Type AB air gap app
links updated 25 Jun 2021
WRAS (now Water Regs UK) has recently published a second plumbing design app, this time to help designers and installers with type AB air gap arrangements. The Air Gap Calculator replaces the former Microsoft Excel spreadsheet-based tool on the website.
The new calculator is based on the ‘calculation methods’ set out in British Standard BS EN 13077:2008, and provides a guide to help ensure an arrangement satisfies the associated dimensional requirements and validation criteria. 
The Air Gap Calculator can only be used for systems which have, or are designed to have, rectangular shaped overflows supplied via a single inlet valve. It cannot be used with alternative water supplies such as greywater or where multiple inlet valves are to be used.
Once the dimensional requirements and validation criteria have been satisfied, additional onsite tests will be required. These are to ensure there are no other factors (such as splashing, foaming etc.) which may contaminate the water inlet valve or reduce the air gap so it no longer provides the required backflow protection.
The Air Gap Calculator is free to use on the website without the need for any specialist applications.
There is a user guide which provides more information on the calculator: User guide
Why not take a look 
