Natural Draft 1 : A current of air produced by the difference in the weight of a column of flue gases inside a chimney and a corresponding column of air of equal dimensions outside the chimney or vent.
Ralph W. Burklin & Robert G. Purington, Fire Terms: A Guide to Their Meaning and Use, National Protection Association, Boston, MA 1980 USA. (Guide: Meaning and Use, Nat Prot Association Fr Srvc - Amer) {457}