No C Wire (older system)

[Ewing33855]

Hi,

I have a 3rd gen nest thermostat that I’ve been using since 2017 (8 years). Just recently, my system has been short power cycling. I would have to take it off base and manually charge it via the micro usb port. The current set up is wires in Y1, G and RC. This is cooling only, no heating. I purchased a plug in 24v transformer but am unsure how to connect the wires. The transformer has 2 terminals. I assume one goes into the c but where does the other go into? Any help is appreciated.

[sicsacol]

Hi @Ewing33855,

Thanks for reaching out to the community. I understand you're looking for guidance on how to install a plug-in 24V transformer into the wiring of your Nest Learning Thermostat (3rd gen) due to your cooling system recently short power cycling. No worries, I'd be more than happy to share more details with you.

Adding a 3rd-party C wire or transformer can potentially cause problems or damage your thermostat and your system, even if they claim to be Nest compatible. In this case, the best recommendation is to install a Nest Power Connector or a C wire to ensure consistent power delivery. For more information, please refer to this article: Learn about the common or C wire. 

Hope this information helps.

Cheers,
Jonathan

[Patrick_Caezza]

@Ewing33855 

Since you have a Learning Thermostat, you can connect the transformer leads to the C and Rh terminals. This is the old way of providing external power.

The new and better way is to use the Nest Power Connector. If there is a C terminal in the AC unit, you won't need the external transformer. If there isn't one. You will need it. The Power Connectors instruction will guide you through the correct installation process.

In the future, ask your question in the Nest Thermostat forum, as I don't check this forum very often.