Fix a Nest Thermostat C Wire Issue

3 Materials
1 Hour

So you got a fancy Nest, right? Suddenly it stops working! What? Are you missing a C wire? Fix it here!
This Nest Thermostat, I tell ya, while it does have some perks, it has been a handful since I got it. Luckily for me, I managed to get one for free.

Unluckily for me, it's stopped working on numerous occasions and I've frozen my rear end off as of course it waits until the weather is sub-zero.

This time was different though, everything stopped working. As you can see, we have a four wire system.
As it turns out, despite what Nest has said for years that you don't need a C wire (essentially a power wire) to run one of these things, they lied. Or stretched the truth. Either way, turns out, over time without a C wire, eventually it'll stop working. Which is a hassle at bare minimum.

Here's some fancy trickery a technician did to get the heat and a/c working again, but this doesn't allow usage of the fan setting on its own which I like to use. He also rewired the furnace which I was not privy to.

Wholly dissatisfied, I contacted someone else.
What's needed is a Venstar Add-a-Wire that adds a C wire through some creative splitting in its gizmo.

Now, this is definitely a DIY-able task but as Nest was paying for my repair, I went pro. Know your limits though; messing with furnace wiring looks terrifying.

You wire this thing at the furnace and a little splitter at the thermostat. All in all, it's very simple and quite genius.
As you can see, my pro was wow, very pro and wired this thing back up in a very tidy fashion! And as you can see too, there's now a blue wire going into the C slot.
Proof in the pudding, the C wire is detected and all the numbers were reading properly. Haven't had a stitch of an issue since!

Be sure to click the link below to read all the details about this and so many other cool projects around the house!

Suggested materials:

  • Venstar Add-a-Wire  (Amazon)
  • Wire strippers  (Amazon)
  • Wire nuts  (Amazon)

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Becky at Flipping the Flip

Want more details about this and other DIY projects? Check out my blog post!


Have a question about this project?

1 question
  • Alysia Croft
    on Feb 3, 2019

    I’m having this same problem right now . Been trying to figure it out for two days. So why did nest pay for a pro installer? Is that normal

    • Be sure to get in touch with Nest. I go into more detail in the blog post so please feel free to come read it. Essentially Nest had claimed their thermostats could be installed without a C-wire. Turns out that’s false. They paid for someone to come out and fix the issue. I don’t know if them doing so is normal but see what they say.

Join the conversation

  • Jan Marie
    on Aug 30, 2018

    That was one of the reasons I went with the Honeywell Color Change Programmable thermostat. I had to move one wire on the furnace. It was straight forward and matched the instruction manual diagram. Plus, it gives me both inside and outside temps and humidity (all on the home screen) along with the ability to change the color of the screen and font so that it blends into the wall color. Having a traditional house and the Nest is round and modern looking so your eye is drawn to it. It's great that you were able to get it fixed and that they footed the bill. It's always nice to have the furnace running in the winter.

    • Becky at Flipping the Flip
      on Aug 31, 2018

      Right, I’ve heard the Honeywell and Ecobee have different wiring to manage the C wire issue. I’m glad it worked out for you! Thanks!

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