A B vent pipe does not actually belong to any particular class or a specific category. They are simply classified as B vent pipes. The specific appliances that are to be vented are classified into various classes or categories, however. B vent pipes are typically used in the venting of category 1 appliances. Generally speaking, in fact, a B vent pipe is only allowed to be used on a category 1 appliance. In some unusual circumstances, though, the manufacturer of a particular appliance may have certified the use of a specific B vent system as an alternative venting system for that particular appliance. This is not a common occurrence.