I was changing the filters in my grandfather's HVAC and noticed that despite the fact the filters are changed regularly things were very dusty. I looked up and noticed that the part of the unit that goes in to the return vents opens into an area that mostly plaster, lath and asbestos rock. The outside of the return air vents in the other rooms have to be dusted almost every three days due to the amount of dust. Now I've rehabed my older similarly aged home before but the HVAC systems are something I've always contracted out. So my question is-is that kind of set up normal? I guess I made the assumption that it was probably code to have the kind of thing sealed off before venting into that area due to the asbestos rock. For reference my grandpa's home is a 130 year-old American Four Square and the HVAC is only set up to cool/heat the downstairs. The HVAC is in the closet of one room and the closet exterior walls where the return air duct work sits touch the interior walls of the three main rooms that make up the rest of that part of the house. The entire HVAC system was replaced and installed new about 7 years ago.