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Any HVAC guys around? Stupid “c” wire question for google nest

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Doing a simple nest thermostat upgrade for a friend and he assumed there was no common wire. He ordered the Nest power adapter and I went over to install.

Didnt have much time but after looking at his current setup it looked like it had the blue c wire but it was connected the aux of the original thermostat.

I installed the nest and used the blue as the common. I have included images. I had to run quickly but doing a quick setup everything looked good but I think I got a c wire missing message. 

Anyways everything running but will go back (with more time) to install the nest power adapter if in fact the blue wire isnt common.


Any help appreciated. 



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  • 1 year later...

Just wanna add that you might consider taking a multimeter and checking for 24 volts between the R and C terminals to confirm the blue wire is, in fact, a common wire. If not, then installing the Nest power adapter as you initially planned would be the right way to go. But if you're not 100% confident or run into other issues, sometimes it's best to bring in the pros. Companies that specialize in AC repair like ac repair in Riverview, FL are well-versed in HVAC systems and smart thermostat installations. They can provide expert services that go beyond just a quick fix, ensuring that everything is set up properly and functioning efficiently.

Edited by AlmaMatter
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