Terminology

• Backflow: reverse flow from backpressure or backsiphonage
• Cross connection: link between potable water and a possible contaminant source
• Air gap: vertical open space between an outlet and a fixture's flood rim
• Atmospheric vacuum breaker (AVB) / pressure vacuum breaker (PVB): backsiphonage protection
• Double check assembly (DCVA): two check valves; low hazard
• Reduced pressure principle assembly (RP or RPZ): checks with a relief valve; high hazard
• Dual check: small nontestable device, such as at meters
• Degree of hazard: low (pollutant) or high (contaminant)
• Containment / isolation: protection at the service or at individual equipment

Standards

Ratings

• RPZ assemblies acceptable for high or low hazard; double check assemblies for low hazard only
• RPZ relief openings discharge through an air gap to an indirect waste receptor or outdoors, sized for the maximum relief flow
• Pressure vacuum breakers set at least 12 in. above downstream piping and outlets
• Boilers need a backflow preventer with intermediate atmospheric vent, or an air gap or RPZ where chemicals are added
• Lawn irrigation uses AVB, PVB, or RPZ protection, with RPZ where chemicals are injected
• Fire sprinkler systems use double check or RPZ assemblies, with RPZ where antifreeze or additives are used
• Testable assemblies tested at installation, after repair or relocation, and at least annually

Test standards

• ASSE 1013 / AWWA C511: reduced pressure principle assemblies
• ASSE 1015 / AWWA C510: double check assemblies
• ASSE 1020: pressure vacuum breakers
• ASSE 1012: backflow preventers with intermediate atmospheric vent
• ASSE 1024: dual check valve backflow preventers
• ASME A112.1.2: air gaps

Components

• Assembly: two independent check valves, shutoff valves, and test cocks; plus relief valve on RPZ
• Strainer: upstream protection
• Enclosure: insulated and heated outdoor enclosure where exposed
• Drain: floor drain or air gap funnel for RPZ relief discharge

Detailing

• Locate RPZs where relief discharge will not cause damage; provide adequately sized drains
• Install assemblies with test access and clearance, not in pits subject to flooding
• Protect outdoor assemblies from freezing with heated enclosures
• Coordinate service-entry assemblies with the water utility and fire department connection

Uses

Specific uses

• Building water service containment
• Fire sprinkler and standpipe connections
• Boiler and chilled water makeup
• Lawn irrigation
• Beverage dispensers and coffee machines
• Laboratories and healthcare equipment

Pros and cons

Pros

• Protects public water from contamination
• Testable assemblies verify performance
• Air gaps are simple and reliable

Cons

• RPZ relief discharge needs drainage and can flood rooms
• Annual testing adds operating cost
• Pressure loss through assemblies

Manufacturing and cost

Construction

• Factory-assembled bronze or stainless steel assemblies; installed in mechanical rooms or enclosures and tested by certified testers

Main manufacturers and suppliers

• Watts
• Zurn Wilkins
• Apollo Valves
• MIFAB

Trade associations: ASSE International and American Backflow Prevention Association.

Sources

• UpCodes: 2024 IPC Chapter 6 water supply and distribution (Section 608 protection of potable water; GSA adoption without amendments), https://up.codes/viewer/general-services-administration/ipc-2024/chapter/6/water-supply-and-distribution
• UpCodes: 2024 IPC Chapter 3 general regulations (backflow assembly testing), https://up.codes/viewer/general-services-administration/ipc-2024/chapter/3/general-regulations
• Duke University: 22 11 19 domestic water specialties, https://facilities.duke.edu/sites/default/files/22 11 19 Domestic Water Specialties.pdf
• Wikimedia Commons: Back siphon example hosepipe up hill.svg, by Stannered (Public domain), https://commons.wikimedia.org/wiki/File:Back_siphon_example_hosepipe_up_hill.svg