Cross-Connection Air Gap (IPC 608.15.1)
The minimum air gap for the most positive backflow protection there is -- the physical gap between a supply outlet and the flood-level rim, which nothing mechanical can defeat. IPC 608.15.1: the gap is twice the effective opening diameter but never less than 1 in, and three times the opening within three diameters of a wall. A 1 in opening needs a 2 in gap (3 in near a wall); a 1/2 in opening computes 1 in where the absolute minimum governs. Checks a measured installed gap against the requirement. For indirect-waste and tank-fill protection. A design aid, not a substitute for the plumbing code adopted by your AHJ.
Formula and source
air_gap = max(2 x opening, 1); air_gap_wall = max(3 x opening, 1); required = near_wall ? air_gap_wall : air_gap; passes = measured >= required.
IPC 2021 ยง608.15.1 (air gaps) and ASME A112.1.2 (Air Gaps in Plumbing Systems) by name.
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