Abstract: Damage due to implosions occurring within the furnace chambers of balance draft vapor generators is prevented by the provision in the downstream combustion gas duct of one or more openings that are closed during normal operation by doors which are automatically released upon the occurrence of an inordinately high negative fluid pressure atmosphere in the furnace. The doors, when released, operate to obstruct the flow of combustion gas out of the furnace chamber and thereby, in association with operation of the forced draft fan, overcome the excessive negative pressure in the furnace chamber. Also, the exposure of the induced draft fan to outside air upon release of the door prevents the fan from producing an inordinate negative pressure adjacent the door that would otherwise tend to close the door and also detrimentally affect the furnace pressure.