I just bought a home that was built in 1940. It has an old oil furnace which employed separate return vents in each room of the home, all joined to one plenum box in the basement.

My questions are in whether or not there are any down-sides to this design and if it's worth converting to one centralized return vent between the two floors. The walls are plaster and there isn't much room (if any) in the apparently hardwood studded walls. The upstairs bedrooms are in a former attic, hence no space above them either.

My other question is in how to filter the air. I know about putting a filter rack on the furnace at the return air plenum, but what about at the return vents themselves? Their sizes are about the size of the supply vents.

Thanks for your input!