The idea of the heater is to get the engine up to normal operating temperature as quickly as possible and allow warm/hot air from the heater units in the car. Due to the size of the cooling/heating system and these efficient engines producing little heat they need the help of the booster heater to achieve this. Replacing the thermo switch with your own switch wired inside the car is a good idea and has been done by members and stops the heater firing up for short journeys which is not good for these heaters.
Fuse 12 is the one to remove to disable the booster.
