Eheh, the hot water is actually fluctuating and the pipes need time to get to the full temperature.
The amount of cleanliness depends also on the flow of water, the more water you manage to keep flowing, the faster you clean :)
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Ah, after reading your comment I tried again and finally managed to beat the game. What I was missing is that you can turn the knobs more than by 1/8th of a turn (which is what you get when you drag the "slider" down/up), and can actually do a 1/4th of a turn (sliding twice). It's not obvious as is (at least on PC, might be more intuitive on mobile).