"TheScullster" wrote in message
...
If it's a combi, aren't the heating and DHW sides separate?
How does the internal state of the CH system affect the DHW side of the
boiler heat exchanger?
Sorry if this doesn't answer or clarify the original question.

I have the same issue of "magnetite" (as a visiting plumber called it).
Looks a bit like fine(ish) coal fragments.
My system is 20 years old though and has suffered pump over problems etc.
It is not clear from the OP whether the CH side of his system is sealed or
open.

Fit a Magnaclean or the Fernox version on the return top the boiler. That
will get rid of it.