Nest - C Wire ins't getting power

[rogeriogcamargo]

Hi, 

I've installed my Nest with R, W and C wires connected. I got an error saying that the  C Wire isn't getting power. 

Using a multimeter, I've checked the AC voltage from R to C and I'm getting 27.97 V (from the terminal of the Nest Base), so I'm assuming the voltage is there. 

Does anyone know what I might be doing wrong? 

[rogeriogcamargo]

I've some pictures, but I'm not getting permission to post it. 

[Markjosephp]

Hello rogeriogcamargo,

Thank you for reaching out and providing detailed information about your Nest Thermostat installation.

I appreciate that you've taken the initiative to troubleshoot the issue with your thermostat installation. Since you've confirmed the presence of voltage (27.97 V) between the R and C wires using a multimeter, it indicates that there is power reaching the thermostat.

To better assist you, could you please provide additional details about the specific message or wiring report you're encountering? Additionally, it would be helpful to know if there are any other wires connected to your Nest Thermostat and their corresponding voltages. To insert an image in a post:

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I'll look forward to your response.

Best regards,
Mark

[rogeriogcamargo]

Thanks for your answer, Mark! Very appreciated. 

Please find below the message that I'm getting. Also, I tried with another brand new thermostat that I had here (same model) and I got the same error, so it's not a malfunctioning unit issue. 

Wires Connected are - R, W and C

Voltages:

R-C = 27.97 VAC

R-W = 27.40 VAC

W-C = 2.60 VAC

Thanks again! Looking forward to your comments

[Markjosephp]

Hi there,

You're welcome! Thank you for providing additional information. It's helpful that you've tested with another thermostat of the same model to rule out a unit-specific issue. Based on the voltages you've shared, it seems there might be an issue with the W-C connection, where the voltage is significantly lower than R-C and R-W.

This lower voltage could indicate a problem with the wiring or the connection itself. I suggest double-checking the connections to ensure they are securely fastened and that there are no loose wires. Additionally, it could be the green wire itself. Do you still have a photo of your old thermostat's wiring? Usually, the green wire is dedicated to fans. If you no longer have the original photo, a picture from your system's control board might help.

Keep me posted.

Best,
Mark

[rogeriogcamargo]

Thanks Mark. 

The old system was an old Honeywell analog thermostat (heat only) with 2 wires (white and red). I've used the green one because was a spare wire in the same cable that is running to control this zone. I simply connected the green wire in the C terminal from the transformer. 

The wire connection is good.

So W-C voltage should be around 27 VCA as well?  

What else I'm doing wrong? 

Thanks again!

[Markjosephp]

Hi rogeriogcamargo,

You've made a good effort in connecting your thermostat to the existing system, and using the green wire as a common (C) wire is a common practice when upgrading from older systems. The voltage between the W (heat) and C terminals should indeed be around 27 VAC.

If you're still encountering issues, here are a few things you might want to check:

1. Confirm that the transformer you connected the C wire to is indeed the correct one and that it's providing a stable voltage. If the voltage is too low, the Nest Thermostat may not function properly.
2. Check if there is a fuse in your HVAC system that might be blown. If the fuse is blown, it could be disrupting the power supply to the thermostat.
3. If you've checked all of the above and are still having problems, it might be helpful to consult an HVAC technician for a proper C-wire installation.

Regards,
Mark