ERD50
Give me a museum and I'll fill it. (Picasso) Give me a forum ...
The space under the furnace. The furnace sits on a "filter box" that is not a part of the furnace and holds a 4"x20"x25" pleated filter. The house slab is the floor of the space.
OK, but is the combustion (outside) air isolated from the house air ducts? You really don't want those two mixing. Let the cold outside air go to the combustion chamber, keep the treated air from the heat exchanger cycling through the house.
Otherwise, you are pulling outside air into your ducts, and/or pulling the flame outside the combustion chamber (the suction from the air duct return). But "open ceiling" and the duct open to the floor (inside that closet?) sounds like they are mixed?
Not ideal, and possibly dangerous.
-ERD50