I'm unfamiliar with modern thermostatic taps, but if they work on the bi-metallic strip principle, there are very small movements indeed (by the bi-metallic strip) actuating other bits. In my limited experience, hard water eventually builds scale deposits sufficient to prevent these small movements. I had a Mira shower operating on this principle in SE England where the water is so hard it's barely liquid. It had to receive 6-monthly dismantling/ descaling to work correctly.
Colin
January 2010
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