Backflow Prevention and Cross-Connection Control
Backflow Prevention and Cross-Connection Control is heavily tested because it is a public health function: the exam wants the hazard, the device, and the test.
Hazard and device matching
- High hazard, meaning anything that could cause illness or death such as sewage, chemicals, antifreeze, or pesticides, requires an air gap or a reduced-pressure principle assembly (RPZ, ASSE 1013).
- Low hazard uses a double check valve assembly (DCVA, ASSE 1015), pressure vacuum breaker (PVB, ASSE 1020), or atmospheric vacuum breaker (AVB, ASSE 1001).
- An AVB cannot be under continuous pressure and cannot have a downstream shutoff. A PVB can be under continuous pressure but cannot see backpressure. Neither belongs on a high hazard.
- An air gap is the best protection and needs no testing, but the gap must be at least twice the effective opening of the supply pipe and above the flood level rim.
Testable assemblies
- An RPZ maintains about a 5 psi differential across the first check; the relief valve discharges when the differential falls to roughly 2 psi. A dripping relief usually means a fouled first check or a leaking second check.
- Test annually or as the jurisdiction requires, using a certified tester, and file the report with the water purveyor; tag the assembly with the test date.
- Install above grade, drain the relief, protect from freezing, and never bypass.
Worked example
A car wash injects wax concentrate with a venturi downstream of a hose bibb. The wax is a high hazard and a downstream pump can create backpressure. A hose-bibb vacuum breaker fails on both counts, so install an RPZ or provide a physical air gap into the holding tank.
Traps
Specifying a DCVA on a high hazard, burying an RPZ in a pit that can flood, installing an AVB with a shutoff downstream, and forgetting the thermal expansion the device creates.