Which type of backflow prevention device is the least reliable?

The check valve is considered the least reliable type of backflow prevention device. This device functions by allowing water to flow in one direction and closing to prevent reverse flow. However, it is prone to failure due to the possibility of debris or sediment accumulating within it, which can prevent the valve from sealing properly. Additionally, check valves can wear out over time, leading to potential leaks or backflow situations.

In contrast, an air gap provides a physical separation between potable water and potential contaminants, making it a very effective form of backflow prevention with no moving parts that could fail. The hose-bib vacuum breaker and atmospheric vacuum breaker also employ mechanisms designed to prevent backflow, but they tend to have fewer reliability issues compared to check valves because they are engineered to function effectively in a wider range of operating conditions without the wear and tear that can affect check valves.