6.2 Backflow Prevention Assemblies & Devices

6.2 Backflow Prevention Assemblies & Devices

Exam Focus: Matching the correct backflow prevention assembly or device to a specific plumbing installation is among the most heavily tested subjects on the Michigan Journeyman Plumber exam. Candidates must memorize the standard designations (ASSE 1013, 1015, 1020, 1056, 1001, 1011), internal mechanical components, hazard ratings (high vs. low), hydraulic capabilities (backpressure vs. backsiphonage), and minimum critical installation heights.

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1. Hierarchy of Protection & National Standards

The Michigan Plumbing Code recognizes two major classifications of backflow hardware:

1. Backflow Prevention Assemblies: Complete factory-manufactured field-testable units that incorporate tight-closing resilient seated shutoff valves (isolation valves) at each end and test cocks for differential diagnostic field testing. Examples: RPZ (ASSE 1013), DCVA (ASSE 1015), PVB (ASSE 1020), and SVB (ASSE 1056).
2. Backflow Prevention Devices: Mechanical units that do not incorporate shutoff valves or test cocks and are non-testable in the field. Examples: AVB (ASSE 1001), Hose Connection Vacuum Breaker (ASSE 1011), Dual Check Valve (ASSE 1024), and Carbonator Backflow Preventer (ASSE 1022).

National standards governance is established by the American Society of Sanitary Engineering (ASSE), the American Water Works Association (AWWA), and the Canadian Standards Association (CSA).

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2. The Air Gap (MPC Section 608.16.1 & ASME A112.1.2)

The Air Gap is the unobstructed vertical distance through the free atmosphere between the lowest point of a water supply inlet, pipe, or faucet and the flood level rim of the fixture, tank, or receptor into which it discharges.

                  POTABLE WATER SUPPLY PIPE (Diameter = D)
                                |       |
                                |   v   |
                                |       |
                                +-------+  <-- Lowest point of water outlet
                                    |
                                    |  VERTICAL AIR GAP SEPARATION:
                                    |  - Unobstructed (min. 2 x D)
                                    |  - Absolute minimum = 1.0 inch
                                    v
  FLOOD LEVEL RIM  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  <-- Top overflow point
  OF RECEPTOR      |                           |
                   |   NON-POTABLE FLUID       |
                   |   OR CONTAMINATED VAT     |
                   +---------------------------+

Engineering Attributes & Sizing Rules

• Hazard Level: High Hazard (Contamination) & Low Hazard (Pollution).
• Flow Condition: Protects against BOTH Backpressure and Backsiphonage.
• Ultimate Safety: The air gap is the single absolute physical barrier against backflow; because it has no mechanical diaphragms, springs, or check discs that can corrode or foul on debris, it cannot fail mechanically.
• Dimensional Standards (MPC Table 608.16.1):
  • Standard Sizing Rule: The vertical air gap must be not less than two times the effective opening diameter ($2 \times D$) of the water supply pipe.
  • Absolute Minimum Distance: Under no circumstances shall an air gap be less than 1.0 inch (25.4 mm).
  • Wall Proximity Multiplier: If the water supply outlet is installed close to a vertical wall (closer than 3 times the diameter of the effective opening), or close to two intersecting vertical walls (closer than 4 times the diameter), the air gap must be increased to not less than three times the effective opening diameter ($3 \times D$), with an absolute minimum of 1.5 inches (38 mm). This accounts for surface tension, splashing, and wall-adhering siphon currents.

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3. Reduced Pressure Principle Backflow Preventer (RPZ / ASSE 1013)

The Reduced Pressure Principle Backflow Preventer (RPZ) is the highest level of mechanical protection available in plumbing engineering. It is certified under ASSE 1013, AWWA C511, and CSA B64.4.

                      RPZ ASSEMBLY CROSS-SECTION (ASSE 1013)

          Shutoff #1           Check #1            Check #2          Shutoff #2
           [VALVE]           [SPRING 1]          [SPRING 2]           [VALVE]
       ----[     ]-------------( \ )-----------------( \ )------------[     ]---->
              |       TC #1       |         TC #3        |      TC #4
              |       ( O )       |         ( O )        |      ( O )
              |                   |                      |
              |             +-----+----------------------+---+
              |             |   INTERMEDIATE ZONE            |
              |             |   (Hydraulically Reduced)      |
              |             +-----+--------------------------+
              |                   |
              |            DIFFERENTIAL RELIEF VALVE
              +--- SENSING ------>[   ]
                  LINE            |   | <--- Dumps to atmosphere if zone
                                  v   v      pressure approaches inlet within 2.0 psi

Internal Components

1. Two Independently Acting Check Valves (Check 1 & Check 2): Internally spring-loaded to normally closed positions.
2. Intermediate Hydraulically Dependent Relief Valve: Located between Check 1 and Check 2. Operates on differential pressure. Spring-loaded to pop OPEN to atmosphere. It is held CLOSED by incoming supply pressure routed through an internal hydraulic sensing line acting on a diaphragm.
3. Resilient Seated Isolation Shutoffs: Two full-flow ball valves or OS&Y resilient-seated gate valves flanking the assembly.
4. Four Test Cocks: Numbered 1 through 4 in the direction of flow, used for diagnostic pressure testing.

Operating Mechanics

• Normal Flow: Water passes through Check 1. The strong internal spring of Check 1 causes a deliberate friction head loss of 6.0 to 10.0 psi. Consequently, the water pressure inside the intermediate zone is always maintained at least 6.0 to 10.0 psi lower than the upstream supply pressure. This differential pressure exerts a force against the relief valve diaphragm, keeping the relief valve tightly shut against its internal spring.
• Backsiphonage Response: If supply pressure drops (e.g., to negative or atmospheric), the differential between the inlet and intermediate chamber vanishes. The relief valve spring overcomes hydraulic force and snaps the relief port wide open, dumping the water in the zone onto the floor/drain and admitting air, creating a physical air gap between the two checks.
• Backpressure Response: If downstream pressure rises and forces Check 2 backward, Check 2 seats. If Check 2 fouls on scale, sand, or pipe dope and leaks, high downstream pressure enters the intermediate chamber. As intermediate chamber pressure rises to within 2.0 psid of inlet supply pressure, the relief valve instantly vents the backflowing fluid to atmosphere.
• Ratings: High Hazard & Low Hazard; Backpressure & Backsiphonage; Continuous Pressure Rated.

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4. Double Check Valve Assembly (DCVA / ASSE 1015)

The Double Check Valve Assembly (DCVA) is certified under ASSE 1015, AWWA C510, and CSA B64.5.

Internal Components & Operation

• Consists of two independently acting, spring-loaded check valves in series, flanked by two resilient-seated shutoff valves and equipped with four test cocks.
• Check 1 requires a minimum of 1.0 psid to open; Check 2 requires a minimum of 1.0 psid to open.
• Under backflow conditions, both check valves close tightly. If one check valve is held open by debris, the second check valve holds the line.

Critical Code Limitation: Low Hazard Only!

Exam Trap Alert: The DCVA does NOT incorporate an atmospheric relief valve. If both check valves foul or fail simultaneously, backflowing liquid passes silently into the potable supply with zero external evidence or leakage. Therefore, under MPC Section 608.16 and EGLE Part 14, DCVAs are strictly prohibited for high-hazard (contamination) applications. They are approved exclusively for Low Hazard (Pollution) systems subject to backpressure or backsiphonage under continuous static pressure.

• Common Applications: Commercial fire sprinkler systems with standard wet pipes containing no antifreeze or chemicals; multi-story closed hydronic heating systems using untreated potable water; commercial aesthetic fountains without chemical additives.

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5. Vacuum Breakers: Backsiphonage Protection Only

Vacuum breakers are engineered to prevent backflow caused exclusively by backsiphonage (negative pressure).

Core Rule: Under no circumstances can any vacuum breaker (PVB, SVB, or AVB) be installed where it can be subjected to backpressure from downstream pumps, boilers, elevation head, or thermal expansion!

+-------------------------------------------------------------------------+
|                        VACUUM BREAKER COMPARISON                        |
+-------------------------------------------------------------------------+
|                                                                         |
|  PRESSURE VACUUM BREAKER (PVB - ASSE 1020)                              |
|  - Backsiphonage ONLY.                                                  |
|  - Continuous Pressure ALLOWED (can stay pressurized > 12 hours).       |
|  - Downstream Shutoffs / Solenoids PERMITTED.                           |
|  - Minimum Elevation: 12 INCHES above highest downstream piping/outlet. |
|                                                                         |
|  ATMOSPHERIC VACUUM BREAKER (AVB - ASSE 1001)                           |
|  - Backsiphonage ONLY.                                                  |
|  - Non-Continuous Pressure ONLY (MAXIMUM 12 HOURS in any 24-hr period). |
|  - Downstream Shutoffs / Solenoids STRICTLY PROHIBITED.                 |
|  - Minimum Elevation: 6 INCHES above highest downstream outlet/rim.     |
+-------------------------------------------------------------------------+

Pressure Vacuum Breaker (PVB / ASSE 1020) & Spill-Resistant (SVB / ASSE 1056)

• Anatomy: Contains an independently operating spring-loaded check valve and an independently operating spring-loaded air inlet valve admitted to atmosphere, flanked by two isolation valves and two test cocks.
• Operating Mechanics: During normal flow, water pressure forces the check open and holds the air inlet disc closed against atmospheric port seals. When line pressure drops to 1.0 psi, the internal spring forces the air inlet valve wide open, breaking the vacuum.
• Continuous Pressure Rating: The spring-loaded design allows the PVB to remain under continuous static operating pressure indefinitely.
• Elevation Mandate: Must be installed at least 12 inches (305 mm) above the highest downstream piping, sprinkler head, or water outlet.
• Downstream Valves: Downstream control valves (such as automatic irrigation zone solenoid valves) are permitted.
• Spill-Resistant Vacuum Breaker (SVB / ASSE 1056): Specifically designed for indoor locations. Unlike a standard PVB that spits water from its air vent during pressurization, an SVB utilizes an internal sliding mechanism that mechanically seals the air vent before the check valve opens, preventing water damage to finished interior spaces.

Atmospheric Vacuum Breaker (AVB / ASSE 1001)

• Anatomy: Contains a lightweight gravity float check disc and poppet. It has no test cocks, no shutoff valves, and no springs.
• Operating Mechanics: When water flows, upstream dynamic pressure lifts the float poppet up, sealing the air inlet vent and allowing water to discharge through the outlet. When flow stops, the float disc drops by gravity to seat against the water inlet, admitting atmospheric air to break any siphon.
• Non-Continuous Pressure Limitation (12-Hour Rule): Under MPC 608.16.4 and ASSE 1001, an AVB shall not be subjected to continuous pressure for more than 12 hours in any 24-hour period. If left pressurized continuously, the rubber float seal adheres or vulcanizes to the upper atmospheric metal seat, causing the device to fail to drop when backsiphonage occurs.
• Absolute Ban on Downstream Shutoffs: A shutoff valve, solenoid, nozzle, or throttling cock CAN NEVER BE INSTALLED DOWNSTREAM OF AN AVB. If a downstream valve is closed, the supply line remains pressurized, trapping pressure beneath the float disc.
• Elevation Mandate: Must be installed at least 6 inches (152 mm) above the highest downstream outlet, fixture rim, or piping.

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6. Hose Connection Vacuum Breakers (HCVB / ASSE 1011 & 1052)

Outdoor sillcocks, boiler drain valves, laundry tub faucets, and utility sinks with threaded garden hose spouts present the most common cross-connections in residential and light commercial buildings. An open garden hose submerged in a swimming pool, detergent bucket, or pesticide sprayer will siphon poisons straight into the domestic plumbing if line pressure drops.

• ASSE 1011 (Standard Hose Vacuum Breaker): Single check valve and atmospheric vent. Backsiphonage protection only; non-continuous pressure. Features a freeze-thaw drain screw and a breakaway locking set screw that shears off upon installation to prevent unauthorized removal.
• ASSE 1052 (Dual-Check Hose Vacuum Breaker): Incorporates two independent check valves and an atmospheric vent. Rated for continuous pressure and light backpressure, permitting the use of shutoff spray nozzles at the end of the garden hose.

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7. Master Assembly & Device Selection Matrix

The following table represents the core selection standard required for passing the Michigan licensing exam:

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8. Realistic Exam Application Scenarios

Scenario A: Commercial Greenhouse Lawn Irrigation

Exam Scenario: A plumbing contractor in Ann Arbor is installing a commercial lawn and crop irrigation system for an agricultural research facility. The system utilizes automated zone solenoid valves and includes a downstream chemical injector (chemigation pump) that introduces liquid nitrogen fertilizer into the piping grid. The piping runs underground and feeds drip lines elevated 4 feet on greenhouse tables.

Which backflow prevention assembly is required by the Michigan Plumbing Code?

Code Analysis:

1. Hazard Level: Chemical fertilizer injection introduces toxic nitrates into the system, establishing a High Hazard (Contamination).
2. Hydraulic Forces: The chemical injection pump generates mechanical head, creating potential backpressure. In addition, the irrigation loops are on tables 4 feet above grade, creating static elevation head.
3. Elimination of Vacuum Breakers: Vacuum breakers (PVB, SVB, AVB) are strictly rated for backsiphonage only and are prohibited where backpressure can occur. An AVB is doubly illegal because the system has downstream automated zone solenoid valves.
4. Elimination of Double Check: A DCVA is illegal because it is rated strictly for low hazards (pollution).
5. Required Protection: The contractor must install an Air Gap or an ASSE 1013 Reduced Pressure Principle Backflow Preventer (RPZ).

Scenario B: Residential Laundry Tub Faucet

Exam Scenario: An apprentice plumber is installing a rough-in for a laundry tray in a Warren, Michigan basement. The laundry faucet spout has standard 3/4-inch male hose threads. The homeowner plans to attach a 6-foot garden hose to fill mop buckets and soak clothes.

What must be installed on the faucet to satisfy MPC Section 608.16.4.2?

Code Analysis:

1. Cross-Connection Risk: Leaving a hose attached to the threaded spout creates an indirect cross-connection when the hose sits submerged in dirty laundry water. If a main breaks, backsiphonage pulls detergent, graywater, and bacteria into the potable supply.
2. Required Device: The faucet must be protected by an ASSE 1011 or ASSE 1052 Hose Connection Vacuum Breaker.
3. Installation Mandate: Under MPC Section 608, the vacuum breaker must be secured with a breakaway set screw that shears off during tightening, ensuring it cannot be removed by the homeowner.