Air gap (backflow prevention)
Air gap is a physical vertical separation between a water outlet and the highest possible flood level. Backflow protection method under AS/NZS 3500.1.
Ask Chalkline about this →An air gap in residential plumbing is a physical vertical separation between a water outlet and the highest possible flood level (HPFL) of the receiving vessel, used to prevent backflow of contaminated water from the vessel into the supply line. It is one of the backflow prevention methods listed in AS/NZS 3500.1:2021 (Plumbing and drainage, Part 1: Water services). An air gap is mechanism-free: as long as the physical gap exists and is unobstructed, backflow cannot occur (verified 2026-05-16).
The principle is simple: if the water supply pipe ends well above the rim of the sink, bath, dishwasher tray or other receiving vessel, no siphon or pressure event can pull contaminated water from the vessel back into the pipe. Compare with mechanical backflow preventers (RPZ, double-check valves) that achieve the same outcome through mechanical valves but introduce a moving part that can fail.
Common residential applications:
| Fixture | Air gap requirement (typical) |
|---|---|
| Kitchen sink tap | The tap spout must terminate above the sink rim (the highest flood level the sink can reach) |
| Bath tap | Tap above bath rim |
| Dishwasher | Discharge hose looped above the rim of the cabinet or into an air-gap fitting |
| Washing machine | Discharge hose into a laundry tub or stand-pipe at the proper height |
| Garden hose tap | If a hose is attached and might be left submerged in a bucket or pool, an air gap or hose-tap vacuum breaker is required |
| Cold-water tank | Inlet pipe ends above tank’s overflow level |
Minimum gap dimensions under AS/NZS 3500.1:
- Commonly 25 mm, or twice the diameter of the supply pipe, whichever is greater.
- The gap is measured vertically from the lowest point of the outlet to the highest flood level of the receiving vessel.
- The gap must remain unobstructed: no flexible hose extending down into the vessel, no kitchen-sink rinser that can fall into the sink and be submerged.
Common defects on a residential plumb-up:
- Kitchen sink rinser that hangs in the sink without a proper return; potentially submerged in dishwater.
- Dishwasher discharge hose run direct to the waste under the sink without an air-gap loop or fitting; if the drain blocks, dishwasher water can be siphoned back.
- Washing machine discharge into a tub set lower than the machine; flood-back risk.
- Garden tap fitted with a permanent hose connection without a vacuum breaker on a non-test-cock layout.
When the air gap method does not apply:
- High-hazard applications (e.g. dental chair, medical equipment) need mechanical backflow prevention with annual testing, not air gap.
- Where space is constrained and an air gap cannot be maintained, a double-check valve assembly or RPZ device is installed instead.
Also known as: air break; physical separation; backflow air gap.
Category: Plumbing.
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Last updated: 2026-05-16. Verified: 2026-05-16. Quarterly review for currency.