Nest Learning Thermostat (3rd Generation) A/C will not turn off

[kpcbills]

On multiple occasions, the A/C will not turn off, regardless of the settings on the Nest device.  The display can show "off", "eco" mode or a temperature that is higher than the actual room temperature, but the fan will not turn off. 

We have three circuits to support the HVAC system: one for the upstairs zone, one for the downstairs zone and another for a ZTech Smart Vent.  I have to reset all three breakers for the fan to turn off and for the system to reboot.  This is very frustrating because we are spending a bunch of money on electricity but also because the house is unnecessarily freezing.  

We have the nest learning thermostat (gen 3).   Please propose a solution.  Thanks. 

[CoolingWizard]

@kpcbills ,

When this will not shut off problem occurs, did you go outside and verify the outdoor unit was still running or are you only describing the indoor blower fan that is not turning off?

AC Cooling Wizard

[kpcbills]

Thanks for your response.  Yes, the outdoor unit also remains on when this happens.  

[kpcbills]

@CoolingWizard does this give you any ideas by chance?  

[CoolingWizard]

@kpcbills ,

Sometimes the electrical contractor remains stuck. To best identify this, you need to be at the air handler and and be measuring the AC voltage on the Y wire and the C terminal.  When the thermostat is satisfied, the thermostat should disconnect the R internally from the Y.  If you see the power go to near zero, and the system stays on, this will indicate a electrical problem win the control board.  If the power on the Y wire remains, it is a thermostat issue.  

AC Cooling Wizard

[kpcbills]

Thank you for the additional information.  Will try to see this all the way through when I spot the problem happening again.