Read the five bands from wholesome water to serious hazard, the protection each band usually needs, and the air-gap drawing beside them.
The figure's five bands run down the left, from category 1 at the top to category 5 at the bottom, with backflow risk rising as you go down. The drawing on the right illustrates an unrestricted air gap above a vessel’s spill-over level.
Reading the bands
Each band gives the hazard and a hint at the protection. The typical protection for each:
- 1, wholesome water, no health risk: water as supplied by the water undertaker. No backflow device is needed.
- 2, aesthetic change: taste, odour or temperature only. A single check valve, type EA or EB, is enough.
- 3, slight health hazard: the figure notes that double check protection is often used. A double check valve, type EC or ED, is rated to category 3.
- 4, significant health hazard: higher protection is needed, such as a reduced pressure zone (RPZ) valve, type BA.
- 5, serious health hazard: the key idea is a physical air gap. Types AA and AB are rated for category 5 against both backpressure and backsiphonage.
The air-gap drawing
The outlet stays above the spill-over level of the vessel below it, leaving a visible gap. Even with the vessel full to the brim and the main under suction, the used water cannot reach the outlet to be drawn back, and nothing mechanical has to work.
For the unrestricted gap shown, the vessel’s rim must remain free to spill. Arrangements that use an overflow, such as Type AB, depend on that overflow staying clear and carrying the fault inflow. Splashing, foam or a hose must not bridge the required separation. Not every air-gap type reaches category 5.
Exam cues
- The higher the category, the more stringent the protection. Never protect a higher band with a device rated only for a lower one.
- For category 5, choose a recognised arrangement rated for the fluid and pressure conditions. AA/AB air gaps are examples; single check, double check and RPZ valves do not reach category 5.