TDD beat me to the reply. Another possibility of flue gas not venting
properly even with the vent pipe clean and intact is that the room is in
a negative pressure and the gases are being sucked out of the draft
diverter. The air going up the vent pipe has to be replaced by outside
air somehow. Usually, especially with a small furnace like this one,
enough air leakage occurs naturally. However, a fireplace or anything
else that pulls air out of the house can cause a negative. Larry