4 Backflow Preventers To Keep Your Irrigation System Safe

Landscape architects are charged with protected the health, safety, and welfare of the public. Safe drinking water is a universal concern. If contaminated water were to enter the municipal drinking water system, many people could become ill…or worse.

Backflow prevention devices are specified by landscape architects to prevent contaminated water from entering the drinking system. There are questions about protecting drinking water by using backflow preventers on the California Supplemental Exam for landscape architects. This article is a brief overview of the different types of backflow prevention devices used to protect potable water that are used in California.

4 Types of Backflow Preventers

There are a four different types of backflow preventers available for use in irrigation systems. Select a backflow preventer based on the severity of hazard presented if irrigation water contaminated potable water and project size. Here are a few backflow options arranged from most expensive to least expensive:

  • Reduced pressure type backflow preventer (abbreviated as RP)
  • Double check valve type backflow preventer (abbreviated as DC)
  • Pressure vacuum breaker (abbreviated as PVB)
  • Atmospheric vacuum breaker anti-siphon valve (abbreviated as ASV)

Let’s examine the benefits of each type of backflow preventer. Some backflow preventers are more suitable for some projects than others. The following list is arranged by the level of protection provided by the device.

Reduced Pressure Principle Backflow Prevention Device

The reduced pressure backflow preventer is the best (and most expensive) backflow prevention device and provides a great deal of safety in high hazard applications. This is the most common backflow device used in commercial developments. When backflow is detected by the device, water will drip out of the valve and prevent possibly contaminated water from entering the potable water system. The following regulations are from the 2010 California Plumbing Code: “A required reduced pressure principle backflow prevention device (RP) shall, as a minimum, conform to the AWWA Standard C506-78 (R83) adopted on January 28, 1978 for Reduced Pressure Principle Type Backflow Prevention Devices…. A reduced pressure principle backflow prevention device shall be located as close as practical to the user’s connection and shall be installed a minimum of twelve inches (12″) above grade and not more than thirty-six inches (36″) above grade measured from the bottom of the device and with a minimum of twelve inches (12″) side clearance.”

Double Check Valve Assembly

Double check valve backflow devices use two spring-loaded check valves to prevent irrigation water from flowing back into the potable water system. The double check valve assembly also has test cocks to test the efficacy of the device periodically. Unlike every other backflow device, double check valve assemblies do not vent to the atmosphere. Because of this feature, some municipalities prohibit double check valve assemblies from being used for irrigation water while others encourage their use. When in doubt, check with the permitting authority prior to designing the irrigation system. Double check valve assemblies are less expense than reduced pressure backflow devices and can be installed below grade (unlike any other backflow device).

The following regulations are from the 2010 California Plumbing Code: “A required double check valve assembly (DC) shall, as a minimum, conform to the AWWA Standard C506-78 (R83) adopted on January 28, 1978 for Double Check Valve Type Backflow Preventive Devices…. A double check valve assembly shall be located as close as practical to the user’s connection and shall be installed above grade, if possible, and in a manner where it is readily accessible for testing and maintenance.”

Pressure Vacuum Breaker

A pressure vacuum breaker (PVB) is similar to the atmospheric vacuum breaker (AVB) (discussed below) because it creates an atmospheric gap between irrigation water and potable water. “A pressure vacuum breaker back- flow prevention assembly consists of a loaded air inlet valve, an internally loaded check valve, two (2) properly located test cocks, and two (2) isolation valves.”

If backflow occurs, the possible contaminated water is vented out of the system at the pressure vacuum breaker. An individual pressure vacuum breaker is more expensive than one atmospheric vacuum breaker, but only one PVB is required for a system no mater how many valves are on the system. One AVB is required for each valve. So, if more than half a dozen valves are on the system, a PVB quickly becomes the more cost effective solution.

Pressure vacuum breakers must be installed at least 6 inches higher than the highest sprinkler. Some jurisdictions require PVBs to be install up to 18 inches above the highest irrigation head. If the project site slopes, install the PVB at the top of the slope and run the mainlines down hill.

Atmospheric Vacuum Breaker (AVB)

An atmospheric-type vacuum breaker consists of a body, checking member and atmospheric port and is usually incorporated into a valve. Since no valves may be placed downstream of an AVB, one atmospheric vacuum breaker is required for each valve. AVBs are the least expensive backflow prevention devices but they have their limitations. First, one is required for every valve. Second, the AVB must be installed at least 6 inches above the highest head in the irrigation system. As a result, the valve cannot be installed below grade and can be aesthetically unpleasing from a design perspective. You may construct and enclosure to screen atmospheric valves from view, but they must be located above grade in order to work and protect the potable water system from cross-contamination. AVBs are usually installed in small areas and residential landscapes.

Selecting The Right Backflow Preventer for a Site

The type of backflow preventer you specify as a landscape architect depends on the size of a project and the potential threat level posed to the potable water system.

Commercial landscapes typically use reduced pressure (RP) systems because the threat and liability are quite high. Protecting the public’s health and safety is worth the extra expense and superior protection provided by RP backflow preventers.

If any chemicals, such as fertilizers and pesticides, are added to irrigation water, then a reduced pressure system is required to provide the maximum level of protection to the public. Golf courses, nurseries, or other agricultural sites may require this degree of protection.

In small areas, such as typical residential landscapes, a pressure vacuum breaker, double check valve backflow preventer, or atmospheric vacuum breaker are acceptable choices.

If the backflow device must be installed below ground, the only choice is a double check valve backflow preventer.

Backflow preventers are required for all irrigation systems. Landscape architects specify different types of backflow preventers based on a project’s size and potential threat for contamination of the potable water supply. Backflow preventers keep irrigation water from contaminating potable water and protect the public’s health, safety, and welfare.


  1. Potable water regulations
  2. Recycle water regulations
  3. Using graywater in the landscape

John is a landscape architect who is currently preparing to take the California Supplemental Exam to become licensed in California. He is currently a licensed professional landscape architect in Georgia and Florida. John graduated from California State University, Pomona with a BSLA degree in landscape architecture in 1997 and has extensive practice experience in residential and commercial landscape design.

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