In cross-connection control, what is the minimum distance for an air gap between the fill pipe and the receiving tank rim when using an air gap?

• A. One times the inside pipe diameter
• B. Half the inside pipe diameter
• C. Equal to the inside pipe diameter
• D. Two times the inside pipe diameter

Answer: (D)

An air gap protects the potable water supply by creating a vertical separation that cannot be overcome by pressure changes, stopping backflow from the receiving tank into the fill pipe. The distance is set to scale with the pipe size because larger pipes can push a stronger water column, increasing the potential for siphonage or backflow if the gap is too small. Requiring a gap of two times the inside diameter provides a robust, conservative barrier that reduces the chance of backflow under varying pressures. For example, a 1-inch pipe needs at least a 2-inch gap, a 2-inch pipe needs at least a 4-inch gap, and so on. Distances smaller than this—equal to, half, or one times the diameter—do not offer the same level of protection and could allow backflow under some conditions.