air gap

A clear vertical space between a water or drain line and the flood level of a receptacle to prevent back-flow or siphoning from the receptacle in the event of negative pressure or vacuum. Most plumbing codes require the air gap to be at least twice the diameter of the water or drain line, with a minimum of 1-1/2 inches (3.8 cm). (See vacuum breaker or back-flow preventer.)

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