Nest will over-temp without shutting off the heat request (white wire)

[tommar]

I have a Nest (3rd generation) learning thermostat. Five or six times in the last year (especially yesterday), the thermostat will not shut off the heat request to the furnace (white wire). Yesterday, we had the heat set to 72 degrees and the thermostat showed the temperature at 78 degrees. I checked the white wire from the thermostat at the furnace terminal (interlock disabled) and it was reading 26 VAC. I turned the thermostat off and the reading was still 26 VAC. I disconnected the thermostat white wire from the furnace terminal. The terminal on the furnace read 0 VAC, but the white wire still read 26 VAC with THE THERMOSTAT OFF! The only way to reset the "send heat" signal (white wire) was to flip the breaker for the HVAC.

I'm worried I may not be at home and the furnace will over heat or cause a fire. 

Any and all suggestions would be greatly appreciated.

[Markjosephp]

Hello tommar,

I'm sorry to hear about the issues you're experiencing with your Nest Learning Thermostat. It sounds like a concerning situation, especially with the risk of the furnace potentially overheating or causing a fire. Here are a few suggestions to troubleshoot and address the problem:

1. Ensure that your thermostat has the latest firmware updates installed. Sometimes, issues like this can be resolved through software updates that address bugs or glitches.
2. Perform a reset on your thermostat to see if it resolves the issue. You can do this by going to the thermostat settings and selecting "Reset".
3. Double-check the wiring at both the thermostat and furnace ends. Make sure there are no loose connections or exposed wires. If there are any damaged wires, they may need to be replaced. Gather the power readings in your thermostat's Technical Info settings. The absence of a C wire could be a potential cause of the issue, as it appears to be having power issues.
4. If you're not comfortable dealing with the electrical components or if the problem persists, it might be wise to consult with a professional HVAC technician. They can assess the wiring, thermostat, and furnace to identify and resolve any underlying issues.

As a precaution, you may want to install an emergency shut-off switch for your furnace. This switch can be used to cut power to the furnace in case of emergencies.

Let us know how it goes.

Best regards,
Mark

[tommar]

Thanks, Mark. The issue was definitely the thermostat and was still under warranty via Best Buy and the Geek Squad. I had them replace the thermostat (same type) and I haven't had the issue since.

[Markjosephp]

Hi there,

I appreciate the update. It's great to know that the replacement thermostat has been working well, and you haven't experienced any further issues. If you have any more questions or need assistance in the future, feel free to reach out.

Best regards,

Mark