You may have noticed that your ecobee thermostat sometimes behaves in one or more of the following ways while calling for heating or cooling:
 

• The thermostat screen periodically goes completely black, or powers off and then on again, often with a delay of a few minutes before power is restored
• The thermostat screen frequently displays "Calibrating: Heating and Cooling Disabled", indicating the ecobee has regained power and is calibrating its temperature sensor
• The ecobee app or ecobee Web Portal frequently display a temperature value of -500F or "N/A", indicating temperature readings are invalid because the ecobee is calibrating its temperature sensor

All of these are symptoms of the ecobee thermostat frequently losing power on heating or cooling calls.
 

ecobee thermostats require a constant 24VAC power supply to operate. If this power supply is interrupted, the ecobee will not power back on until 24VAC is restored. When the ecobee thermostat loses power during heating or cooling, it is often due to one of the following:
 

• A safety feature on the HVAC (Heating, Ventilation, and Cooling) system triggering, cutting power to the thermostat
• For boiler and zone valve systems with multiple thermostats, the 24VAC transformer may not have a sufficient VA (volt-amperes) rating to handle multiple thermostats and zone valves calling for heat at once