Type AB Air Gap Calculator

Supplying an installation via a type AB air gap is one way of providing fluid category 5 backflow protection.

Type AB air gap is formed by an arrangement of an air gap and spillover slot.

The WRAS type AB air gap calculator tool is an easy-to-use design aid for designers and installers. It provides details of the height of the spillover slot; the air gap distance between the inlet valve and the critical water level; and as the calculation method is used rather than the test method, three validation criteria.

Even when the arrangement satisfies the dimensional requirements and validation criteria, addition onsite tests are required. These are to ensure there are no other factors (such as splashing, foaming etc.) which may contaminate the inlet or reduce the air gap so that it no longer provides the required backflow protection.

Please note the calculator app can only be used for systems with a rectangular shaped overflow supplied via a single inlet valve. It cannot be used with alternative water supplies e.g. as a backup to a greywater system.

This tool uses the calculation methods and associated validation criteria set out in BS EN 13077:2008 - Devices to prevent pollution by backflow of potable water - Air gap with non-circular overflow (unrestricted) - Family A - Type B.

Use the 'AB Launch tool' button to access the tool via your web browser,

User guide

The tool is also available free to download onto your mobile device from the following stores:

download from Windows Phone storedownload from Google playdownload from App Store

Storage cisterns supplied via a type AB air gap are sometimes used to store water that is required to be wholesome. The spillover slot can allow stored water to become exposed to light and is known to promote the growth of algae. Where this occurs it would not only cause taste and odour problems but also provide nutrients which can support the growth of bacteria.

The most common way of addressing light ingress is to fit a cowl or shroud that covers the spillover slot. This addition may impede, or in extreme cases prevent, water escaping during fault conditions. The following design advice has been developed to minimise this.

Light cowl design - installation information note