Watts Brass & Tubular #9D-M3 1/2 1/2" Backflow Preventer 1/2" female pipe thread union inlet x 1/2" female pipe thread union outlet With intermediate atmospheric vent Typically installed on boiler feed lines to prevent backflow caused by back siphonage or back pressure For systems without antifreeze Rust inhibitors or other chemicals which will contaminate the potable water system