Backflow Prevention, Cross-Connection Control, and Potable Water Protection
Backflow Prevention, Cross-Connection Control, and Potable Water Protection
This domain is about the boundary between potable water and everything else. Expect hazard recognition, correct device selection, and installation details that decide whether the device can work at all.
Concepts
A cross-connection is any actual or potential link between potable water and a contaminant source. Backflow occurs by backsiphonage (negative supply pressure, as during a main break or heavy draw) or by backpressure (downstream pressure exceeding supply). A high hazard can cause death or serious illness - sewage, chemicals, glycol, pesticides; a low hazard is objectionable but not dangerous.
Protection, from best to most limited
- Air gap: complete protection and the preferred method for high hazard; at least twice the effective opening of the supply, never less than 1 in., above the flood-level rim.
- Reduced-pressure zone assembly (RPZ): testable, works under continuous pressure, protects against both backpressure and backsiphonage; the usual choice for boiler feed, irrigation with chemical injection and carbonators.
- Pressure vacuum breaker (PVB): low hazard only, no continuous pressure, at least 12 in. above the highest downstream point.
- Double check valve assembly: low hazard, testable, continuous pressure allowed.
- Atmospheric vacuum breaker: no downstream shutoff, no continuous pressure, and 6 in. above the highest downstream outlet.
Devices must be listed, installed in a permitted orientation, accessible for testing, and tested by a certified tester on schedule, with records kept.
Worked example. A hydronic boiler with glycol feed and a chemical pot feeder ties into the potable supply. That is a health hazard with backpressure, so a double check is inadequate; the correct device is an RPZ (or an air gap), tested annually.
Traps. Using an AVB under constant pressure or with a shutoff downstream; assuming a double check protects against a health hazard; missing the annual test record; setting a PVB below the highest downstream outlet.