The Basics of Backflow Prevention

By Dale Stewart on August 29, 2019 in Blog10 Comments

The Basics of Backflow Prevention graphic

A well-designed irrigation system is important for the health of plants in a landscape but it’s also important for human health. A successful irrigation system keeps the water for landscapes separate from potable drinking water, preventing contamination that can cause illnesses. That’s why a backflow preventer is a necessity on any irrigation project.

Why do I need a backflow preventer and how does it work?

According to the Uniform Plumbing Code, an irrigation system must have an approved backflow prevention device to help prevent pollution or contamination of public water supplies from backflow.

Backflow is a reversal of water flow, caused by unintentional or unwanted actions when an irrigation and municipal water cross-connection is made. Backflow creates the potential for fertilizers, herbicides and other unwanted harmful substances to move from an irrigation system and into the public or potable water supply.

In an irrigation system, backflow has two causes: back siphonage or back pressure.

Back siphonage occurs when water is drawn or pulled backward because of negative or decreased pressure in the supply side of the water system. If the irrigation line does not have a backflow preventer at the cross-connection, then contaminated water can be pulled into the supply line from sprinklers or other emitters.

For example, a 2-inch service line connected to a 6-inch supply line will have the same pressure. If there is a break or high demand for the water downstream from the 6-inch service line, then negative or reduced pressure in the 2-inch supply line will follow and cause a vacuum or siphonage from the service line.

Back pressure is caused when system pressure is higher than the supply line’s pressure. A pump connected downstream of a service line can create this additional pressure, leading to back pressure.

Four backflow prevention devices to know Febco Backflow Device

Due the potential to potential for backflow to cause illness, injury or even death, the Uniform Plumbing Code considers landscape irrigation systems to be high hazard, but by installing approved backflow prevention devices at cross-connections you can prevent backflow and protect your irrigation clients and community.

These are the four most common backflow preventers used in residential sprinkler systems:

Atmospheric Vacuum Breaker (AVBs)

A Watts Atmospheric Vacuum Breaker

These valves have a lot of flexibility. They’re the least expensive of the backflow prevention options, but they also provide the least amount of protection. An atmospheric vacuum breaker, also known as an anti-siphon valve, stops back siphonage with a floating disc. This disc rises and seals off the air inlet when pressurized and drops to allow air to enter the downstream piping when depressurized.

AVB’s can be plastic or brass and either manual or electric. Unlike other backflow preventers, AVBs are installed on each zone of the irrigation system, immediately after the zone control valve.

They also must be installed at least six inches above the highest emission point. They are not intended to have continuous pressure on the downstream side and cannot be used as a master valve.

Note: If you use anti-siphon valves for backflow prevention all of your zone valves must be anti-siphon valves. The only exception is a drain valve for winterizing the system.

Pressure Vacuum Breaker (PVB)

PVB Backflow Device

The next level up, in terms of price and protection provided, is called a pressure vacuum breaker, or PVB.  This device prevents back siphonage only. A pressure vacuum breaker has a spring-loaded check valve that will close whenever water stops flowing with an air relief valve that opens to break the siphon when pressure drops to 1 PSI.

A PVB must be installed 12 inches above the highest emission point. The advantage of this device is that you can install multiple zone valves in valve boxes underground after the device.

Reminder: Both anti-siphon valves and pressure vacuum breakers cannot prevent back-pressure.

Double Check Valve Assembly (DCV)

A Wilkins (Zurn) Double Check Backflow Preventer

A double check backflow preventer features two spring-loaded check assemblies to prevent back siphonage and back pressure on non-health hazard (low hazard) systems.

One of the nice benefits of a DCV is that it can be installed in a valve box underground. Be sure to check your codes at the local level, as a DCV doesn’t always meet the requirements for an irrigation system. This is because they are not designed for health hazard situations, such as an irrigation system with a fertilizer injector.

Reduced Pressure Assembly (RP or RPZ)

A FEBCO Reduced Pressure Zone Assembly

The last device provides the highest degree of protection. It is called a reduced pressure assembly, also known as a reduced pressure zone. This is the only mechanical assembly allowed for use in high hazard applications (such as commercial sites or fertigation systems) with back pressure.

RPs have a pressure differential relief valve in a zone between two spring-loaded check valves, which can discharge water to reduce pressure.

Like the double check assembly, RPs don’t need to be higher elevation than sprinklers and other emitters. The last thing to note, and this is important, is that the RP reduces water pressure, by about 10 to 14 psi as the water passes through the device. Take this into consideration with your design.

Professionally maintain systems with Ewing’s help

A healthy, beautiful landscape is not the only reason to professionally maintain an irrigation system. Protecting public water sources from potential contamination also matters. Except for the atmospheric vacuum breaker, most backflow devices need to be inspected and certified on an annual basis. In some areas, they must also be installed by a certified backflow technician. So be sure to check your local codes.

To learn more about how you can prevent system issues like backflow, go to your local Ewing store. To find a location near you or to shop online, visit us at

Dale Stewart photo
Dale Stewart
Author Dale Stewart is a service professional at Ewing's Santa Fe, NM, location. He can be reached at

10 Responses to The Basics of Backflow Prevention

  1. Mir Sarfaraz Ali Khan April 24, 2016 at 3:32 am #

    Good article on the Backflow Prevention Devices. There is one more device Hose Vacuum Breaker (HVB) which should be installed on all garden hose bibs to prevent contaminated lawn water being sucked in to the drinking water supply network. It is a small brass unit that can be directly threaded on to the hose bib.
    I don’t know about the other parts of the world but here in the Kingdom of Saudi Arabia I have seen in many places garden hoses connected directly to the hose bibs with out HVB. There are instances of people getting sick or killed after drinking water from hose bib.

  2. Sarah Smith July 5, 2016 at 12:35 pm #

    My sister recently had a backflow problem. Thanks for the advice about how you can install devices to prevent backflow and help protect your home and yard. I’ll have to see about getting a company to test my backflow and install a device to prevent it.

  3. Kate Hansen September 3, 2019 at 11:18 am #

    I never knew that you have to have a prevention device for an irrigation system. Last week my husband was telling me about getting one and I didn’t know if it was necessary to get one. I appreciate you helping me learn more about why you need a prevention device!

  4. Adam Golightly August 17, 2020 at 11:34 am #

    Thanks for your tips about how a disc can rise and deal with the air in an AVB system and is immediately after the zone control valve. We are having some trouble with our pipes and my daughter said that something was coming up the sink instead of going down it. Getting it fixed by a professional could be really useful and allow us to get an installation to prevent any more backflow.

  5. Access Doors and Panels August 19, 2020 at 7:27 am #

    The facts that have been discussed here are really important. Thank you so much for sharing a great post.

  6. Ashok Kethiguntla September 13, 2020 at 6:37 am #

    thank you for explaining about the backflow preventer, this is too good the parts were explained very well.

  7. Derek Campbell February 5, 2021 at 3:25 am #

    This is a good source of having a running water clean. Thanks for sharing this important info.

  8. Corten February 21, 2021 at 4:44 am #

    I’ll have to see about getting a company to test my backflow and install a device to prevent it. We are having some trouble with our pipes and my daughter said that something was coming up the sink instead of going down it. Thanks for the advice about how you can install devices to prevent backflow and help protect your home and yard.

  9. Joel March 24, 2021 at 3:33 pm #

    What type of irrigation doesn’t need a backflip prevention device?

    • Robyn Hazen March 25, 2021 at 3:09 pm #

      Hi Joel – It depends. It’s illegal not to have one in some cases. Local authorities determine where they’re required and sometimes the exact type of backflow preventer that has to be used. It might not be required by law in situations where the irrigation system is not linked to potable, drinking water but check with local authorities for confirmation.

Leave a Reply