Backflow Preventer Reference
Overview
Backflow preventers stop non-potable water from contaminating the supply. Required on irrigation systems, boilers, fire-sprinkler systems, and any fixture where the supply outlet could be submerged in non-potable water. Most jurisdictions require annual testing by a certified tester. This reference covers identification, common installations, test procedure, and the failure modes a service tech needs to recognize.
Specifications
Common backflow preventer types in residential / light commercial:
| Type | Use | Test required | Notes |
|---|---|---|---|
| Atmospheric Vacuum Breaker (AVB) | Hose bib, low-hazard outdoor | No | Can't be under continuous pressure; non-testable |
| Pressure Vacuum Breaker (PVB) | Irrigation supply, low-hazard | Yes, annual | One-way check + air-inlet |
| Double Check Valve Assembly (DCVA) | Boiler feed, low-to-medium hazard | Yes, annual | Two independent check valves |
| Reduced Pressure Zone (RPZ) | High-hazard, irrigation w/ chemicals, fire sprinkler | Yes, annual | Two checks + relief valve; spills water when failing |
| Hose Bib Vacuum Breaker | Outdoor spigot (the small attachment) | No | Required on every residential hose bib in modern code |
Materials
Standard installation requirements:
- Pressure rating: matched to system pressure plus safety margin (typically 175 psi for residential)
- Materials: brass body, stainless springs, EPDM or NBR seals
- Orientation: most assemblies are horizontal-only; some have vertical options noted by manufacturer
- Drain provision: RPZ assemblies WILL discharge water during fail-safe activation; provide a floor drain or air-gapped discharge
Performance
Standard test procedure (RPZ - most demanding):
- Pressurize the assembly at full system pressure for 5 minutes to confirm no immediate discharge.
- Test check valve #1 - close shut-off valve #2, open test cock #2. Pressure differential between check valves should be ≥ 1 psid (per ASSE 1013 standard).
- Test check valve #2 - close shut-off valve #1, open test cock #3. Pressure differential should hold.
- Test relief valve - relieve pressure through test cock #3 slowly; relief valve should open by the time differential drops to ~2 psid.
- Document readings on the certified test form. State + local authority typically requires submission within 10 days.
Safety
RPZ backflow preventers discharge water when the relief valve activates - sometimes dramatically. ALWAYS verify a drain or air-gapped discharge exists before any test or service. An RPZ installed in a finished basement without drain provision is a flood waiting for the first activation.
Notes
- Annual test: required by most jurisdictions; the certified tester (often a licensed plumber with backflow endorsement) submits results to the water authority.
- Failed test resolution: most failures are repairable - replace internal o-rings, check spring tension, clean check valve seats. Whole-assembly replacement is rare on modern brass bodies.
- Common failure cause: debris on check-valve seats. Even one grain of sand prevents proper seal and fails the test.
- DIY-installed PVB on irrigation: very common, often installed below required elevation. PVB must be 12" above the highest downstream outlet. Below that, it's non-compliant regardless of test result.
- Test cock placement: the test cocks (small ball valves on the assembly) make testing possible. Without them, the assembly is testable only by removal - a waste-of-water fail-mode that costs the customer.
Troubleshooting
- RPZ discharges continuously: relief valve seat fouled or check #1 not holding. Disassemble + inspect.
- PVB sprays during normal operation: air-inlet stuck open (debris) or downstream backpressure exceeds spec.
- DCVA fails test: one or both checks worn. Replace internal cartridges.
- No data plate visible: assembly may be old enough that the manufacturer is out of business. Identify by physical features and check current alternatives for replacement parts.
References
- ASSE 1013 (Performance Requirements for RPZ Backflow Preventers)
- USC FCCCHR Manual (current edition)
- Local water authority requirements (vary widely by jurisdiction)