1½" (1.1/2") TeeVent domestic appliance connector with Non-Return Valve for use with WM4 trap Size: 1½" (1.1/2") Please Note: For use with WM4 Trap. The Tee Vent, which incorporates a Non-Return Valve, provides an air break and enables the waste system to be sealed. Should a blockage occur, the Non-Return Valve prevents flooding. Supplied with Blank Plug.