The function of a chimney or an exhaust pipe is based on the chimney effect. The heated air from a fireplace and hot exhaust gases expand, have less density and are therefore lighter than the surrounding air. The light air climbs up and go out through the chimney.
The flow of exhaust gases creates a negative pressure in the chimney, which prevents the flue gases from entering the living area from the fireplace. To compensate the negative pressure, colder combustion air flows into the fireplace and supplies the fire with fresh oxygen.
Depending on the height and diameter of the chimney as well as on the temperature and density difference between exhaust gas and combustion air temperature, the chimney effect will be stronger or weaker.
Therefore, a chimney must always be optimally adapted to the quantity, speed and temperature of the exhaust gases to be extracted ("design" or "cross-section calculation")