Natural Draft 2 : Draft produced by the difference in the weight of a column of flue gases within a chimney or vent and a corresponding column of air of equal dimension outside the chimney or vent.
NFPA 97 Standard Glossary of Terms Relating to Chimneys, Vents, and Heat-Producing Appliances, 2003 ed. http://travelwidpinx.info/nfpa-97-standard-glossar y-of-terms-relating-to-chimneys-vents-and-heat-pro ducing-appliances-2003-edition.html, (Std: Glossary of Terms Relating to Chimneys, Vent, and Heat-Producing Appl - Amer) {241 NFC}