Fan starts despite no schedule setup and no learning.

[fahedz]

I have two nest thermostats 3rd generation, one located upstairs and one downstairs. The upstairs one turns on the fan for hours despite no schedule setup and despite learning turned off. 
The only way I could stop the fan from continuing to run is by turning on the AC for like 10 seconds or 20 seconds then turn it off. The fan stops shortly after that, 

And one more thing, the fan is started daily at the same time looks like. I go to bed around 11:30 PM and find it is running around 2 or 3 AM.

there were few occasions depending on weather when it started in the evening and continued running till I stop it few hours later with the above technique

what’s the cause and solution? 

[chaconmorales]

Hi fahedz,

Thanks for reaching out to the Community. I am sorry that you are having issues with your Nest Thermostat features for fan control. But no need to worry, help is here.

Before we get started with further troubleshooting, let me ask you some questions

1. When did the issue start?
2. What other strange symptoms have you experienced since the problem started?
3. Does the thermostat display show any error codes or messages?
4. Do you notice any intervals or specific patterns between these strange behaviors?
5. When was the last maintenance check for your HVAC system?
6. Has there been any recent changes or upgrades to your HVAC system?
7. Have you checked the power readouts on the Nest thermostat?
8. Do you have a common ground wire or C wire?"
9. Are you able to share a picture of your wiring?

In the meantime, here are some steps you can try to troubleshoot the issue:

1. Ensure your thermostat is set to the correct mode (heat or cool).
2. Check for any advanced settings or schedules that might be causing conflicts. You can check the following guide to Troubleshoot strange heating or cooling behavior. 
3. Please check the following article to Restart or reset a Nest thermostat depending on your device’s model and make sure the correct process is followed.
4. Check that your system doesn't need maintenance.

Please review this guide for further details related to the Fan control with a Nest thermostat. If necessary, you can also check the following article to verify the Fan compatibility with Nest thermostats.

Please keep me posted. 

Best regards,

Mario

[fahedz]

Hi Mario

thanks for the reply. 

the issue started a little while after the installation of my thermostat so I disabled learning. It continued to happen.  It continues to happen

No other strange behaviors were noticed. 

it occurs almost around the same time, shortly before midnight or shortly after midnight. Pretty consistent and not random 

I noticed it is happening when the temperature of the room is higher than what we typically set the thermostat to when we turn AC on, so it seems as if like the thermostat is still learning and still trying to help cool down by turning in the fan 

last HVAC preseason tuneup was March or April, all went well. Next pre HVAC tuneup is next week 

no error messages on the thermostats

Not able to attach photo but here is the wiring 

Y1 cool

W1 Heat

G fan 

Rc power 

C power 

rest are empty 

[chaconmorales · Accepted solution]

Hi fahedz,

Thank you for following up and sharing more details about your issue including your wiring configuration. I understand your frustration with the device. I apologize for the inconvenience you're experiencing. 

Please make sure to verify the following settings on your Nest Learning Thermostat 3rd Generation as the issue is consistent and it starts malfunctioning at a specific time: 

1. Go to Settings > Nest Sense > Auto Schedule. Select No and then tap Done.
2. Go to Settings > Eco. Then, under Heat to and Cool to, select one and turn the dial until you see Off. Finally, tap Done.
3. Go to Settings > Home Away Assist. Select Stop Using.
4. Finally go to Settings > Reset > Schedule > Reset and turn the ring to reset.

Please keep in mind to verify the Thermostat safety temperatures that also can change your system functionality depending on your safety. 

You can check the following guide to change Temperature schedules for the Nest Learning Thermostat. Or the following article to Reset a Nest Learning Thermostat.

Please keep me posted. 

Best,

Mario

[fahedz]

Thank you. Did and will let you know if the problem persist 

[chaconmorales]

Hi fahedz,

Thanks for getting back to me!

Thank you for your best effort while following the troubleshooting steps and instructions to resolve the issue. 

I'll be here if you need anything else from our side. I'll be happy to answer any question you may have!

Regards,

Mario