i don't know how often it happens but i have fitted 3 so far and had one stick in the on/dhw demand potsion. asked about it when i was at telford last month.
they said that they have changed the design as the old one could stick if muck or grit got in it. the one i had was of the new design so god knows if the design change helped at all!!
good thing i guess is that it is a easy to take out and give it a good shake and put water thought it(no pun intended)
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