Abstract: In the heating chamber of the furnace are a number of heat-absorbing elements which serve as a thermal storage, absorbing some of the heat which is developed during the regular burning cycle and then releasing such heat for additional heating effect. Stored heat is available for additional and therefore more efficient heating. A control system is used to conserve heat by selectively and automatically opening and closing the stack to prevent heat loss in timed relation with the burning cycle and adapted to provide sufficient purge time. The stack valve opens automatically at the commencement of a burning cycle and thereafter automatically closes after a purge interval. In the event of a power failure, the stack valve automatically closes and reopens when power is restored prior to the commencement of any succeeding burning operation.