air to enter DWV systems to equalize pressure when water drains out of the system ; eliminate the need for high-priced roof penetrations and vent piping. 1ATD5 up to 500 DFUs on a stack ; 1ATD6 vents up to 160 DFUs on a branch and up to 24 DFUs on a stack. 15W940 a similar cabon filter to reduce plumbing drainage system odors.