In my moms house, there is a large return grill in the ceiling at the
top of the
stairs with a 16x20 filter that feeds directly into a large return
plenum. The filter is mounted just above the grill, i.e the grill
holds the filter in place. Just above the filter is a large plenum
that feeds to the furnace return.

Problem is all the individual return ducts from the various rooms all
converge in this same large duct. Therefore the return air from the
individual rooms does NOT pass through the filter. Only the return
air from the main grill passes through the filter.

Am I missing something or was this how the installers built it?

Is the filter supposed to be installed somewhere up in the plenum and
not right at the intake grill or ????

I looked for but did not find any other filter at the furnace or
elsewhere in the system.

thanks

Mark