The chimney effect, also known as the stack effect, is the tendency of heated air or gas to rise in a duct or other vertical passage, as in a chimney, small enclosure, or building, due to its lower density compared to the surrounding air or gas.

 

The presence a chimney enhances the effect for several reasons:

 

The lower effective section increases the air speed in the chimney; this lowers the pressure in the chimney, creating a greater difference in pressure with the rest of the interior space, thus accelerating the air movement (Venturi effect).

Being tall, the chimney increases the temperature difference between the air intake and exhaust points, increasing the Venturi effect.

Even a slight wind lowers the pressure at the chimney exhaust, and consequently increases the efficiency of air extraction.