Natural Draft: This is obtained by the "stack effect" or (natural draft)of a heated chimney, in which the flue gas is less dense than the ambient(outside)air surrounding the boiler. The more dense column of ambient air forces combustion air into and through the boiler. The other method(induced draft)is by simply using an induced draft fan (ID fan) which sucks flue gases out of the furnace and up the stack. Almost all induced draft furnaces have a negative pressure.
