-
Usually installing a filter means a drain down and then either a power flush or run some cleaner in the system for a bit and then clean filter a few times and see what comes out.
A power flush is just the pump being removed and the system circulation a solution to clean the system and then back to a larger filter system. That sometimes doesn't help so removing rads and flushing them out.
How would a filter close to the boiler get gunk that is not returning up the side of the house?