I see far too many videos from gas engineers (and gas trainers) cleaning boiler heat exchangers with a drill and nylon brush.
This is NOT the way to do it. Manufacturers' instructions specifically state not to. It scratches the heat exchanger which will eventually cause corrosion.

I use water and a Viessmann cleaning too for cleaning between the grooves. For very dirty heat exchangers I use a specific combustion side cleaner that I bought from Canada, Axi-Therm 'Clean F Steel' for stainless steel heat exchangers.

https://viessmanndirect.co.uk/Catalog...