There are two possible reasons why tado° would start heating before the time set in your Smart Schedule:
You activated Early Start.
You activated Preheat Before Arrival.
When you activate Early Start, tado° ensures that it reaches and maintains the target temperature by the time set on the Smart Schedule. So, it starts heating your room earlier than the time set on your Smart Schedule.
To start heating at the right time, tado° takes many factors into account like current temperature, target temperature, your heating system characteristics, building insulation, and so on.
For instance, suppose there is a time block in your Smart Schedule that starts at 7:00 am with a target temperature of 21 °C, and you've activated Early Start. If the temperature at night is 18 °C, then tado° starts heating your home earlier to make sure that the room will be at 21 °C by 7:00 am.
Early Start has been designed for slow heating systems, like underfloor heating. If you have a radiator heating system, it usually heats up quickly, so you won’t need to activate Early Start.
You can deactivate this in the Smart Schedule settings (gear icon). When Early Start is deactivated, tado° only starts heating your room at the scheduled time.
Early Start is not available for the Smart AC Control.
Preheat Before Arrival
If you have Auto-Assist and Preheat Before Arrival is activated, tado° may start heating even when it is in Away Mode.
To start heating at the right time, tado° takes many factors into account like current temperature, target temperature, your Preheat Before Arrival setting, your heating system characteristics, building insulation, and so on.
You can set Preheat Before Arrival to ‘Off’ using the slider in the Away tab of the Smart Schedule. When Preheat Before Arrival is off, tado° maintains the Minimum Away Temperature while in Away Mode and starts heating only when it is back in Home Mode.
You can choose to turn off Preheat Before Arrival for some or all of your rooms.
As it takes very little time to heat or cool via your heat pump or air conditioner, preheat/pre-cool before arrival is not available for the Smart AC Control.
Please note: if a Home member has manually set tado° to Away or Home Mode, automatic Geofencing and Preheat Before Arrival will be deactivated. To reactivate automatic Geofencing and Preheat Before Arrival, you need to switch tado° back to Auto.