Boiler draught may be defined as the small difference between the pressure of outside air and that of gases within a furnace or chimney at the grate level, which causes flow of air or hot flue gases to take place through boiler.

Steam jet draught takes place when a small portion of steam is passed through a jet or nozzle, pressure energy converted to kinetic energy and steam comes out with a high velocity, which carries a large amount of air or flue gases and makes if to flow through boiler.