Boiler draught is the pressure difference between the atmosphere and the pressure inside the boiler. Generally a modern coal fired boiler has balanced draught. Draught is maintained inside boilers using fans.
Induced draught — This is produced by the help of an induced draught fan, it is installed before the chimney and it maintains negative pressure inside the furnace, the disadvantage is the air ingression inside the furnace due to improper sealing, thus leading to losses.
Forced draught — It is produced by the help of a forced draught fan installed before the furnace, air blown inside the furnace, thus producing positive pressure inside furnace, disadvantage is that hot flue gases will tend to leak into atmosphere and flames come out from the inspection holes in the furnace.
Balanced draught — It has both ID as well as FD fan and the draught or pressure difference produced is balanced or almost equal to atmospheric pressure.