Thanks for the opportunity to explain this. Gas furnaces are very efficient today. That means that the furnace takes most of the heat out of the gases before they go up the chimney - therefore they are comparatively cool gases around 300F or so. A wood stove can not be made that efficiently because wood (and coal) burning produces fly ash that will clog up small passages inside a furnace. Therefore, the gases going up the chimney are much hotter.
For these reasons, the National Fire Protection Association makes suggestions that are followed by the heating industry. They suggest (and their suggestions in effect become law) that you not heat aluminum gas vent beyond its duty rating by using it for wood fires. Aluminum-sleeved bvent for gas use is slid together in sections and has places for air to enter and leave at each junction. Smoke and fire from wood fires can pass out of the loose joints.
The task is not to seal the air gaps in the bvent to continue using it for wood; but to follow the local codes, which are the laws that affect your safety and insurance payments - by not using gas chimneys for wood fires. You will need to make sure your chimney is adequate for wood fires, which, today means getting stainless-steel all-fuel chimney or a wood-rated masonry chimney. DO NOT USE A GAS CHIMNEY FOR WOOD.