thanks ChrisR for this. And Wilhelm is right, if I were mad enough to ring a manufacturer's "help" line, they would suggest replacing the HX
ChrisR said:
Best stuff is fernox DS-3 (sulphamic acid) for limescale, though citric (DS-40 works too and would be better on the primary side.
It can be very hard to get all the limescale out though.
Looking a the diag it's an easy one to remove
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