Appreciate you advices! Will try it with a Scotch-Brite. Unfortunately I saw too late that I didn't remove all of this nasty plastic stuff from the heater so there came some of it into the bowl and built partially a thin plastic film at the inside. Not so easy to clean but will do my very best. @Alexis, I think my controller is a normal one. It isn't turning by itself for sure. But it happened a few times that something got in soft touch with the dial and it moved up or down. Would be really nice if you would remember to speak about with Sebastien. Best whishes and thanks to him.