What’s an Air Gap?
Air gap fittings are plumbing code required to mount in the drain line (waste line) of water treatment equipment. Their primary purpose is to prevent non-potable water from flowing backwards, possibly mixing and contaminating potable (drinking) water. One could think of the function of an air gap fitting as a no-fail check valve but without an internal seat or any moving parts. Properly designed and installed, air gaps, like check valves, allow water to flow only in the desired direction.
The key characteristics for an air gap is protection from back flow and back siphonage. Back flow occurs when there is a clog or blockage downstream in the drain line which with more water entering causes the nonpotable water to back up, fully engulfing the air gap unit or, at least, reaching the back siphonage critical level (C/L) of the air gap unit. Back siphonage happens when there is suction on the potable water line and the air gap inlet that sucks non-potable water port backs into the potable (drinking) water line. The ideal air gap would provide protection from back flow and back siphonage.
A true air gap is a physical separation in the drain line that one could pass their hand through. All existing air gap fittings, when properly installed, will protect portable water supply against back siphonage water contamination.