I use a furnace filter (3M brand 2200 (merv 13) or even 2800 (merv 14)), and I tape smaller merv 13 filters on the outbound vents in the bedrooms. Those latter

I use a furnace filter (3M brand 2200 (merv 13) or even 2800 (merv 14)), and I tape smaller merv 13 filters on the outbound vents in the bedrooms. Those latter ones clearly need changing every couple of months as they become quite dark. So clearly the furnace filter is not sufficient, at least in my case.