What is an air gap?

Drawing of an Air GapAn air gap is a vertical, physical separation between the end of a water supply outlet and the flood-level rim of a receiving vessel. This separation must be at least twice the diameter of the water supply outlet and never less than one inch. An air gap is considered the maximum protection available against backpressure backflow or back siphonage but is not always practical and can easily be bypassed.

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1. What is a cross-connection?
2. What is backflow?
3. What is backpressure backflow?
4. What is backsiphonage?
5. Why do water suppliers need to control cross-connections and protect their public water systems against backflow?
6. What should water suppliers do to control cross-connections and protect their public water systems against backflow?
7. What is a backflow preventer?
8. What is an air gap?
9. What is a reduced principle assembly (RP)?
10. What is a double check valve assembly (DC)?
11. What is a residential dual check valve (RDC)?
12. Why do backflow preventers have to be tested periodically?
13. Where can I get more information about cross-connection control?