5.2 Backflow Assemblies and Devices: RP, DC, PVB, SVB, AVB, & Installation Rules (UPC 603.3, 603.4, Table 603.2)
Key Takeaways
A reduced pressure principle assembly (RP, ASSE 1013) protects against both backsiphonage and backpressure for both pollution and contamination hazards (UPC Table 603.2).
A double check valve assembly (DC, ASSE 1015) is limited to pollution (low hazard) but protects against both backsiphonage and backpressure (UPC Table 603.2).
A PVB (ASSE 1020) or SVB (ASSE 1056) must be installed upright at least 12 inches above all downstream piping and the flood-level rim (UPC Table 603.2).
An atmospheric vacuum breaker (ASSE 1001) must be upright, have no valve downstream, sit at least 6 inches above downstream piping, and not be under pressure more than 12 hours in any 24 (UPC Table 603.2).
Backflow assemblies need at least 12 inches of clearance below them, and a permanent platform where they are more than 5 feet above the floor (UPC 603.4.3).
Choosing a Device: Three Questions
- What is the degree of hazard? Pollution (low) or contamination (high)?
- What kind of backflow can occur? Backsiphonage only, or backpressure as well?
- Will the device be under continuous pressure? AVBs may not be.
The Main Devices (UPC 603.3 and Table 603.2)
| Device | Standard | Construction | Pollution | Contamination | Installation (Table 603.2) |
|---|---|---|---|---|---|
| Air gap | ASME A112.1.2 | Unobstructed vertical separation | Backsiphonage and backpressure | Backsiphonage and backpressure | Per Table 603.3.1 |
| RP (reduced pressure principle) | ASSE 1013 | Two check valves, a differential relief valve, 4 test cocks, 2 shutoffs | Both | Both | Horizontal unless listed; 12-inch bottom clearance; may discharge water |
| DC (double check valve) | ASSE 1015 | Two check valves, 4 test cocks, 2 shutoffs | Both | Not permitted | Horizontal unless listed; 12-inch bottom clearance; does not discharge |
| PVB (pressure vacuum breaker) | ASSE 1020 | Loaded air inlet, loaded check, 2 test cocks, 2 shutoffs | Backsiphonage only | Backsiphonage only | Upright; may have valves downstream; at least 12 inches above downstream piping and flood rim; may discharge water |
| SVB (spill-resistant vacuum breaker) | ASSE 1056 | One check, air inlet vent, test cock, shutoffs | Backsiphonage only | Backsiphonage only | Upright; at least 12 inches above downstream piping and flood rim |
| AVB (atmospheric vacuum breaker) | ASSE 1001 | Body, checking member, atmospheric port | Backsiphonage only | Backsiphonage only | Upright; no valve downstream; at least 6 inches above downstream piping and flood rim; not under pressure more than 12 hours in 24 |
| Double check detector (fire) | ASSE 1048 | DC with a metered bypass DC | Both | Not permitted | Fire systems under continuous pressure |
| Reduced pressure detector (fire) | ASSE 1047 | RP with a metered bypass RP | Both | Both | Fire systems under continuous pressure |
Other Table 603.2 devices, all backsiphonage-only and not for continuous pressure unless noted:
- Antisiphon ballcock (ASSE 1002): critical level 1 inch above the overflow.
- Hose connection vacuum breaker (ASSE 1011): no valve downstream.
- Hose connection backflow preventer (ASSE 1052).
- Freeze-resistant wall hydrants (ASSE 1019 and 1053).
- Freeze-resistant sanitary yard hydrants (ASSE 1057).
How an RP Works
- The first check creates a pressure drop, so the zone between the checks is always lower than supply pressure.
- A differential relief valve senses supply pressure against zone pressure. If the difference shrinks, the valve opens and dumps the zone to atmosphere. That happens if the first check leaks, if backpressure pushes past the second check, or if supply pressure falls.
- Water from the relief port shows that the assembly is protecting the system. It is a normal response to a pressure change, not a nuisance leak. Provide drainage sized to the manufacturer's discharge rate (603.4.8).
Because the relief port must stay open to air, 603.4.9 prohibits devices with atmospheric vents or ports in pits, underground, or submerged locations. They are also not allowed in areas with toxic, poisonous, or corrosive fumes. Washington's DOH rule (WAC 246-290-490) also requires installation so the assembly cannot become submerged by flooding.
Why AVBs Have the Tightest Limits
An AVB's float drops by gravity to open the air port. Continuous pressure can hold the float up against its seat until it sticks. A downstream valve would trap pressure on it. That is why an AVB must have no shutoff downstream and may not be under pressure for more than 12 hours in any 24-hour period. A PVB or SVB has a spring-loaded air inlet and check, so it tolerates continuous pressure and downstream valves.
A PVB may spill water when it is repressurized, so 603.3.5 allows it indoors only where provisions for spillage are provided. An SVB is designed to avoid that spill.
General Installation Requirements (UPC 603.4)
| Section | Rule |
|---|---|
| 603.2 / WA 603.1 | Devices must be approved by the AHJ. In Washington, assemblies protecting the public water system must be models on the Department of Health approved list |
| 603.4.1 | Multiple backflow valves at one location on a premises must each be permanently identified |
| 603.4.3 | Access and clearance per the manufacturer, but at least 12 inches between the lowest part of the assembly and the grade, floor, or platform. Installations more than 5 feet above floor or grade need a permanent platform that can support a tester |
| 603.4.5 | Devices on hot water above 110°F must be rated for that temperature |
| 603.4.7 | Protect from freezing, for example with an ASSE 1060 enclosure |
| 603.4.8 | Drain lines sized to the device's discharge rate |
| Table 603.2 note 2 | Installation in a pit or vault requires prior AHJ approval; never for devices with atmospheric ports (603.4.9) |
Quick Selection Examples
| Application | Minimum protection |
|---|---|
| Lawn irrigation, no pumps or chemicals (WA 603.5.6) | AVB, PVB, SVB, or RP; DC only if the purveyor and the AHJ approve |
| Irrigation with chemical injection (603.5.6.3) | RP |
| Boiler makeup (WA 603.5.10) | Air gap or RP |
| Carbonated beverage dispenser (WA 603.5.12) | Listed RP |
| Fire sprinkler, no chemicals (WA 603.5.14) | DC, double check detector, RP, or RP detector |
| Fire sprinkler with antifreeze or chemicals (603.5.14.2) | RP or RP detector |
| Pure water process system, such as dialysis (603.5.18) | RP |
| Pool, spa, or hot tub with a submerged fill line or a direct connection to the circulation system (603.5.20) | Air gap or RP |
Which statement about atmospheric vacuum breakers is correct under UPC Table 603.2?
They must be upright, have no valve downstream, be at least 6 inches above downstream piping, and not be under pressure more than 12 hours in 24
They may be installed in an irrigation valve box below grade
They may be installed with a shutoff valve downstream if the valve is left open during business hours
They protect against backpressure for low hazards
A reduced pressure principle assembly is installed 7 feet above the floor in a mechanical room. What does UPC 603.4.3 require?
Relocation into a pit for easier access
A second assembly in parallel
Nothing, as long as a ladder is available
A permanent platform capable of supporting a tester or maintenance person
Which device may protect a contamination (high) hazard that is subject to backpressure?
Spill-resistant vacuum breaker
Double check valve assembly
Pressure vacuum breaker assembly
Reduced pressure principle assembly
Sections you finish are checked off in the contents.