The stack or flue effect occurs when the outdoor temperature is lower than the indoor temperature. The warm indoor air is lighter than cooler outside air.
The air flow can be estimated as
Q = 9.4 A (h (tr - to))1/2 (1)
Q = rate of air flow (ft3/min)
A = free area of inlets and outlets - assumed equal (ft2)
h = height from inlets to outlets (ft)
tr = average room temperature at height h (oF)
to = outdoor temperature (oF)
Flow of Air in Natural-Draft Flues
Systems for ventilation and air handling - air change rates, ducts and pressure drops, charts and diagrams and more.
Online calculator, figures and tables showing density, specific weight and thermal expansion coefficients of air at temperatures ranging -100 to 1600 °C (-140 to 2900 °F) at atmospheric and higher pressure - Imperial and SI Units.
Air flow - volume and velocity - due to stack or flue effect caused by indoor hot and outdoor cold temperature difference.