What is the significance of draft in boiler combustion ?

Draft or draught is an important factor in the combustion of fuel in the boiler.  Draft refers to the difference in pressure in the boiler furnace to the pressure on the top of chimney.

This pressure difference is necessary for the flow of fresh air into the boiler and for the removal of flue gases out of the boiler.  The draft should be optimal.  The draft has a direct influence on the fuel/air mixture in the furnace.  A higher draft will result in more air being sucked into the furnace.  This will result in higher combustion. 

A lower draft will not be able to remove the flue gases properly from the furnace.  This will result in less air entering the furnace.  This, in turn, will result in incomplete combustion which affects the efficiency.  Incomplete combustion will also result in more pollution.