Replaces any secondary plumbing vent protecting the traps of kitchen sinks, bathrooms, basins, tubs, showers, washing machines, etc. Breaks siphoning action caused by draining water. The differential created by draining water automaticallyopens sealing disc and as drain flow stops, spring mechanism automatically closes sealing disc to keep water seal in trap constant and prevent sewer gas from escaping drain. MUST be located at least 6" above trap arm top. 13/16 thread.