Air-gap types and their ratings, tap-gap sizes over basins and baths, where a gap is measured from, and what can bridge or drown it.
An air gap is a physical break: water falls from the outlet through open air, so there is no continuous path back into the supply and no moving seal to stick or wear. That makes it the protection relied on for category 5, but only for the right type, correctly installed.
The type sets the rating
- AA and AB: category 5 against both backpressure and backsiphonage, when their installation conditions are met.
- AG: category 3 against both, used for example at a cistern's float-operated inlet.
- AUK3: category 5 against backsiphonage only, for taps over higher-risk appliances where category 4 or 5 may be present.
Not every air gap reaches category 5, and a visible space below an outlet is not a rated air gap until it meets that type’s requirements.
Tap gaps over domestic appliances
A fixed tap over a domestic basin, bath, bidet or shower tray can use a specified tap gap above the appliance’s spill-over level. Keep the required clearance and ensure splash, foam or a hose cannot bridge it. Under the AUK2 rules, the gap from the lowest point of the outlet to the spill-over level is at least:
- 20 mm for taps up to G½
- 25 mm for taps above G½ up to G¾
- 70 mm for taps above G¾.
An AUK3 gap is at least 20 mm or twice the inlet pipe diameter, whichever is greater.
Mixers have another return path
On a single-flow mixer, the hot and cold paths join before water leaves the tap. If one feed has a higher pressure, it can drive water into the other. Provide suitable single check valves on each feed as well as the required tap gap. Keeping the outlet clear of basin water does not separate the two feeds inside the mixer.
Measure from the right level
An AA gap is referenced to the receiving vessel's spill-over level, and its rim must allow unrestricted spillover. An AB gap uses the defined critical level of its weir overflow. A type AG inlet uses the defined critical water level under fault conditions, not the normal float setting or the lip of the overflow pipe. For AB and AG arrangements, the overflow must carry the fault inflow without compromising the specified gap.
What defeats an air gap
- Restricting the required spillover or overflow path so that fault inflow compromises the gap.
- Splashing or foam reaching the inlet.
- A hose or spray attachment that puts the outlet below the spill-over level.
- A later alteration that bridges the gap.
Leave access to inspect it: no moving parts does not mean no maintenance.