Backflow Prevention

Backflow Prevention

Learning Objectives

By the end of this chapter, you should be able to:

1.1 The Hydraulic Principles of Backflow

Backflow is the undesirable reversal of water flow in a potable water distribution system. It occurs when the pressure in the downstream (non-potable or potentially contaminated) side exceeds the pressure in the upstream (potable) supply side. The 2018 IPC defines two distinct hydraulic conditions that cause backflow: backpressure and backsiphonage.

Backpressure is a reversal of flow caused by an increase in downstream pressure above the supply pressure. This can be created by a pump, a boiler, elevated tanks, or any other source of pressure in the customer’s system. It is not dependent on a loss of pressure in the public main. For example, a high-rise building’s booster pump can create backpressure against the city supply if a check valve fails.

Backsiphonage is a reversal of flow caused by a negative pressure (a vacuum or partial vacuum) in the supply piping. This negative pressure can result from a main break, firefighting operations drawing water at a high rate, or a sudden shutdown of a high-demand pump. When the supply pressure drops below atmospheric, water from the customer’s system can be siphoned back into the public main.

Code Navigation: The definitions for backflow, backpressure, and backsiphonage are found in IPC Chapter 2 (Definitions). The general prohibition against contamination of the potable water supply is established in IPC Chapter 6, Section 601.2, which requires that the potable water system be protected against backflow in accordance with the provisions of that chapter.

1.2 The Concept of Hazard: Health vs. Non-Health

The selection of a backflow prevention method is fundamentally driven by the degree of hazard posed by the potential contaminant. The code categorizes hazards into two primary classes.

A health hazard is any condition, device, or practice that could introduce a contaminant capable of causing illness or death. This includes sewage, chemicals, and any substance that could make the water unsafe for human consumption. A non-health hazard (or aesthetic hazard) is one that could make the water unpalatable or aesthetically objectionable (e.g., discoloration, odor, or taste) but does not pose a threat to human health.

The code’s approach is risk-based. For a health hazard, you must provide a degree of protection that physically separates the potable water from the source of contamination. For a non-health hazard, a lesser degree of protection—such as a check valve or a vacuum breaker—may be sufficient.

Practical Field Point: As the master plumber, you must make a defensible hazard assessment on every job. When in doubt, the code and the AHJ will always require the higher degree of protection. Installing a reduced pressure principle assembly (RP) where a double check is required is acceptable; the reverse is a violation and a potential liability.

1.3 The Hierarchy of Protection: Air Gaps and Assemblies

The code establishes a hierarchy of protection, with the air gap being the highest and most reliable method. An air gap is a physical separation between the free-flowing end of a potable water supply line and the overflow rim of a receiving vessel or tank. The 2018 IPC specifies a minimum vertical distance for this separation, which is generally twice the inside diameter of the supply line, but never less than one inch. For example, a 2-inch supply line requires a minimum 4-inch air gap.

Below the air gap in the hierarchy are mechanical devices and assemblies. It is critical to understand the difference between a backflow prevention assembly and a backflow prevention device. An assembly (e.g., a double check valve assembly or an RP) has test cocks and shutoff valves, making it field-testable. A device (e.g., an atmospheric vacuum breaker) is not testable and is generally considered less reliable. The code requires assemblies for higher hazards and devices for lower hazards or specific, isolated applications.

1.4 Specific Backflow Preventers: Selection and Installation

The 2018 IPC, through its reference to ASSE and other standards, recognizes several specific types of backflow preventers. The master plumber must know not only what each does, but the precise installation rules.

Reduced Pressure Principle Assembly (RP): This is the most robust mechanical protector and is used for health hazards under both backpressure and backsiphonage conditions. It consists of two independently acting check valves with a pressure differential relief valve located between them. The relief valve is designed to discharge water if the pressure between the checks drops to a point indicating a leak or a backflow condition. The RP must be installed with adequate clearance for testing and maintenance, and its relief valve opening must be at least 12 inches above the flood level rim of the drain or grade to prevent flooding. The code requires the RP to be tested at least annually.

Double Check Valve Assembly (DC): This assembly consists of two independently acting check valves. It is used for non-health hazards under continuous pressure. It is not approved for health hazards because it has no means of discharging or indicating a failure. Installation requirements are similar to the RP, but it can be installed below grade in a vault, provided the vault is drained and accessible.

Double Check Detector Assembly (DCDA): This is a DC with a bypass meter and a small DC on the bypass. It is specifically used on fire sprinkler systems to detect leakage or unauthorized water use while still protecting the potable supply from a non-health hazard.

Pressure Vacuum Breaker (PVB): This assembly includes a spring-loaded check valve and an independently acting air inlet valve. It is designed to protect against backsiphonage only; it is not approved for backpressure conditions. The PVB must be installed with its air inlet at least 12 inches above the highest downstream outlet. It is commonly used on irrigation systems.

Atmospheric Vacuum Breaker (AVB): This is a simple device with a floating disc that seals against a seat under pressure and opens to admit air when pressure drops. It is for backsiphonage only and must not have any shutoff valve or other obstruction downstream of it. It must be installed at least 6 inches above the highest outlet it serves. Because it is a device, it is not testable.

Spill-Resistant Vacuum Breaker (SVB): This is a newer type of assembly that combines the features of a PVB with an internal check valve, allowing it to be installed under continuous pressure without spilling water. It is used for backsiphonage protection on health hazards.

Code Navigation: The specific installation requirements—including minimum clearances, elevation above flood level rims, and the prohibition of valves downstream—are detailed in IPC Chapter 6, Section 603. The code references ASSE standards for the performance of these devices. The table in IPC Section 608.1 (or the referenced appendix) is the primary guide for selecting the correct device based on the hazard and the type of backflow condition.

1.5 The Master Plumber’s Responsibility: Testing and Records

The 2018 IPC and the Maryland State Plumbing Code place a significant burden on the master plumber regarding the testing of backflow prevention assemblies. The code requires that all backflow prevention assemblies be tested at the time of installation and at least annually thereafter, or more frequently if required by the AHJ. Tests must be performed by a certified backflow prevention tester.

As the responsible licensed professional, you are accountable for ensuring that these tests are performed and that the results are documented. This is not merely a technical task; it is a legal and business responsibility. Failure to maintain records can result in fines, loss of license, and liability in the event of a contamination incident.

Practical Field Point: On a new installation, you must schedule the initial test before the system is put into service. You must also provide the owner with a copy of the test report and, in many Maryland jurisdictions, file a copy with the local water authority. You should maintain your own permanent log of all tests for every project you have signed off on.

1.6 Code Navigation: Where to Find It

For an open-book exam, knowing where to look is as important as knowing the content. Use this guide to navigate the 2018 IPC and Maryland regulations quickly.

1.7 Common Exam Traps and Field Pitfalls

1.8 Business and Liability Considerations

For the master plumber, backflow prevention is a critical area of professional liability. A single cross-connection incident can sicken hundreds of people and result in catastrophic legal and financial consequences. Your responsibility extends beyond the installation.

You must ensure that your employees are properly trained to identify cross-connections and to install devices correctly. You must maintain a robust record-keeping system for all test reports. You must also be prepared to defend your hazard assessments to the AHJ. In Maryland, local jurisdictions often have their own cross-connection control programs, and you must be familiar with the specific requirements of the jurisdiction in which you are working. This is not just a code issue; it is a public health issue, and the code holds the licensed professional to the highest standard of care.