Fired furnace operation and apparatus

In a fuel fired furnace having a heating section containing at least one burner for firing said furnace and an air plenum for admitting combustion air to said at least one burner, said furnace having a blower for forcing combustion air into an air duct which is operably connected between said blower and said plenum, said blower having a first power source, said duct having a long axis extending from said blower to said plenum, said duct having a normally closed drop door that can be opened to admit ambient air into said duct, the improvement comprising at least one ventilator operably connected to said duct downstream of both said blower and said drop door and upstream of said plenum to force ambient air into said duct toward said plenum, said ventilator having a second power source, said ventilator having a movable close-off plate that is normally closed but which can be opened when said ventilator is in operation to admit ambient air to said ventilator and by way of said ventilator to said duct, said second power source being different from said first power source so that said ventilator is capable of operation independent of said blower and said first power source.