Type AB Airgap Tanks - Cat 5 - Water Tanks with Backflow Prevention
Type AB Airgap Break Tanks (Category 5 Backflow Prevention) also know as Cat 5 Tanks.

The AB Airgap is the Screened Spillover Weir either on the side of the tank wall or on a Raised Ballvalve Housing ontop of the tank.

Every tank should also have a screened overflow to meet water regulations, and then the AB Airgap is designed to prevent the inlet ever being able to become submerged and have a possibility of contaminating the mains water supply if there was a backflow pressure drop in the mains that would cause it to suck the water from the tank into the mains.

The Airgap is screened with stainless steel mesh, and is sized based on the inlet size, on larger inlets there will be two or more screened spillover weirs.