Backflow preventer
Also known as: RPZ, backflow assembly
Plumbing device that prevents contaminated water from flowing backward into the potable water supply. Required by code on irrigation, fire suppression, and many commercial connections.
A backflow preventer is a mechanical device installed at the connection between a building's potable water supply and any system that could potentially contaminate it (irrigation, fire sprinklers, boilers, commercial dishwashers, etc.). Common types include reduced pressure zone (RPZ) assemblies, double-check valve assemblies (DCVA), and pressure vacuum breakers (PVB).
Backflow can occur from back-pressure (the downstream system's pressure exceeds supply pressure) or back-siphonage (negative pressure in the supply line, e.g., during a water-main break). Either can pull contamination from the secondary system into the drinking water supply. Backflow preventers physically prevent this with check valves, air gaps, and pressure-relief mechanisms.
Most municipalities require annual testing of backflow preventers by a certified tester. Test results are submitted to the local water authority. Failed devices must be repaired or replaced. Backflow testing and certification is a recurring revenue stream for plumbing operators willing to obtain the certification (typically a 16-hour course plus exam).
Related terms
Irrigation backflow assembly
Backflow preventer specifically required at the connection between potable water supply and irrigation system. Annual testing required by most municipalities.
Fixture units (DFU)
Standardized measure of expected water demand from plumbing fixtures. Used to size drain and supply piping per the plumbing code.