OK, when you say the furnace inatakes and exhuasts outside, I think you
are refering to the combustion air. OK fine. You also need to look
at the air in the ducts that are in the house. If you have an open
return in the basement, it will suck air (not combustion air) into the
hot air system and expel it upstairs. It will naturally flow back down
through the doorway. Close off the all the rerun ducts and look for
hidden opens. My return ducts are just flat sheet metal over the 2x4s.
Sometimes they leave the ends open by mistake or whatever?