blank off plate facing by a steel band 84 laid down the center of the double tadpole and fastened by bolts 86 the closure device 34 can be seen in fig7 a threaded shaft 90 is supported by shaft enclosure 92 in the flue 14 wall or the outlet duct wall 116 a foot 94 is attached to the internal end of the shaft 90 and a handle 96 is located on the external portion of the threaded shaft which also serves orientation purposes the foot 94 pulls against the blank off plate to provide positive closure or locking in the same fashion the foot 94 may pull against a tab not shown on the top of the blank off plate for closure this tab may then be used as a pushing surface by the foot 94 to assist in opening the blank off plate a tightening nut 91 on the threaded shaft 90 is used to tighten the foot 94 against the blank off plate thereby compressing the tadpole gasket 82 the tightening nut may be a spinner hex nut or other form of tightening nut also shown are nylon bearings 93 at the ends of the shaft enclosure 92 for interfacing between the shaft enclosure 92 and the threaded shaft 90 a gasket 95 may also be used to minimize flue gas leakage as the flue gas duct 14 narrows.