When an air gap is used, what is the minimum gap between the bottom of the fill pipe and the floor level rim of the receiving tank?

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

Answer: (C)

The minimum air gap is twice the inside diameter of the fill pipe. This creates a reliable atmospheric break between the supply line and the receiving tank so that, if pressure changes or a siphon tendency develops, contaminated water cannot be drawn back into the supply. Keeping the gap at 2D prevents backflow effectively; for example, a 1-inch pipe needs a 2-inch gap, a 2-inch pipe needs a 4-inch gap, and so on. Choices that propose equal to, 1x, or 3x the diameter don’t align with this standard minimum, which is set to ensure a sufficient air break.