Draft

Q. What function does draft play in the combustion of fuel?
A. Draft provides the differential  pressure in a furnace to ensure the flow of the gases. Without draft, stagnation in the burning process would result and the fire or process of combustion would die from lack of air. Draft pushes or pulls air and the resultant flue gas through a boiler and up into the stack. The draft overcome the resistance to flow of the tubes, furnace walls, baffles, dampers and the chimney lining.

Natural  Draft
Natural draft is produced by the chimney into which the boiler exhausts. The cool air admit to the furnace, rushes in to displace the lighter hot gases in the furnace. Thus the hot gases rise (chimney effect), causing the natural draft.

Mechanical Draft
Mechanical draft  is produced artificially by mean of forced or induced draft fans. The chimney is still necessary on mechanical draft installation for venting the produced of combustion high enough not to be offensive to the surroundings.