Short definition
A plumbing air gap is the vertical separation between a faucet or supply outlet and the flood-rim level of the basin below it. Code requires the gap to be at least twice the pipe inside diameter, and never less than one inch. Because it’s geometric — empty air between two pipes — it can’t be defeated by pressure inversion the way a mechanical valve can. It’s the most reliable form of backflow protection.
What it is
If a kitchen sink fills past its rim while connected to a faucet whose spout dips below the water surface, and the supply line then loses pressure (a utility main break upstream, for example), the dirty sink water can siphon back into the supply through the spout. An air gap prevents this by ensuring the spout can never sit below the water — there’s always unobstructed atmosphere between the outlet and the flood rim.
The geometry rule: the gap must be at least 2× the inside diameter of the supply outlet, and never less than 25 mm (1 inch). This shows up at every code-compliant kitchen and bathroom faucet (the spout must rise above the basin rim by the air-gap distance), at every dishwasher (a countertop air-gap fitting or, where allowed, a high-loop on the discharge hose), and at the atmospheric vent ports of certain backflow assemblies.
Standards include ASME A112.1.2 and ASSE 1001; the UK Water Regulations classify air-gap configurations as types AA, AB, AD, AG, AUK1–3, and DC, each suited to a different hazard category.
Why it matters to a homeowner
The air gap is what protects your drinking water from being contaminated by your own fixtures. It’s almost invisible when designed in — the spout is just where a normal faucet spout sits — but it’s load-bearing for water safety. Three places to recognize it:
- Kitchen and bathroom faucets. The spout’s neutral position must clear the basin rim by the air-gap distance. Aftermarket spray-hose attachments that drop the outlet below the rim during use can defeat the gap.
- Dishwasher discharge. Either a small chrome-cap fitting on the countertop near the faucet (a true air gap) or a high-loop in the discharge hose where it rises above the disposal connection. WA jurisdictions vary on which is acceptable; some require both.
- Backflow assembly vents. RPZ assemblies discharge to atmosphere through an air-gap relief, which is why they can’t be installed in flood-prone pits.
The opposite of an air gap is a submerged outlet — a hose dropped into a pool, a sink filled above the spout, a chemical-mix tank with a permanent supply line entering below the water surface. These are exactly the cross-connections backflow programs are designed to find and eliminate.
Common variants and what an air gap isn’t
- Air gap vs. air break. An air gap has unobstructed vertical space; an air break has structure between outlet and receiver, with a smaller gap. Air gap is the higher-protection geometry.
- Air gap vs. vacuum breaker. Air gap is positive — geometry that never fails. A vacuum breaker is a mechanical valve that can fail. When both are options, air gap is always preferred.
- Dishwasher countertop air gap vs. high loop. WA jurisdictions generally accept either, with local variation. Some inspectors require both for new installations.
Common failure modes
- Aftermarket pull-down spray hoses that extend below the basin rim during use — eliminates the air gap during the moment they’re below water level.
- Dishwasher countertop air-gap fitting clogged with food residue — water backs up and discharges through the gap, indicating a downstream blockage.
- Submerged hose-end below water level (garden hose dropped in a pool, fertilizer applicator with hose-end dipping into the tank) — eliminates the air gap; relies on hose-bib vacuum breaker for protection.