The Oatey® Sure-Vent® 1.5" - 2" 160 Branch, 24 Stack DFU Air Admittance Valve with ABS Schedule 40 adapter is a one-way valve designed to allow air into the plumbing drainage system when negative pressures develop in the piping system, eliminating the need to tie into a stack vent. Superior engineering along with stringent quality standards ensure unmatched performance as a secondary vent. Oatey products have earned the trust of plumbing professionals for over 100 years.