It happens that Dave Plowman (News) formulated :
The obsession with power flushing etc on a system which worked OK before
is merely a way of hyping up the costs.

That was what my installer said and the way I too see it. There is
always be some sediment in a system and some magnetite, no matter how
frequently it is flushed out. Regular flushing and refilling will cause
its own problems, in as much as the fresh water will have some air and
oxygen in it initial+ly, which can cause some rusting until all the
oxygen and air is out.

In the 30 plus years since the system was installed, I have never
suffered a blockage. Poor flow to some ads due to lack of balance, but
never a blockage or any sort of dirt come out of the system.