A convection heating system for vacuum furnaces is described. The convection heating system including a hot zone enclosure defining a hot zone, a plurality of gas injection nozzles for injecting a cooling gas into the heat treatment zone of furnace, an exit port for permitting discharge of the cooling gas from the hot zone, and means for circulating a process gas within the hot zone to provide convective heating and cooling. Each gas injection nozzle has a flap disposed pivotally supported therein for substantially preventing the escape of heat from the hot zone during a heating cycle, but for permitting the injection of the cooling gas into the furnace hot zone during a cooling cycle. The gas exit port includes a flap pivotally mounted therein for impeding the unforced outward flow of a gas from the heat treatment zone during a heating cycle