This is a very dangerous condition. The gases emitted by a furnace are supposed to go up the flue or vent pipe. When the internals are cracked (heat exchanger) they go right into your duct line and they ar blown all over your house.
Another thing we have found is the combustion air. The flames in your furnace need oxygen to burn. we have found that several furnaces are pulling the air from inside the home and not the attic. This is a pretty quick fix. let us know!
If you have a 100,00 BTU furnace, the Code says you need at least 25 square inches of combustion air vent. on a 40,000 BTU water heater, you need at least 20 square inches.
Call me today if you think you have a problem.