3.3 Cross-Connection Control & Backflow Prevention (IPC 608 & 312.10)
Key Takeaways
IPC Table 608.1 classifies a high hazard as contamination and a low hazard as pollution. A double check assembly (ASSE 1015) is for low hazards only, while an RP assembly (ASSE 1013) handles high or low hazards under backpressure or backsiphonage.
Pressure vacuum breaker and spill-resistant vacuum breaker assemblies must be installed with their critical level at least 12 inches above all downstream piping and outlets (IPC 608.14.5).
Atmospheric and pressure-type vacuum breakers protecting an outlet must have their critical level at least 6 inches above the flood level rim, and no valve may be installed downstream of an atmospheric vacuum breaker on a lawn irrigation system (IPC 608.16.4, 608.17.5).
IPC Table 608.16.1 requires a 1-inch air gap for openings up to 1/2 inch, 1-1/2 inches for openings up to 3/4 inch, 2 inches for openings up to 1 inch, and twice the opening diameter for larger openings away from walls.
Air gaps and assemblies must be inspected every year, and RP, double check, PVB, spill-resistant and hose-connection assemblies must be tested at installation, after repair or relocation, and at least annually (IPC 312.10).
Why this section matters
Protecting the potable supply is the core public-health duty of a plumber. Backflow questions appear under System Installation and General Knowledge. Section 608 of the 2021 IPC answers most of them, but its numbering is easy to mix up, so tab Table 608.1, 608.14, 608.16 and 608.17.
Key terms
- Cross connection: any connection through which nonpotable liquids, solids or gases could enter the potable supply. Section 608.7 prohibits cross connections unless they are protected by an approved assembly, device or method. A private water supply may never be cross-connected to a public supply (608.7.1).
- Backsiphonage: backflow caused by negative (sub-atmospheric) pressure in the supply. Causes include a main break, heavy firefighting draw or undersized risers.
- Backpressure: backflow caused when downstream pressure exceeds supply pressure. Causes include pumps, boilers, elevated tanks and thermal expansion.
- Contamination (high hazard): an impairment that creates a health hazard, such as sewage or toxic chemicals.
- Pollution (low hazard): an impairment of quality that is not a health hazard.
Devices and where they work (Table 608.1)
| Device | Standard | Hazard | Protects against | Notes |
|---|---|---|---|---|
| Air gap | ASME A112.1.2 (fittings A112.1.3) | High or low | Backpressure and backsiphonage | Physical separation; see Table 608.16.1 |
| Reduced pressure principle (RP) assembly | ASSE 1013, AWWA C511 | High or low | Backpressure and backsiphonage | Two checks plus a relief valve that dumps to atmosphere; may be under continuous pressure |
| Double check (DC) assembly | ASSE 1015, AWWA C510 | Low only | Backpressure and backsiphonage | Two independent checks, shutoffs and test cocks |
| Pressure vacuum breaker (PVB) assembly | ASSE 1020 | High or low | Backsiphonage only | Check valve plus spring-loaded air inlet; may be under continuous pressure |
| Spill-resistant vacuum breaker (SVB) | ASSE 1056 | High or low | Backsiphonage only | Like a PVB but designed not to spill when the air inlet opens |
| Atmospheric vacuum breaker (AVB), pipe-applied | ASSE 1001 | High or low | Backsiphonage only | Float opens to air when flow stops |
| Hose-connection vacuum breaker | ASSE 1011 | High or low | Backsiphonage and low-head backpressure | Screws onto hose threads |
| Hose-connection backflow preventer | ASSE 1052 | High or low | Backsiphonage and backpressure | Testable dual-check type for hose outlets |
| Frost-resistant automatic-draining wall hydrant | ASSE 1019 | High or low | Backsiphonage and low-head backpressure | Self-draining wall hydrants |
| Backflow preventer with intermediate atmospheric vent | ASSE 1012 | Low | Backpressure and backsiphonage | Used on boiler feed lines |
| Dual check | ASSE 1024 | Low | Backpressure and backsiphonage | Often at residential meters |
| Beverage-machine backflow preventer | ASSE 1022 | Low | Backpressure and backsiphonage | Carbonated beverage dispensers |
| Laboratory faucet backflow preventer | ASSE 1035 | High or low | Low-head backpressure and backsiphonage | Lab faucets |
| Barometric loop | — | High or low | Backsiphonage only | Rises 35 feet above the connection (608.14.4) |
Air gaps (608.16.1)
An air gap is measured vertically from the lowest end of the potable outlet to the flood level rim of the receptor (608.14.1). Hose-thread outlets must be protected some other way, because a hose defeats an air gap.
| Fixture opening (Table 608.16.1) | Away from a wall | Close to a wall |
|---|---|---|
| Lavatories and other openings up to 1/2 in | 1 in | 1-1/2 in |
| Sinks, laundry trays, gooseneck faucets, openings up to 3/4 in | 1-1/2 in | 2-1/2 in |
| Over-rim bath fillers, openings up to 1 in | 2 in | 3 in |
| Drinking fountains (single orifice up to 7/16 in) | 1 in | 1-1/2 in |
| Openings larger than 1 in | 2 × diameter | 3 × diameter |
"Away from a wall" means a single wall is more than three opening diameters away, or two intersecting walls more than four diameters away.
Installation rules for assemblies
- RP assemblies (608.14.2): May be under continuous pressure. The relief opening must discharge through an air gap and must not be able to be submerged, so an RP must never sit in a pit that can flood.
- Relief port piping (608.15.2.1): Must run to an indirect waste receptor or outdoors where it causes no damage. The receptor and drain must be sized for the manufacturer's maximum relief flow.
- PVB and SVB (608.14.5): Critical level at least 12 inches above all downstream piping and outlets. A PVB may not be installed where spillage could damage the building.
- Vacuum breakers at outlets (608.16.4): Critical level at least 6 inches above the flood level rim. Never install them under exhaust hoods or other places with toxic fumes. Deck-mounted or integral vacuum breakers need a critical level at least 1 inch above the flood rim (608.16.4.1).
- AVBs: They work only at atmospheric pressure, so a shutoff valve downstream keeps them under pressure. Section 608.17.5 says valves shall not be installed downstream of an atmospheric vacuum breaker on lawn irrigation. ASSE and many water suppliers also limit AVBs to no more than 12 hours of continuous pressure in any 24-hour period. That is a product limit, not a separate IPC section.
- Location and access (608.15): Provide access as the manufacturer requires. Protect assemblies from freezing, or make them removable with unions (608.15.2). Outdoor enclosures must meet ASSE 1060.
- Hose connections (608.16.4.2): Sillcocks, hose bibbs and wall hydrants need a vacuum breaker or a permanently attached hose-connection vacuum breaker. Water heater and boiler drain valves, and washing machine supply valves where protection is built in, are exempt.
- Below-grade outlets (608.8): Potable outlets and stop-and-waste valves may not be underground. A freeze-proof yard hydrant that drains into the ground is allowed only if its supply has an RP or PVB and it is labeled as nonpotable.
Specific connections (608.17)
| Connection | Required protection |
|---|---|
| Carbonated beverage dispenser | ASSE 1022 preventer or air gap. Piping after the second check must be a material not affected by carbon dioxide |
| Coffee machine and noncarbonated dispenser | ASSE 1022 or 1024, or air gap |
| Boiler feed | ASSE 1012 (or ASSE 1081 or CSA B64.3). With conditioning chemicals: air gap or RP |
| Heat exchanger | Toxic transfer fluid needs double-wall construction with an air gap between the walls. Nontoxic fluid may be single-wall |
| Fire sprinkler or standpipe | Double check, DC fire assembly or RP fire assembly. With antifreeze or chemical additives, or a nonpotable secondary supply: RP |
| Lawn irrigation | AVB, PVB or RP. With chemicals injected: RP |
| Potable line to any high-hazard backpressure source | RP |
| Chemical dispensers | Air gap, RP, PVB, AVB or ASSE 1055 |
| Humidifiers without internal protection | ASSE 1012 or air gap |
| Hospital fixtures (608.3.1) | RP, AVB, SVB or air gap. Bedpan washer vacuum breakers 5 feet above the floor; health-care hose vacuum breakers 6 feet |
Other water-quality rules:
- Water used to cool or heat equipment may not be returned to the potable system (608.10).
- Piping once used for anything else may not be reused for potable water (608.11).
- Nonpotable piping must be marked "CAUTION: NONPOTABLE WATER — DO NOT DRINK" at least every 25 feet and wherever it passes through a wall, floor or roof. Reclaimed, rain and graywater piping is purple (608.9).
Inspection and testing (312.10)
- Annual inspection of all backflow assemblies and air gaps (312.10.1).
- Testing: RP, double check, PVB, RP-detector and DC-detector fire assemblies, spill-resistant vacuum breakers and hose-connection backflow preventers must be tested at installation, after repair or relocation, and at least annually. Use ASSE 5013, 5015, 5020, 5047, 5048, 5052 or 5056 procedures (or CSA B64.10) and gauges that meet ASSE 1064 (312.10.2).
- In Alabama, a person who only tests backflow devices does not need a plumbing certificate (Code of Ala. §34-37-15(a)(4)). Water suppliers usually require tester certification.
Choosing a device: a logic path
- Is the hazard high or low? For high hazards, eliminate the double check and dual check.
- Can backpressure occur? If yes, eliminate all vacuum breakers. You are left with an air gap or an RP (or a DC for low hazard).
- Backsiphonage only, with continuous pressure or downstream valves? Use a PVB or SVB, not an AVB.
- Check the location: avoid flood-prone pits for RPs, and avoid spill-sensitive spaces for PVBs.
A pressure vacuum breaker assembly protects a lawn irrigation system. Under IPC 608.14.5, where must its critical level be?
Between 12 and 60 inches above grade, regardless of downstream piping
At least 12 inches above all downstream piping and outlets
At least 1 inch above the flood level rim
At least 6 inches above the highest sprinkler head
An atmospheric vacuum breaker protects a lawn irrigation system. Which installation detail violates the 2021 IPC?
Using an RP assembly instead because fertilizer is injected
Installing a zone shutoff valve downstream of the atmospheric vacuum breaker
Setting the vacuum breaker's critical level 6 inches above the highest outlet
Installing the vacuum breaker so its air inlet is open to the atmosphere
A potable water line supplies a heating boiler whose water is treated with conditioning chemicals. What protection does IPC 608.17.2 require?
A hose-connection vacuum breaker
A double check backflow prevention assembly
An ASSE 1012 intermediate-vent backflow preventer only
An air gap or a reduced pressure principle backflow preventer
Sections you finish are checked off in the contents.