[rohrmann]

Your inside air is circulated through the heater which constantly adds heat to the inside air instead of trying to heat much colder outside air, and all that air must return to the furnace via those slots in the stair risers. For each series furnace, there is a minimum area of opening to allow that air to flow unrestricted so the maximum efficiency or the furnace is realized. This is a page from an Atwood furnace manual, and I believe the newer furnaces are labelled as Dometic who bought out Atwood, but still essentially the same furnace, and the lower part of the page shows the minimum amount of area required for these furnaces. Ours is an 85 series furnace, which I think most of them are, and for that series furnace, 65 square inches of area of an opening is required. I would not close off the area below the stairs.