What is the term for the draft created by the weight difference of flue gases and outside air?

The term for the draft created by the weight difference of flue gases and outside air is natural draft. This phenomenon occurs due to the difference in temperature between the hot gases produced by combustion inside a flue or chimney and the cooler outside air. As the flue gases rise, they create a lower pressure area in the flue, which in turn allows cooler outside air to be drawn in. This process relies solely on the natural buoyancy of the gases and the laws of physics concerning temperature and pressure without the aid of mechanical means.

In contrast, forced draft systems utilize fans to push air into a combustion area, while induced draft systems draw flue gases out of the combustion area using fans. Mechanical draft typically involves additional equipment that actively moves air or flue gases. Therefore, natural draft is specifically identified as the passive process depending solely on temperature differences, distinguishing it from the other methods that require mechanical assistance.