Air gaps are usually specified by local code as being two times the diameter of the discharge pipe.

• A. False
• B. True
• C. Depends on pipe material
• D. Depends on local code variations

Answer: (B)

Air gaps are a backflow prevention feature that provides a vertical separation between a potential source of contaminant and the potable water supply. In many local codes, the required air gap is set relative to the discharge pipe diameter to ensure the gap scales with the size of the flow path. The common rule you’ll see is that the air gap should be at least twice the diameter of the discharge pipe. This proportional requirement helps prevent siphoning or backflow by creating a sufficiently large clearance for any upward pressure or flow to break the channel back into the supply.

Because this rule is widely used by codes to standardize backflow protection, it’s correct to say the statement is true in practice. There can be variations between jurisdictions, but the two-times-diameter guideline is a typical and practical standard that many codes reference. The material of the pipe doesn’t change this requirement, since the focus is on preventing backflow rather than on the pipe’s composition.