Fire Blocking Furnace and Water Heaters

I have two questions. When the furnace is in the hall closet, what is the rule for fire blocking? Can sheetrock touch the B vent or is clearance observed and metal is used? Some I can see into the attic. Also, some of the closets have separate round flashings to the side of the unit that is an open passage into the attic. Is this for supply air and is it allowed?
Thanks so much for your assistance.