A fuel-heated boiler must provide air to oxidize its fuel. Early boilers provided this stream of air, or draught, through the natural action of convection in a chimney connected to the exhaust of the combustion chamber. Since the heated flue gas is less dense than the ambient air surrounding the boiler, the flue gas rises in the chimney, pulling denser, fresh air into the combustion chamber.
a fuel-heated boiler must provide air to oxidize its fuel early boilers provided this stream of air or draught through the natural action of convection in a chimney connected to the exhaust of the combustion chamber since the heated flue gas is less dense than the ambient air surrounding the boiler the flue gas rises in the chimney pulling denser fresh air into the combustion chamber