What is a Type AB air gap?

A Type AB air gap is a non-mechanical backflow prevention arrangement comprising of an inlet which discharges water into a cistern, vessel, fitting or appliance (receiving vessel) fitted with an outlet and a rectangular weir or ‘slot’ overflow. Depending on the outcome of an assessment by the local water undertaker it can feed a single or multiple installations.

A Type AB air gap is rated by the Regulators as suitable backflow protection against both back siphonage and back pressure at the highest level of contamination risk, fluid category 5.

A summary of some of the key requirements applicable to a Type AB air gap is given below:

1. The air gap is an unobstructed and complete physical break between the lowest point of discharge and the critical water level of the weir overflow. Measured vertically it must be no less than 20 mm or twice the internal diameter of the supply whichever is the greater.

2. The critical level (sometimes referred to as h) is the fluid level in the receiving vessel under fault conditions i.e. when the outlet is closed but the inlet continues to discharge. It is measured at least 2 seconds after closing the water inlet

3. Whether sited internally or externally (as shown in the diagram opposite) the weir overflow must be rectangular (non-circular) and capable of accommodating discharge under fault conditions. Where a screened mesh is installed consideration should be given to the impact this may have on discharge flow. The air gap can be confirmed by test or calculation using the Type AB air gap calculator.

4. Neither the fluid pathway to the overflow nor the discharge from it should be restricted. For example, there should be a sufficient gap between the overflow and any surface to accommodate full discharge unimpeded during fault conditions.

5. Submerged supply pipes are not permitted. If the supply pipe feeding the inlet or the inlet itself comes into contact with the contents of the receiving vessel, for example due to splashing or foaming, then the air gap is considered to be compromised and must be increased to the point no contact occurs.

6. Type AB air gap installations, including screened mesh, should be inspected, cleaned and as necessary maintained every 6 months (BS EN 806: 5)