Backflow Prevention and Cross-Connection Control - Hazards, Assemblies, and Testing
Domain 4 - Backflow Prevention and Cross-Connection Control
This domain tests hazard recognition and assembly selection. Expect questions on backsiphonage versus backpressure, actual versus potential cross-connections, and the installation and testing rules for backflow assemblies.
The two backflow mechanisms
Backsiphonage occurs when supply pressure drops below atmospheric and a vacuum pulls nonpotable liquid into the potable line. Backpressure occurs when downstream pressure exceeds supply pressure, such as a boiler or chemical feed system. A device approved only for backsiphonage cannot protect a backpressure hazard.
Assembly selection
- Air gap: best protection; suitable for high hazards where physically practical.
- Reduced-pressure principle (RPZ) assembly: high hazard, testable, protects against both backsiphonage and backpressure.
- Double check valve assembly: low hazard only.
- Pressure vacuum breaker (PVB): backsiphonage only; not for backpressure.
- Spill-resistant vacuum breaker and atmospheric vacuum breaker: limited fixture or branch applications.
Installation and testing
Assemblies must remain accessible for testing and maintenance, with adequate drainage and freeze protection. Testable assemblies are tested at installation and retested at the interval required by the AHJ, by a certified tester, with results reported.
Worked example
A chemical injection system feeds a closed heating loop connected to the potable supply by a make-up line. The hazard is high because chemicals can enter the line, and backpressure is possible when loop pressure exceeds supply pressure. A PVB or double check is inadequate; specify an RPZ or air gap. Add thermal expansion control because the check valve can create a closed system.
Traps that cost marks
Using a PVB for backpressure; burying an RPZ; omitting a drain; skipping annual testing; using a double check for a high hazard; forgetting thermal expansion.