[HayZee518]

natural flow through a chimney or "flue" is afforded due to the fact that heat rises. burnt gases are lighter than air so naturally float upward. because of the fact that you have sealed off the wall thimble, there is still a backwards effect of air flow which is make-up air being drawn into your basement through whatever leaks you have in your trash [ash door] if you put a roof cap on the unused flue and seal around it and gasket your ash door then it would be as if it wasn't there to begin with and you shouldn't get a reverse flow