Chimneys operate on the principle that hot air increase above cold air. The movement of hot gases orient from the fire creates a pressure difference between the inside of the flue and the room. It is called a “draught” and it forces air into the fireplace, These air feeds the flames as it rushes past the fire. The hotter the hearth , the faster the air increase and the better the chimney works. The motion of air through the chimney is helped by the fact that air in the house is warmer than the air outside.
If the chimney is not working perfectly then sufficient draught will not be generated inside the flue and subsequently not enough air will be sucked into the fire for it to burn perfectly.
Though a chimney appears to ‘suck’, & smoke appears to naturally rise, it is more accurate to think of the weight of thick outside air pushing down to force air into the building then push the warmer , lighter, waste gases out up the stack, somewhat like an bubble rises in water.
This tiny difference in pressure between inside & outside the flue is what makes a chimney function. Predicting the standard of flue draught for a chimney is extremely difficult and requires experience and a degree of technical knowledge.
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