The Oatey® Sure-Vent® 1.5" – 2" 160 Branch, 24 Stack DFU Air Admittance Value with PVC Schedule 40 adapter's one-way valve designed to allow air to enter the plumbing draining system when negative pressures develop in the piping system. Superior engineering along with stringent quality standards ensure unmatched performance. Oatey products have earned the trust of plumbing professionals for over 100 years.