Smoke will rise up the chimney or through the nearest opening it can find because it is hotter and, therefore, lighter than the air in the room.
Before the days of chimneys smoke was allowed to escape through vents or open turrets in the roof. Chimneys were introduced to induce a draught, thus providing more air for the fire. The hot smoke passing up the shaft made room for cool air, of higher density.
Thus a chimney would not only carry away the smoke and gases from a fire but also act as a ventilator enabling a change of air in the room.
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