What is a Type AF air gap?

A Type AF air gap is a non-mechanical backflow prevention arrangement comprising of an inlet which discharges water into a cistern, vessel, fitting or appliance fitted (receiving vessel) fitted with an outlet and circular overflow.

A Type AF air gap is rated by the Regulators as suitable backflow protection against both back siphonage and back pressure for contamination risks no greater than fluid category 4.

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

1. The air gap is an unobstructed and complete physical break measured downwards between the lowest point of discharge from the inlet and the critical water level. It must be no less than 20 mm or twice the internal diameter of the supply whichever is the greater.

2. The critical level 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. The overflow must be circular and of a minimum size throughout its length (four times in the inlet pipe cross sectional area). It must be capable of draining the maximum inflow of water under fault conditions i.e. when the outlet is closed but the inlet continues to discharge.

4. 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.

5. Where they discharge to drain the overflow and warning pipe must be fitted with an air break to drain or equivalent prior to the drain connection.

6. Type AF air gap installations should be inspected, and as necessary maintained every 12 months (BS EN 806: 5)