What is the recommended distance to maintain for an air gap to prevent back-siphoning?

The recommended distance to maintain an air gap to prevent back-siphoning is at least twice the diameter of the discharge pipe. This distance ensures that there is enough air space to prevent any backflow of contaminated water into the clean water supply. An air gap functions as a physical barrier that effectively prevents any negative pressure created in the system from drawing back potentially polluted water.

The reasoning behind this specific measurement relates to how a back-siphon can occur. If the drained liquid were to reach the level of the discharge pipe, it could create a vacuum effect that pulls fluid back into the supply line. By having an air gap that is at least twice the diameter of the discharge pipe, it minimizes the risk of this happening and offers a safe distance to prevent any such back-flow scenarios.

The other potential answers suggest distances that do not adequately protect against back-siphoning. Relying on the length of the hose or setting a one-foot or three-foot gap either provides insufficient safety or is arbitrary without considering the size of the discharge pipe. Therefore, the specification of twice the diameter provides a scientifically sound measure for protection against this risk.