The Oatey® Sure-Vent® 1.5" 20 Branch, 8 Stack DFU Air Admittance Valve with PVC Sched. 40 Adapter is a one-way valve designed to allow air into the plumbing drainage system when negative pressures develop in the piping system. 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.