What is the most effective and dependable method of preventing backflow?

The air gap is considered the most effective and dependable method of preventing backflow because it creates a physical separation between the potable water supply and any potential contaminants. This separation ensures that under no circumstance can contaminated water flow back into the clean water system. By maintaining a gap, gravity helps to keep the non-potable water from entering the drinking water system, effectively eliminating the risk of contamination.

In contrast, other methods, while beneficial, do not provide the same level of protection as an air gap. Check valves and vacuum breakers are mechanical devices that can fail or become obstructed, which could allow backflow to occur under certain conditions. A backflow preventer is also a mechanical device designed to stop backflow, but like the others, it might not be entirely foolproof and can require maintenance or proper installation to function effectively. Therefore, the air gap stands out as the most reliable method by creating an unbreakable physical barrier against backflow.