Prevents backflow - comes with full instructions. This Water Softener Air Gap is glued into the tap of the stand pipe Air Gap 3/8 inch - It enables a 3/8" connection with a air break. This fitting can be glued onto the top of the 1½" stand-pipe. Air Gap 1/2 inch - It enables a 1/2" connection with a air break. This fitting can be glued onto the top of the 1½" stand-pipe. When you connect the drain to the waste system you must protect the water softener and the water supply from backflow. When you connect the drain to the waste system you must protect the water softener and the water supply from backflow. The waste pipe is classed as a category 5 risk (sewage) and therefore it requires an air break. A non-return valve on its own is not enough., Weight: 0.110231131 pounds, Manufacturer: Harvey