IF you have the model that has the 4 inch exterior, you need to twist the center exhaust pipe back and forth to release it. There is NO other attachment on the outside holding the furnace inside. NOW, some OEM's will use screws on the inside furnace angle bracket/s that mount to both the floor and outside wall. When you get the outside vent pulled loose and the furnace still does not pull inward, then they have used one or more of those mount brackets. Doug