If you have already set up your installation via the Home + Control app, the number is available in the app settings: select "Manage my home", select the room where the thermostat is installed and select the thermostat. You will find the serial number there. Otherwise, follow these steps to find the serial number of your thermostat: 1) Turn off your thermostat directly from your electrical panel. 2) Take a screwdriver and insert it into the hole located under the thermostat. Gently press to remove the front panel of your thermostat. 3) On the back, you will find the serial number starting with 00:03:50.