Back-siphonage vs. back-pressure, cross-connection hazard levels, the four core backflow preventer types and where each is required, and testing and certification requirements.
Backflow prevention protects a building's potable water from reversing direction and pulling or pushing contamination backward into the supply. This module builds the two-mechanism model — back-siphonage (vacuum-driven) versus back-pressure (positive-pressure-driven) — that every device selection decision in the field actually depends on, then works through how atmospheric vacuum breakers, pressure vacuum breakers, reduced pressure zone assemblies, and double check valve assemblies each stop one or both mechanisms.
By the end of this module you should be able to explain why a device rated only for back-siphonage provides zero protection on a connection exposed to back-pressure, and why code reserves reduced pressure zone assemblies for health-hazard connections specifically because of how a double check valve assembly fails.