Fan runs off and on even when system is off

[Lindsey]

My fan runs on and off at random times even when the nest system is off. It runs with both the heat and cool mode. My nest thermostat doesn't register when the fan is on either. The fan just runs by itself at random times. Please help! Thanks.

[Ashepherdson]

Do you have anything attached to your HVAC system that would call for the fan to run?  For example I have a whole home humidifier that has a fan control and can turn the HVAC fan on without the nest "knowing" it 

[Lindsey]

This is a picture of my thermostat wiring 

[Lindsey]

No I do not. But that was a good thought! Could it be the wiring?

[Ashepherdson]

I mean, yes it could possibly be the wires (https://support.google.com/googlenest/answer/9235232?hl=en-CA#zippy=%2Cmake-sure-all-thermostat-wire...) 

Can you confirm that you don't have the fan set to circulate air ?  Settings, Fan schedule (should be off)

[Lindsey]

Confirmed. The fan schedule is off. 

[Ashepherdson]

AH .. I skipped by the obvious ... was looking for a Zebra while it was a horse all along... 

You dont have a C wire... not having a C wire can cause some strange behaviour with nest thermostats, as they only get power when the system is "running".. I think it it a way for the nest save itself by drawing power , turning on the fan will put power back into the thermostat.. 

So if you are looking to fix this, adding the power adapter will be the way to go, 

https://store.google.com/product/nest_power_connector?hl=en-GB

[Lindsey]

okay I hope this works. I will try, thanks.

[Ashepherdson]

It should work, this issue has been around for a while, even looking at this posting from 2019 on reddit

https://www.reddit.com/r/Nest/comments/9jx21f/nest_thermostat_turning_fan_on_randomly/

[Lindsey]

The fan even runs when the faceplate of the nest is taken off.  Do you think this is still a c wire issue? I'm hesitant to install only to find out its my furnace. 

[Ashepherdson · Accepted solution]

Okay that changes everything, I am going to take back my suggestion.  If the fan is running when the nest isn't even connected to the base then there is something going on at the furnace itself that is causing this.  I think I'm going to have to suggest that you get an HVAC specialist to come and take a look at your furnace as it could be a faulty relay, board, or any number of things. 

[Lindsey]

Thank you for your suggestion. I had an HVAC specialist come in and he replaced the control panel. Everything is working fine now. I appreciate your help!

[Jake]

Hey all,

It has been a few days since the last reply, and I am going to lock the thread at this time. If you have any questions, please feel free to create a new post. 

Best regards,
Jake