Air gap
An air gap is an unobstructed vertical space between a water outlet and the flood level rim of the fixture or tank it fills. The standard minimum is twice the diameter of the outlet, and never less than 1 inch (25 mm); the gap increases when a nearby wall could affect it. Because nothing mechanical can fail, it is the most reliable backflow protection.
Updated 2026 · Researched from published standards, not professional or legal advice.
Air gap sizing basics (ASME A112.1.2 and model plumbing codes)
| Item | Requirement |
|---|---|
| Minimum gap | 2 times the effective outlet opening diameter |
| Absolute minimum | 1 inch (25 mm) |
| Outlet near a side wall | Larger gap required (3 times the opening in most tables) |
| Measured from | The end of the outlet to the flood level rim |
| Protects against | Backsiphonage and backpressure, any hazard level |
An air gap is the vertical, unobstructed open space between the end of a water supply outlet and the highest possible water level of the receiving vessel, known as the flood level rim. It is not a mechanical device at all; it is an arrangement of the piping itself. The classic example is a faucet discharging above a sink: even if the sink overflows, the water cannot climb through open air back into the spout.
The air gap is considered the gold standard of backflow protection because there is nothing to wear out, foul, stick, or fail. No pressure condition, whether siphon or push, can move contaminated water across a properly sized gap of open air. That is why air gaps are favored at the highest hazard connections, such as sewage-related equipment and chemical tanks, and why an air gap is often the benchmark other protection methods are measured against.
Testers and property owners should know the air gap's weaknesses are practical rather than mechanical. It breaks the pressure of the system, so anything downstream needs a tank and often a pump, and the gap itself can be quietly defeated in the field when someone attaches a hose or extension that bridges the open space. Surveys should verify that required gaps still exist, remain unobstructed, and have not been converted into direct connections.
Common questions
What is the recommended distance for an air gap?
At least twice the effective diameter of the supply outlet, and never less than 1 inch, measured vertically from the outlet to the flood level rim. Local codes can require more.
Does an air gap need to be tested?
It is not tested with gauges like an assembly, but inspectors check it visually. The common failure is someone extending the outlet with a hose or pipe, which removes the gap.
Sources
- ASME A112.1.2, Air Gaps in Plumbing Systems
- Model plumbing codes (IPC and UPC) air gap tables
- FDA Food Code, air gap provision for food establishments
- U.S. EPA, Cross-Connection Control Manual (EPA 816-R-03-002)
Related terms
| Cross-connection | Any actual or potential physical link between a potable water system and a source of contamination or pollution. |
| Degree of hazard | The classification of a cross-connection by the severity of harm its contaminant could cause, typically as a health hazard or a non-health hazard. |
| Backflow | The undesirable reversal of water flow in a piping system that can carry non-potable water or other substances back into the potable supply. |
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