Backflow Prevention and Cross-Connection Control
Two mechanisms
Backsiphonage is caused by negative pressure in the supply; backpressure is caused by downstream pressure exceeding supply pressure. Both can pull contaminants into potable water. The degree of hazard governs the device: high hazard (toxic, sewage, chemicals) versus low hazard (aesthetic only).
Assemblies and devices
- Air gap - the best protection, a physical separation; typically at least twice the pipe diameter, minimum 1 in.
- RPZ (reduced pressure zone) - high hazard, testable, with relief port.
- DCVA (double check valve) - low hazard, testable.
- PVB (pressure vacuum breaker) - cannot be under continuous pressure and cannot have downstream shutoff valves.
- AVB (atmospheric vacuum breaker) - no downstream shutoff, no continuous pressure.
- Hose bibb vacuum breaker - at every threaded hose outlet.
Testable assemblies require annual testing by a certified tester, with records kept. Devices must be accessible for testing and repair. Thermal expansion control is required downstream of a backflow preventer on a closed system.
Common traps
- Choosing a DCVA for a high-hazard application when an RPZ is required.
- Installing a PVB under continuous pressure.
- Putting a shutoff valve downstream of an AVB.
- Forgetting annual testing.
- Confusing a backwater valve (drainage) with a backflow preventer (supply).
Rule of thumb
High hazard → air gap or RPZ. Low hazard → DCVA or PVB. If it must be tested and repaired in place, it must be accessible.
Worked example: A chemical-mixing station fed from a potable line is a high hazard; install an RPZ at the branch and test it annually.