To factory reset a Room Thermostat:
On the back of the Room Thermostat behind the wall mounting plate, there is a row of 8 white DIP switches.
Put switch 1 (the left most switch), to the UP (on) position.
Rotate the dial on the front of the Room Thermostat, until 'P' is displayed on the LCD screen.
Press the dial twice in succession.
The LCD screen will read 'PP'.
The Room Thermostat is now in a factory reset default state, to add it back onto the system again see this link.
Please note that once the device is included, it must be assigned to a room by going to the 'setup' page, selecting the relevant room, and then pressing 'Add a device'. You can then choose the new device and press OK to assign it to the room.
You just need to add the device again as if it is a brand new device to the 'Whole House'