I'll just add to this. A high efficiency furnace or water heater passes the hot gas through the heat exchangers more than once. That is why they are called high effeciancy. A "high efficiency" boiler or tankless heater do not do this. They modulate the flame this is where they get that "high efficiency" type status. These are different systems and different venting needs. PP or cpvc can supposedly vent them, but again we'll see on that.True, this is why power-vented heaters have plastic blower housings, direct vented condensing units do not dilute exhaust.