The chimney effect, also referred to as the stack effect, is the ventilation in buildings and flue gas stacks or chimneys that results from thermal differences between indoor and outside temperature as well as from the form and height of the structure which conducts the updraft.
Since buildings are not totally sealed (at the very minimum, there is always a ground level entrance), the chimney effect will cause air infiltration. During the heating season, the warmer indoor air rises up through the building and escapes at the top either through open windows, ventilation openings, or leakage. The rising warm air reduces the pressure in the base of the building, forcing cold air to infiltrate through either open doors, windows, or other openings and leakage. During the cooling season, the chimney effect is reversed.
The chimney effect in industrial flue gas stacks is similar to that in buildings, except that it involves hot flue gases having large temperature differences with the ambient outside air. Furthermore, an industrial flue gas stack typically provides little obstruction for the flue gas along its length.
The two main processes driving the chimney effect are convection and the Venturi effect.
The draft or draught flow rate induced by the chimney effect can be calculated with the equation presented below. The equation applies only to buildings where air is both inside and outside the buildings. For buildings with one or two floors, h is the height of the building and A is the flow area of the openings. For multi-floor, high-rise buildings, A is the flow area of the openings and h is the distance from the openings at the neutral pressure level (NPL) of the building to either the topmost openings or the lowest openings. Reference explains how the NPL affects the chimney/stack effect in high-rise buildings.
