What is a Type AC air gap?

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

A Type AC 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 3 .

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

1. The air gap is an unobstructed and complete physical break between the lowest point of the vent (air inlet) and the critical water level of the weir overflow. 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. Supply pipes maybe submerged but adjustable or dismantlable joints on the are not permitted below the critical water level.

4. The overflow shall be not less than 19mm (internal diameter).

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