The "furnace" is a boiler and with its bowerfull burner fan motor and a "tight"(er) house a portion of the air required to make up the combustion air for the boiler is being sucked down the adjacent chimney. One solution -lower cost- is to have a "frash air boot" system installed on the burner, this system provides a direct OUTSIDE air supply for the boiler and effectively stops the depresurization of the COMBUSTION AIR ZONE in this case the basement.