Backflow Prevention and Cross-Connection Control
Two mechanisms, one problem
Backflow occurs by backpressure (downstream pressure exceeds supply pressure) or backsiphonage (negative pressure in the supply). A cross-connection is any actual or potential link between potable water and a nonpotable source. Pollution is a health hazard that degrades quality; contamination is a hazard that can cause illness or death. The exam often asks you to classify the hazard before choosing the device.
The device ladder, best to least
- Air gap - the only absolute protection; the gap should be at least twice the effective opening of the outlet, minimum 1 inch.
- Reduced pressure zone (RPZ) assembly - protection against both backpressure and backsiphonage; used for high hazards.
- Double check valve assembly (DCVA) - low hazard, backpressure and backsiphonage, no discharge.
- Pressure vacuum breaker (PVB) - backsiphonage only; must be installed above the highest downstream point (commonly 12 inches) and cannot be buried.
- Atmospheric vacuum breaker (AVB) and hose bibb vacuum breaker - limited low-hazard, backsiphonage-only use.
Testable assemblies (RPZ, DCVA, PVB) require annual testing by a certified tester and may not be bypassed, removed, or installed where they cannot be tested.
Worked example
A lawn irrigation system fed from the potable main is a high hazard. The correct answer is an RPZ assembly, installed where it is accessible and testable, tested annually. A hose bibb vacuum breaker on a garden hose outlet is fine for a sillcock but not for the buried irrigation main.