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How to choose and install a backflow prevention assembly

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Learn more about different backflow prevention assemblies.
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Residential properties

Air gap

example of approved air gap above tank or reservoir
An air gap is the simplest way to ensure backflow does not happen.

One step you can take to protect our drinking water is to keep the ends of hoses out of pools or other containers while you are filling them. For example, if a hose is submerged in the pool, the chlorinated pool water may enter the drinking water system during a backflow condition. When filling pools, keep the end of the hose out of the pool and have a gap between the end of the hose and the surface of the water. This gap must be twice the diameter of the hose and never less than one inch.

Hose bib backflow protection

Atmospheric vacuum breaker on hose bib
A hose bib backflow preventer keeps water from flowing backward up the hose.

Some residents choose to install a backflow device on their hose bib. These are called vacuum breakers and they do not need to be tested. You can purchase them at most plumbing and hardware stores.

Irrigation system

double check valve assembly installed in valve box at property line
Irrigation double check valve assembly installed just after the water meter on private property at the property line.

Your plumber or licensed backflow assembly testing company will likely know which type of assembly to use. Backflow assemblies most commonly installed on residential irrigation systems are the double check valve assembly (DCVA) and the pressure vacuum breaker assembly (PVBA). The DCVA may be installed in a vault below finished grade. The PVBA must be installed at least 12 inches above finished grade.

Commercial properties

The location of the assembly, service size, and hazard level of the connection determine which backflow assembly you should use. If you have questions about which type of assembly you need, contact the Water Bureau Water Quality Inspection staff at 503-823-7480. Below are some assemblies commonly used in commercial applications. More information about the all the following assemblies can be found on our backflow assembly installation requirements page.

Reduced pressure backflow assembly (RPBA)

RPBA installed 12 inches above finished floor
This is a reduced pressure backflow assembly installed inside a building at the water riser. The assembly is four inches from the water and 12 inches above the finished floor. There are no connections allowed upstream of this assembly.

The reduced pressure backflow assembly (RPBA) is designed to prevent backflow caused by backpressure or backsiphonage and may be installed on either low- or high-hazard connections. Backsiphonage is when water moves in the direction opposite its regular flow.

During normal operation, the pressure between the two check valves, referred to as the zone of reduced pressure, is maintained at a lower pressure than the supply pressure. If either check valve should leak, the relief valve is designed to open and discharge water to the outside.

RPBA installed outside of building in insulated enclosure
This is an outside reduced pressure backflow assembly installed just after the water meter on private property.

A properly functioning RPBA will periodically release small amounts of water due to changes in line pressure up or downstream of the assembly.  For this reason, this type of assembly should be installed in areas that are not subject to water damage and have proper drainage. When backflow happens, a properly functioning RBPA may release some water. If the RPBA becomes clogged by debris or is not functioning properly and needs repair, it may discharge a large volume of water.

Double check valve assembly (DCVA)

Double check valve assembly installed inside building
A domestic double check valve assembly installed vertically on a riser inside a building just inside from a water meter. The premises isolation riser is 12 inches from the foundation wall to centerline and 12 inches above the finished floor.

The double check valve assembly (DCVA) is commonly used to protect against backflow hazards that do not pose a health risk. They may be used under continuous pressure and protect against both backsiphonage and backpressure conditions. They can be installed above or below ground, but no more than 24 inches deep or five feet above the floor or grade.

Double check detector assembly (DCDA)

double check detector assembly installed inside a building
Double check detector assembly installation at immediate inside wall of building. This is installed 12 inches above the finished floor.

The double check detector assembly (DCDA) protects the potable water supply line from possible contamination or pollution from:

  • fire sprinkler systems
  • backpressure from fire line booster pumps
  • stagnant water that sits in fire system piping over extended periods of time
  • the addition of nonpotable (not safe to drink) water through outside fire district connections (FDCs)
  • the detection of leaks or unauthorized water use

The assembly consists of two spring-loaded check valves, a bypass assembly with a water meter and double check valve assembly, and two resilient seated indicating shutoff valves.

double check detector assembly installed inside a vault
Double check detector assembly installation in vault for a two-inch dedicated fire service.

Reduced pressure detector assembly (RPDA)

Reduced pressure detector assembly inside a building
Basement installation of a reduced pressure detector assembly in a high-rise building. Backflow assembly lowered to five feet above finished floor.

This assembly is intended to protect the potable water supply from:

  • fire sprinkler systems that contain chemicals or other additives
  • auxiliary water sources that use storage tanks or supply high-rise buildings

This assembly uses a mainline reduced pressure backflow assembly equipped with two resilient seated indicating shutoff valves, a bypass arrangement containing a smaller reduced pressure backflow assembly, and water meter. The RPDA must be installed at least 12 inches above finished grade or the 100-year flood plain, whichever is greater.

Vacuum breaker assemblies

There are two types of vacuum breakers: the atmospheric vacuum breaker (AVB) and the pressure vacuum breaker assembly (PVBA). The Water Bureau doesn't allow either the AVB or PVBA to be used for backflow protection.

The AVB is among the simplest and least expensive mechanical types of backflow prevention device that the end user can buy. The PVBA is an outgrowth of the AVB and evolved out of a need to have an atmospheric vacuum breaker that was testable and could be used under constant pressure.