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 in which the thermostat is installed and select the thermostat. You will find the serial number there. Otherwise, follow these steps to retrieve the serial number of your thermostat: 1) Turn off your thermostat directly in your electrical panel. 2) Take a screwdriver and insert it into the hole located on the underside of the thermostat. Press gently to remove the front face of your thermostat. 3) On the back of it, you will find the serial number starting with 00:03:50