On Sunday, 11 March 2018 10:58:18 UTC, Brian Gaff wrote:
Which was my point. They are not really very good, and of course they rely
on the crud being able to be electrostatically charged or magnetised to
work. Stuff which has neigher quality will just go straight on.
Brian

The crud is iron oxide from radiators. It is magnetic.
If you have an aluminium alloy boiler heat exchanger, it's vital to keep it out.