What is the minimum size for an air gap to prevent backflow of unsafe water?

The correct answer is that the minimum size for an air gap to prevent backflow of unsafe water is twice the diameter of the water supply pipe. An air gap is a crucial safety feature in plumbing systems that prevents contaminated water from flowing back into the potable water supply. When there’s a change in water pressure or system failure, backflow can occur, potentially introducing harmful substances into clean water.

Establishing an air gap that is twice the diameter of the water supply pipe ensures a sufficient vertical distance between the water outlet and the highest possible level of the upstream water source, effectively minimizing any risk of backflow. This guideline is a key aspect of plumbing codes and safety regulations aimed at protecting public health by preventing contamination.

The other options imply minimum clearance that may not adequately safeguard against backflow, thus they do not meet the rigorous standards required for ensuring safe drinking water.