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DC Plumber: Backflow Prevention, Cross-Connection Control, and Potable Water Protection

DC Plumber · Work through your exam, one topic at a time.

All guide sections

Backflow Prevention, Cross-Connection Control, and Potable Water Protection

Two mechanisms, two hazard levels

  • Backsiphonage is caused by negative pressure in the supply; backpressure is caused by downstream pressure higher than the supply. The device must match the mechanism as well as the hazard.
  • High hazard (toxic or biological contamination such as sewage, chemicals, carbonators, or boiler water) requires an air gap or a reduced-pressure zone assembly (RPZ).
  • Low hazard accepts a double check valve assembly (DCVA) or, where backpressure cannot occur, a pressure vacuum breaker (PVB).
  • An air gap must be at least twice the effective opening of the discharge pipe and never less than 1 in.

Devices and where they go

  • Atmospheric vacuum breakers: no downstream shutoff, no continuous pressure, no backpressure - hose bibbs and flush valves.
  • PVB: testable, installed at least 12 in above the highest downstream outlet, and not under continuous pressure for more than 12 hours.
  • RPZ: testable, with a relief port that must discharge to a drain; installed above grade and protected from freezing.
  • DCVA: testable, low hazard only, no relief port.

The District angle

DC Water administers the cross-connection control program. Containment protection at the service is required, and testable assemblies must be tested on the required annual schedule by a certified tester, with results filed. Non-testable devices still must be correct for the application.

Worked example

A restaurant has a carbonator at the bar and a mop sink with a hose-threaded faucet. Reasoning: the carbonator is a high hazard because carbon dioxide can create backpressure, so it needs an air gap or an RPZ at the unit. The hose bibb needs a hose-connection vacuum breaker. Because a high hazard exists inside, the containment device at the service must be an RPZ rather than a DCVA - the highest hazard inside sets the containment level.

Traps that cost marks

Putting a PVB on a line under continuous pressure; installing an RPZ in a pit below grade; assuming a DCVA protects a high hazard; and skipping annual testing because nothing appears to have changed.