Backwater Valves are installed on the drain line of your plumbing system to prevent back flow. It works because of an interior flap that allows water to exit the home and closes to prevent it from re-entering it. Backwater valves shouldn't be confused with backflow preventers. The first prevents raw sewage from entering your home, the second protects your potable water source from being contaminated by reverse flow of dirty water. Canarsee has different PVC Backwater Valves that fit the most popular tubing sizes. Use it with PVC tubing only.