Backflow is the unintended reversal of flow in a potable-water system that can draw non-potable or contaminated water into the drinking-water supply; a cross-connection is any actual or potential connection between potable water and a non-potable source, and both are the focus of local and national control programs because they threaten public health.[1][2]
An air gap is a physical vertical separation between a potable outlet and the flood level of a non-potable receptacle. When sized and installed correctly it prevents both backsiphonage and backpressure and is the most fail-safe method because it has no mechanical parts to fail.[1]
An RPZ (also called RP) uses two check valves with an intermediate relief zone that discharges to atmosphere if either check leaks; it protects against both backpressure and backsiphonage and is intended for high-hazard applications.[1][3]
RPZ assemblies must be installed accessibly (often above grade), have a way to handle relief discharge, and be periodically tested under local rules; they must not be used where they would be continuously submerged.[1][3][7]
DCVAs use two check valves with test cocks and are suitable for low- to moderate-hazard situations; they protect against backpressure and backsiphonage but are not acceptable where a serious health hazard exists.[1][3]
A PVB combines a check valve and an atmospheric vent. It protects against backsiphonage but not backpressure and is commonly used on irrigation systems; PVBs are typically installed above the highest downstream outlet and above ground so they can be tested and maintained.[1][6]
An AVB is a simple vented device that prevents backsiphonage but is not suitable against backpressure and should not be left under continuous pressure.[1] Model plumbing codes require vacuum breakers or anti-siphon devices on many hose bibs; inexpensive hose-bib vacuum breakers (thread-on or integral anti-siphon hose-faucet devices) are common consumer options and illustrate typical price points for intermittent-use fittings.[5][8]
Model plumbing codes (UPC/IPC), American Water Works guidance, and EPA cross-connection guidance form baseline authorities, but local water utilities adopt and enforce rules that can be stricter or more specific; always check your water provider's program for exact obligations.[1][5]
Common situations that commonly require backflow prevention devices include irrigation systems (which often require a testable PVB or RPZ depending on the hazard), boiler and hydronic systems, chemical feed lines, reclaimed or gray-water connections, private wells, and many exterior hose bibs (which typically need a vacuum breaker or anti-siphon device). Local utility pages explain the specific assembly type required for each application in their service area.[1][5][9][10]
Testable assemblies (RPZs, DCVAs, PVBs) have test cocks and must be periodically tested. Many U.S. jurisdictions require testing on installation, after repair or relocation, and at regular intervals thereafter (commonly annually), but some utilities use different intervals such as biennial schedules for certain residential irrigation assemblies—check your local water authority for the exact testing frequency and record-keeping requirements.[1][4][9][10]
Local water agencies run backflow control programs that define required device types, testing schedules, permit and registration processes, and accessibility/installation rules; these pages are also the place to find approved tester lists and instructions for reporting test results.[9][10]
Manufacturer and product listings demonstrate real installation constraints and test features; for example, contractor-grade RPZs are sold for residential-sized services and show the relief-port and test-cock arrangements installers and testers use.[7]
Homeowners typically pay for device purchase/installation and for professional testing; consumer cost guides summarize typical ranges and recommend getting local quotes because prices vary by region and provider.[11]
For inexpensive hose-bib vacuum breakers, retail product listings provide real-world examples and price points that illustrate the low-cost end of required protection for many outdoor hose connections.[8]
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