One of them Polyphospate doodahs.
They do work, though an ex BG teckie told us they had been known to cause blockages.
Very popular in Epsom where the water is very hard.
Sorry, those grey cells must have died. Going back about 7 years.
Precipitation maybe a problem. Nowhere internal and hot cos the crystals would dissolve. CW flow switch maybe. Worcs Junior ones are ok though (though the h/e blocks completely with limescale in Epsom if you don't use one!)
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