Is a backflow preventer the same as an anti-siphon valve?

An anti-siphon valve is the control valve for a sprinkler zone with a vacuum breaker built into the top of it. One part turns the zone on and off and protects your drinking water at the same time. Rain Bird, a major irrigation manufacturer, describes its anti-siphon valves as units to use "in locations where the use of a pressure vacuum breaker (PVB) or a double-check valve is required by city codes to protect against back flow." Its next line is the one worth remembering: "Check your local city codes to determine what type of valves are required."

The protection it offers is narrow. It guards against back-siphonage, which is what happens when supply pressure collapses, say during a main break or a hydrant flush, and the pipes pull water backward like a straw. It does not guard against backpressure, where something downstream pushes harder than the supply. The EPA's Cross-Connection Control Manual says of the atmospheric type that it must not be used for backpressure, and says the pressure vacuum breaker, the next step up, does not protect against backpressure either.

The clearest way to sort them is by what each one stops. There are two families.

Vacuum breakers stop back-siphonage only.

• Hose bib vacuum breaker. The small cap on an outdoor faucet. The EPA manual calls it a specialized application of the atmospheric vacuum breaker, and it is for back-siphonage only. Our page on the hose bib vacuum breaker covers it.
• Atmospheric vacuum breaker. The simplest type. The EPA manual says it must not have shutoffs downstream, which means no valve after it that would keep it under constant pressure. It also cannot be tested once it is installed in the plumbing system.
• Anti-siphon valve. This is the sprinkler valve version of that idea, with the vacuum breaker riding on top of the zone valve.
• Pressure vacuum breaker (PVB). A standalone assembly with test cocks, common in agriculture and irrigation. The EPA manual says it was developed so a vacuum breaker could be used under constant pressure and be tested in line.

Check assemblies stop back-siphonage and backpressure.

• Double check assembly. Two check valves in one body. The EPA manual says double checks protect against both back-siphonage and backpressure and fit low to medium hazard installations. Our page on RPZ vs double check shows where each belongs.
• RPZ (reduced pressure zone) assembly. Two checks with a relief valve between them.

What a device stops is not the same as how much protection it gives. Arizona's backflow rule, Arizona Administrative Code R18-4-215, lists the approved methods "in decreasing order according to the level of protection they provide": an air gap, then the RPZ, then the pressure vacuum breaker, then the double check. So in Arizona a PVB outranks a double check. The double check's advantage is that it also handles backpressure, which the PVB cannot.

The anti-siphon valve belongs to the vacuum breaker family, so it does nothing against backpressure.

For irrigation, the answer usually comes down to what your water provider or city requires, and that is why Rain Bird tells you to check local code. The Arizona backflow rule gives the state's baseline. Under Arizona Administrative Code R18-4-215, a pressure vacuum breaker assembly may be used on a landscape irrigation system only if all of these are true:

• The water use beyond the assembly is for irrigation only.
• The PVB is installed per the manufacturer's specifications.
• The system is designed and built to be incapable of inducing backpressure.
• Chemigation, meaning injecting pesticides or fertilizers into the irrigation water, is not used.

The rule then says: "An RP assembly is required whenever any of the criteria are not met." So an irrigation-only system with no backpressure risk and no chemigation may qualify for vacuum breaker protection, while a system with a fertilizer injector points to an RPZ instead.

One caution. That state rule is written for public water systems, and it does not automatically apply to every single-family residence. Your city or water provider sets the local requirement for homes, so confirm it before you buy parts. Our page on backflow preventers for sprinklers in Arizona walks through what Arizona homeowners typically face.

An anti-siphon valve only protects you if it is installed correctly. Rain Bird's installation notes for its ASVF series say:

• Install it upright.
• Install it at least 6 inches above the highest point of water in the pipe and sprinklers it serves.
• Put no valve downstream of it.
• Do not keep it under operating pressure for more than twelve hours in any twenty-four hour period.

That last point surprises people. A valve that sits pressurized around the clock is the wrong tool. That is the job a pressure vacuum breaker was built for, since the EPA manual describes it as usable under constant pressure. It also explains a common failure. Someone adds a second valve after the anti-siphon valve to split a zone, and the protection is gone. The anti-siphon valve has to be the last valve on that line.

The height rule exists because the device vents to the atmosphere to break a siphon. Sit it below the water it serves and that protection is gone.

Go look at the sprinkler valve area, usually near the house or along a side yard.

1. 1A valve box at ground level with several zone valves inside. Zone valves buried like this are not anti-siphon valves, so the system needs a separate backflow assembly upstream. Find it, because that assembly is the one that matters. Our page on where your backflow preventer is helps.
2. 2A tall brass or plastic valve standing above ground with a cap or vent on top. That is typically an anti-siphon valve or a pressure vacuum breaker. Test cocks on the side mean it is a testable assembly, which points to a PVB.
3. 3A brass assembly with a relief port at the bottom that drips or spits. That is an RPZ, and the discharge is part of how it works. See our page on an RPZ spitting water.

If you cannot tell, read the manufacturer's label or ask a plumber. Guessing is a poor bet when this device is what stands between the irrigation water and your drinking water.

Arizona's rule says each required assembly "shall be tested at least annually, or more frequently if directed by the public water system or the Department," and also after installation, relocation, or repair, when the assembly falls under that section. Testable assemblies, those with test cocks, are the ones a certified tester can check. The EPA manual says an atmospheric vacuum breaker cannot be tested once it is installed in the plumbing system. When one is cracked, stuck, or leaking from the vent all the time, replace it. If your registered assembly fails its test, our page on what happens after a failed backflow test lays out the next steps.

Swapping or installing a backflow assembly is licensed plumbing work, and we handle it as a licensed and insured plumber. If you are not sure which device your system needs, we can take a look and give you a free estimate with the price up front before any work starts. That's the simple version.