Backflow Fundamentals

A cross-connection is any actual or potential physical connection between a potable water supply and a non-potable source. Backflow occurs when contaminated water reverses direction and flows from the non-potable source into the potable supply. Two mechanisms drive backflow:

  • Backsiphonage: Negative pressure in the supply system (caused by water main breaks, high upstream demand, or fire hydrant operation) creates a siphon that draws water backward from the connected piping or submerged hose end.
  • Backpressure: The downstream system operates at higher pressure than the supply (boiler systems, elevated tanks, pressurized chemical injection systems), forcing water backward against normal flow direction.

Hazard Classification

Cross-connections are classified by the degree of hazard:

  • High hazard (health hazard): The contaminant could cause injury, illness, or death if introduced into the potable supply. Examples: irrigation systems with fertilizer or pesticide injection, boiler systems with chemical treatment, medical equipment connections, commercial dishwashers with chemical sanitizers, cooling towers.
  • Low hazard (non-health hazard): The contaminant is objectionable (affects taste, odor, or aesthetics) but not an immediate health risk. Examples: bypass connections, domestic boilers without chemical treatment, auxiliary water supplies from private wells.

Backflow Preventer Types

Air Gap (AG)

The only completely reliable method of backflow prevention. An air gap is the unobstructed vertical distance between the lowest supply pipe opening and the flood rim level of a receiving vessel. IPC §608.15 and ASME A112.1.2 require the air gap to be a minimum of twice the effective opening of the supply pipe (minimum 1 inch). Air gaps cannot be defeated by either backsiphonage or backpressure. They are required for the highest-hazard applications but are impractical for pressurized systems because they interrupt continuous flow and require a break tank and re-pressurization.

Reduced Pressure Zone Backflow Preventer (RPZ / RPBA)

The RPZ (ASSE 1013) provides protection against both backsiphonage and backpressure for high-hazard applications where an air gap is impractical. It consists of two independently acting check valves separated by a reduced pressure zone, with a differential pressure relief valve that opens to atmosphere whenever the zone pressure approaches supply pressure — indicating check valve failure. The RPZ discharges water during a backflow event rather than allowing contamination to pass. Required for irrigation systems, chemical injection points, commercial/institutional boilers, cooling towers, and fire sprinkler systems with chemical treatment (NFPA 13). Must be tested annually by a certified backflow tester.

Double Check Valve Backflow Preventer (DC / DCVA)

ASSE 1015 governs DCs. Two independently acting check valves in series with test cocks between them. Provides protection against backsiphonage and low-hazard backpressure applications. No relief valve — it will not discharge to atmosphere during failure. Approved for low-hazard applications: fire sprinkler systems without chemical treatment, commercial food service equipment, auxiliary supply connections. Not approved for high-hazard applications in most jurisdictions. Must be tested annually.

Pressure Vacuum Breaker (PVB)

ASSE 1020 governs PVBs. A spring-loaded check valve combined with an air inlet valve that opens during backsiphonage conditions, breaking the siphon. Protects against backsiphonage only — does not protect against backpressure. Must be installed at least 12 inches above the highest downstream connection. Common application: residential and commercial irrigation systems where the backflow hazard is backsiphonage only. Cannot be installed in a below-grade vault (the air inlet must remain accessible and cannot be submerged).

Atmospheric Vacuum Breaker (AVB)

ASSE 1001 governs AVBs. Lowest-cost option, protects against backsiphonage only, must be the highest point in the downstream piping, cannot have a valve downstream (must be normally open to atmosphere when not flowing), and is limited to 12 hours per day of operation. Commonly used on individual hose bibs (ASSE 1011 anti-siphon hose bibb) and laboratory faucets. Cannot be used in continuously pressurized systems.

Spill-Resistant Pressure Vacuum Breaker (SVB)

ASSE 1056 governs SVBs. Similar to a PVB but with a diaphragm or check mechanism that prevents the discharge of water when the device vents — useful in freeze-prone locations or where water discharge from the vent would cause damage. Provides backsiphonage protection only; same installation constraints as PVB.

Application Selection Matrix

Selecting the correct device type requires matching the hazard level and backflow mechanism to an appropriately rated device:

  • High hazard + backpressure possible: Air gap or RPZ
  • High hazard + backsiphonage only: Air gap, RPZ, or PVB/SVB (if no backpressure risk)
  • Low hazard + backpressure possible: DC
  • Low hazard + backsiphonage only: PVB, SVB, AVB

Specific applications: (1) Irrigation — RPZ for systems with fertilizer/pesticide injection; PVB or DC for clean water systems (check local ordinance). (2) Commercial boilers with chemical treatment — RPZ. (3) Fire sprinkler (NFPA 13) without chemical additives — DC. (4) Fire sprinkler with antifreeze or chemical treatment — RPZ. (5) Medical equipment (dialysis, sterilizers) — Air gap or RPZ. (6) Chemical injection — Air gap (preferred) or RPZ.

Installation Clearances and Requirements

RPZ and DC assemblies require specific installation clearances for testing and maintenance access: minimum 12 inches below the device, 12 inches above, and 12 inches to each side. They must not be installed in pits, vaults, or below grade where flooding could submerge the assembly unless specifically listed for submerged service (ASSE 1013 and 1015 have submerged-service variants). The relief valve outlet on an RPZ must discharge to an air gap over a drain — not into a drain line that could flood and submerge the outlet.

Annual Testing Requirements

All mechanical backflow preventers (RPZ, DC, PVB, SVB) must be tested at initial installation and annually thereafter by a certified backflow assembly tester (AWWA-recognized certification programs vary by state — California requires state certification; most states accept ASSE, AWWA, or USC/FCCCHR certification). The tester uses a differential pressure field test kit to verify both check valves are holding and, for RPZs, that the relief valve opens at the correct differential pressure. Test reports must be submitted to the local water authority and retained by the building owner.