5.1 Backflow Principles: Backsiphonage, Backpressure, & Degree of Hazard (UPC 602–603; WAC 246-290-490)

Why Backflow Is a Life-Safety Topic

Drinking water is kept safe by moving it only one way: from the source, through the meter, to the fixtures. Any cross-connection creates a possible path for contaminants to enter the potable system. A cross-connection is any link between potable piping and a nonpotable liquid, gas, or used water. The UPC's starting rules are absolute:

• 602.1: Potable piping may not be installed so that used, unclean, polluted, or contaminated water can enter it from a tank, receptor, equipment, or plumbing fixture.
• 602.2: No connection may be made between domestic water and piping carrying water from any other source, used water, or chemicals, gases, or substances unless a backflow prevention device approved for the potential hazard is provided and maintained.
• 602.4: A private water supply may not be connected to any other supply without the approval of the AHJ, health department, or other department having jurisdiction.
• 603.4.4: Direct connections between potable water piping and sewer-connected wastes are not permitted under any condition, with or without backflow protection. Potable water discharging to drainage must pass through an air gap of two pipe diameters, but not less than 1 inch. Alternatively, a vacuum breaker may be connected on the inlet side of a trap at least 6 inches above the flood rim, where it will never see backpressure.

The Two Mechanisms

Why vacuum breakers fail under backpressure: A vacuum breaker works by admitting air when supply pressure drops. Under backpressure, the downstream pressure keeps its float or check closed toward the supply, and nothing prevents the flow reversal. That is why Table 603.2 lists AVBs, PVBs, and SVBs for backsiphonage only.

Atmospheric physics: Atmospheric pressure is 14.7 psia, which can lift water about 33.9 feet (14.7 × 2.31). A full vacuum in a riser can therefore pull contaminated water up from a basin that far below. A hose left in a bucket of chemicals is the classic example.

Degree of Hazard (UPC Table 603.2)

Washington's DOH rule uses the parallel terms "high health cross-connection hazard" and "low cross-connection hazard."

Washington amendments that change hazard assignments

• 603.5.10 Boilers: Potable connections to steam or hot water boilers require an air gap or an RP. The unamended UPC also allows a double check on untreated boilers.
• 603.5.12 Carbonators: The potable supply to carbonators requires a listed RP. Washington deletes the ASSE 1022 carbonated-beverage device line from Table 603.2. Downstream piping may not be copper or other CO₂-affected material.
• 603.5.6 Irrigation (no pumps, no chemicals): An AVB, PVB, SVB, or RP is acceptable. A DC is allowed only when approved by both the water purveyor and the AHJ.

Direct and Indirect Cross-Connections

• A direct cross-connection is a piped connection to a nonpotable system, such as a boiler makeup line or a process tank feed. It can see both backpressure and backsiphonage.
• An indirect cross-connection is an outlet that can become submerged, such as a hose in a bucket or a fill line below a tank's rim. It usually sees backsiphonage only, because an open vessel cannot pressurize the supply.

Two Layers of Protection in Washington

1. In-premises (point-of-use) protection: The UPC, as amended by WAC 51-56, governs devices inside the building at each hazard. Examples include vacuum breakers on hose bibbs, an RP on a boiler, and an air gap on a dishwasher.
2. Premises isolation (containment): The public water system purveyor must run a cross-connection program under WAC 246-290-490. That program includes a device at the service connection sized to the premises' hazard.

WAC 246-290-490 Table 12, premises isolation:

Table 13 lists high-hazard premises that need an air gap or RPBA at the service. Examples include farms and dairies, beverage bottling plants, car washes, chemical plants, commercial laundries and dry cleaners, hospitals and medical and dental clinics, laboratories, metal plating, mortuaries, piers and docks, and irrigation systems with chemical addition. Severe hazards include wastewater treatment plants and radioactive or nuclear facilities. They need an air gap, or an RPBA combined with an in-plant air gap.