Flue gas stack draft (or draught)
The combustion flue gases inside the flue gas stacks are much hotter than the ambient outside air and therefore less dense than the ambient air. That causes the bottom of the vertical column of hot flue gas to have a lower pressure than the pressure at the bottom of a corresponding column of outside air.
That higher pressure outside the chimney is the driving force that moves the required combustion air into the combustion zone and also moves the flue gas up and out of the chimney. That movement or flow of combustion air and flue gas is called “natural draft (or draught)”, “natural ventilation”, “chimney effect”, or “stack effect”. The taller the stack, the more draft (or draught) is created