I recently moved into a home with a Rheem Imperial 90 Plus (RGEB) furnace. In the Maintenance section of the User's Manual it states that I need to clean the "upflow filter" that goes below the blower. Problem is, there is no filter installed! Moreover, I don't see how I could even get a filter into the space; there wouldn't be enough clearance between the top of the nominally 1" thick filter and the bottom of the blower housing. I assume the 16x25x1 filter is supposed to sit on the edges of the sheet metal that forms the opening, as the opening itself is only 15 x 23.5. Do you suppose the blower was replaced at some point and the replacement blower was larger (taller) than the original? What can I do? I think it will be bad practice to leave the unit unfiltered.