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When a vent is less than 10 feet high, the vent should be at least as large as the flue collar outlet.b) Vents for draft hoods or flue collars 12 inches in diameter or less should NOT be reduced more than one pipe size.
If the toilet is going to be Wet Vented the portion of the pipe that is acting as both a drain and a vent must be at least 2 in size but also depends on the number of fixtures(NPC - Table 2.5.2.1.) For a standard 4 piece bath a 2 wet vent will suffice.