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.