. now need to figure out how to hook the furnace end as well.

Will take pics of I figure it out.
Pretty straight forward on the furnace side as there is usually a terminal block with all the outputs labeled on the main board like this
mceclip2.png
Just remember to turn the breaker off to your furnace when you do this.
Sr. Member
Jan 19, 2013
548 posts
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Ottawa
If there is orange wire behind thermostat in the wall. You can use that as common wire.
[OP]
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Aug 22, 2009
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Guelph, ON
1chinaman wrote: If there is orange wire behind thermostat in the wall. You can use that as common wire.
I can try to dig more into the wall to find it. Unfortunately, the wire doesn't pull out of the wall far enough for me to see the sheathing that surrounds all of the coloured wires. Something's preventing me from pulling the wire out of the wall more, but I could drill the hole large to see if I can find it. I suspect that the other end of the orange wire is in there.

Assuming that I can find the orange wire in my thermostat wall, can I simply make the connection shown with the green dashed line in my wiring diagram? Would this mess up my A/C in any way (since there seems to be a common wire going to the A/C for some reason)?

1000x480_smart_fit.jpg

Thanks!
Deal Expert
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Dec 12, 2009
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If there is only 4 wires going to the thermostat, just install a PEK kit. I am not sure what the official name is. The ecobee thermostat come with one and that is what they call it. The install is done at the furnace end and pretty straightforward. I think the concept is to use one of the 4 wires as the common. There is never a need for both AC and heat at the same time.
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Newbie
Mar 27, 2018
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You should be able to add the wire as per your diagram above and run it successfully.

I had a similar issue.
I had a Nest installed as part of one of those promotions, the salesman said he had to install it so I stayed out of his way. I should have checked his work. In the summer, the A/C would not work. It would start, but immediately trip. Since it is almost 20 years old, I looked into getting it replaced. But decided to troubleshoot it first. Opened the AC, cleaned it, did some testing and noticed that it was not getting the proper voltage. Looked into getting a new contractor, but decided to try swapping the stat first. Installed an old mercury stat and it fired right up.

Like yours, I have a black and white wire from the furnace going to the AC.
I had 4 wires going to the stat.
However, at the stat I had 5 wires. I found this strange so I traced the wires. Turns out that the previous owner/furnace installer (from 2007) spliced a 4 wire cable on the 5 wire cable. My guess is that they did that when they replaced the furnace in 2007 as the 5 wire was too short. They spliced the black and red wires together and wired it to the red terminal on the panel.
When the Nest guy came to install the new stat, he did not hook up the 5th wire, he just left it in the wall. He should have said something about it and I would have looked into it.
I was able to pull enough cable to remove the 4 wire cable and wire all 5 wires correctly to the furnace. I tested each one to ensure I was getting continuity at the stat. Hooked up the Nest using the 5 wire technique and it worked.

So, yes, you add that wire.

But, if I was you, I would return the Nest and get an Ecobee.
Sr. Member
Mar 7, 2008
700 posts
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Leask
I would suggest you go with ecobee instead!

I don’t have C wire with my thermostat but with ecobee PEK module, I can use it without a C wire. Problem solved.
[OP]
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Aug 22, 2009
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Guelph, ON
Hi again,

Thanks for the feedback everyone! I did some more invasive digging into my thermostat wall and, sure enough, I was able to find the other end of the orange wire in there!

IMG_4889.jpg

In my utility room, I connected the other end of the orange wire to the Common Wire that's connected to my AC unit. The final connections in the basement look like this:

Thermostat Wiring v2.jpg

I labeled the wires in the utility room in case I ever need to tinker there again:

IMG_4892.jpg

In hindsight, I could have cleaned up the install by swapping the orange and black wires both in the utility room and at the thermostat (so that the colours remain consistent from the furnace all the way to the thermostat with Orange = Y and Black = C). I'll probably clean up this detail later this week :) The Nest seems to be functioning good and looks great, but I wasn't able to test the A/C yet (since you can't run the test when it's below 16C out). I did try manually setting hte temperature to 18C last night, but the A/C compressor wouldn't kick in (even after waiting 10 mins). I hope this is simply because it was cold outside and not because of my wiring? Any ideas?

IMG_4893.jpg

Thanks!
Penalty Box
Jun 24, 2015
4833 posts
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not sure why they swapped orange and black in the utility room, but yes you should fix it up soon,. and great work on the thermostat.
Hi
[OP]
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Aug 22, 2009
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GoodFellaz wrote: not sure why they swapped orange and black in the utility room, but yes you should fix it up soon,. and great work on the thermostat.
I'm not sure why they did that either, but I will tidy it up once I confirm that everything is working :) I really hope I can get the A/C compressor to kick in for a test! Everything else works as expected.
[OP]
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Aug 22, 2009
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It was warmer today and I tested out my A/C -- everything is working good :)
[OP]
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Aug 22, 2009
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I cleaned all of the wiring up last night so that it's now a lot more logical like the diagram below :)

Thermostat Wiring v3.jpg
Penalty Box
Jun 24, 2015
4833 posts
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i have the same wiring as you, im not sure when and why they stopped using yellow and replaced it with orange? yellow would make more sense for the air conditioning wire, my 18-4 had yellow instead
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