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?
Mark