1) A waste pipe may serve as a wet vent provided that, a) There's not too much hydraulic load on the wet vent-- 1.5-- pipe can not serve as wet vent on a toilet and max 2 fixture units. A 2-- pipe can serve a toilet and a maximum 4 fixture units draining into it. (Sinks, tubs, showers all have 1.5 fixtures units each).read more >>