Types of Backflow Preventers Explained

Ask two facility managers what backflow preventer their building needs and you may get two confident answers, both wrong. The reason is that “which type” is the wrong first question. A backflow assembly is not picked from a shelf the way you pick a faucet finish. It is the output of a classification: how dangerous the connection is, and whether water can be pushed backward or only pulled backward at that point. Get those two facts straight and the assembly almost names itself. This guide walks the families of backflow preventers along exactly those two axes, so you can read a requirement and understand why it landed on you.

Two ideas organize everything below. The first is the degree of hazard at the connection. Water authorities and adopted plumbing codes sort cross-connections into low-hazard ones, where the worst case only degrades water quality, and high-hazard ones, where the worst case is a real health threat. The U.S. EPA’s Cross-Connection Control Manual uses the terms pollutant for the low-hazard, non-health case and contaminant for the high-hazard, health case. The second idea is the type of backflow possible at the connection. Back-siphonage is backflow pulled in by a drop in supply pressure, like a vacuum forming when a main breaks. Back-pressure is backflow pushed in when something downstream (a pump, a boiler, an elevated tank) builds pressure higher than the supply. Some assemblies stop both. Some stop only one. Match those two axes and the assembly class follows.

Why the Assembly Is Matched to the Hazard Degree

The assembly is matched to the connection because the level of protection has to match the level of risk, and the device has to survive the kind of backflow that connection can actually produce. A water purveyor will not let a moderate-quality nuisance and a poison share the same hardware. A garden-hose-grade pollutant and a boiler dosed with chemicals are not the same threat, and the rules treat them differently on purpose.

Two questions drive the match. First, what is the degree of hazard: pollutant (non-health) or contaminant (health)? That answer sets the minimum protection tier. Second, what kinds of backflow can occur here: back-siphonage only, or back-pressure too? That answer rules out any device that cannot handle back-pressure when back-pressure is possible. A vacuum breaker is useless against a boiler that can push water backward, no matter how low the hazard is rated, because it was never built to hold against downstream pressure.

This is why you cannot freely choose. The connection has a hazard degree and a backflow profile, and code points to a class of assembly that covers both. The exact determination is made by your water purveyor and the adopted local code, not by preference, and for a specific building it is confirmed by a cross-connection survey. For how that survey assigns protection, see our guide on cross-connection control programs (216).

The Air Gap: The Simplest, Most Absolute Separation

An air gap is a physical vertical space between a water outlet and the flood level of whatever it discharges into, and because nothing connects the two, it is the most complete backflow protection there is. There is no valve to fail and no internal part to wear out. Water that has left the pipe simply cannot climb back up through open air.

The standard dimension is widely stated the same way across water authorities: the gap should be at least twice the diameter of the supply opening, and never less than one inch. The faucet arcing above the rim of a sink is the everyday version. In a building, a properly sized air gap is what protects against both back-siphonage and back-pressure at once, which is why it is treated as the maximum available protection rather than just one option among several.

The trade-off is practical. An air gap only works where you can physically break the line and let water fall through open air, and where nothing downstream needs continuous pressure. That rules it out for most pressurized service connections, where a mechanical assembly has to do the job instead. Where an air gap is feasible, though, no mechanical device beats it.

Vacuum Breakers (AVB and PVB): Back-Siphonage Only

Vacuum breakers protect against back-siphonage only, and that one limit defines where they belong: they do nothing against back-pressure. A vacuum breaker opens a port to the atmosphere when supply pressure drops, breaking the siphon. That mechanism has no answer for water being pushed backward by downstream pressure, so neither type belongs on a connection where back-pressure can occur.

The atmospheric vacuum breaker (AVB) is the simpler one. It has no shutoff valves and no test cocks, which means it cannot be field-tested, and it cannot sit under continuous pressure. It has to be installed above the highest downstream outlet and must not have a shutoff valve downstream of it. Think individual fixture connections and similar low-duty spots where the line is not pressurized for long stretches.

The pressure vacuum breaker (PVB) is the testable upgrade. It is a true assembly: a spring-loaded check, an air-inlet valve, two shutoff valves, and test cocks. Because of that, a PVB can be held under continuous pressure and can be tested in the field, which is why it shows up on irrigation systems and similar connections that stay pressurized. It is installed above the outlets it serves. Both devices still share the same hard limit. They guard against back-siphonage and nothing else.

Double Check Valve Assemblies for Lower-Hazard Connections

A double check valve assembly (DCVA) is the standard mechanical protection for low-hazard (pollutant, non-health) connections that can see back-pressure, and it protects against both back-siphonage and back-pressure. It is built around two independently operating check valves in series, sitting between two shutoff valves, with test cocks so it can be verified. It is the assembly recognized under the ASSE 1015 product standard.

The two checks are the whole idea. If one fails to seat, the second is still holding, and the design works against both backflow types, which is what separates a DCVA from a vacuum breaker. That combination makes it the workhorse for connections where the substance is a nuisance rather than a danger and where downstream pressure is in play. Typical low-hazard uses named by water utilities include fire sprinkler lines without chemical additives and lawn irrigation systems with no chemical injection.

The line a DCVA must not cross is the health hazard. It has no failsafe vent to atmosphere, so if both checks were ever fouled at once there is no third line of defense that dumps the bad water to the outside. That is exactly why a DCVA is acceptable for pollutants but not for contaminants. Where the connection can introduce a true health hazard, the rules step up to a different assembly.

The RPZ for High-Hazard Connections

A reduced-pressure zone assembly (RPZ, also written RP) is the required mechanical protection for high-hazard (contaminant, health) connections, and it protects against both back-siphonage and back-pressure with a failsafe a DCVA does not have. It carries the two independent checks of a double check, but it adds a differential relief valve between them that holds the space between the checks at a pressure below the supply. It is the assembly recognized under the ASSE 1013 product standard.

That relief valve is the reason an RPZ is trusted with poisons. If a check leaks and pressure in the middle zone climbs toward the supply pressure, the relief valve opens and dumps water out the bottom of the device, to the outside, rather than letting it travel back into the building or the public main. Contaminated water exits the assembly instead of the drinking supply. Because that relief port can discharge, an RPZ carries placement rules a sealed device does not: the discharge has to be handled and the port kept clear, which is why this class of assembly is not a drop-it-anywhere install.

This is the assembly you will see called for on the genuinely dangerous connections: boilers with chemical treatment, chemical feed systems, and similar high-hazard equipment. Vacuum breakers and double checks are not accepted substitutes there, because only the air gap and the RPZ are recognized to protect a health-hazard connection against the full range of backflow. The internal mechanics of how the RPZ relief valve behaves under a fault are covered in our guide on how an RPZ works (213).

How the Right Assembly Gets Assigned to a Connection

The right assembly is assigned by reading the connection against the two axes and then deferring to the authority that has the final say. In plain order: identify the degree of hazard (pollutant or contaminant), identify whether back-pressure is possible or only back-siphonage, and the class of assembly follows from there. A health hazard pushes you to an air gap or an RPZ. A pollutant under back-pressure points to a double check. A back-siphonage-only connection can use a vacuum breaker, with a PVB where the line stays pressurized and an AVB where it does not.

The part that surprises building owners is who actually decides. The binding determination is made by your water purveyor and the local adopted plumbing code, and for a specific property it is set by a cross-connection survey performed by qualified people, not by a homeowner or operator reading a chart. The chart tells you why a requirement makes sense. It does not let you self-certify the choice.

A safety note belongs here. Selecting, sizing, and installing a backflow assembly is code-bound, licensed-plumber and certified-specialist work, and a regulated one at that. This guide is meant to help you understand the categories and read a requirement, not to choose or install a device yourself. The standards above (ASSE 1013, ASSE 1015, ASSE 1020) and the protection tiers are the framework professionals work within, and the right answer for your building always comes from your water authority and a qualified specialist.

Frequently Asked Questions

Is the air gap really better than an RPZ?
As pure protection, yes. An air gap is a physical break with no mechanical part to fail, so it covers both back-siphonage and back-pressure absolutely. The catch is that it only works where you can break the line into open air and where nothing downstream needs continuous pressure. Where a connection must stay pressurized, an RPZ is the high-hazard mechanical equivalent.

Why can’t I just use a vacuum breaker everywhere?
Because vacuum breakers stop back-siphonage only. They have no defense against back-pressure, so on any connection where a pump, boiler, or elevated tank can push water backward, a vacuum breaker is not allowed no matter how low the hazard is rated.

What is the real difference between a DCVA and an RPZ?
Both use two checks and stop both backflow types, but the RPZ adds a differential relief valve that dumps water to the outside if a check fails. That failsafe is why a DCVA is accepted for low-hazard pollutants while an RPZ is required for high-hazard contaminants.

Can I decide which assembly my building needs?
No. The degree of hazard and the required assembly are determined by your water purveyor and adopted local code, confirmed by a cross-connection survey. Understanding the categories helps you read the requirement, but the binding call is not yours to make.

Does the same assembly type apply everywhere in the country?
The general families and what they protect against are consistent, but the exact standards adopted and the hazard determinations are set by your jurisdiction’s code and water authority. Always verify the requirement with your local water purveyor.

This article is general information for understanding backflow preventer categories. It is not professional or code advice. Backflow assembly selection, sizing, and installation are regulated, licensed-plumber and certified-specialist work; verify all requirements with your water purveyor and local code.

Sources

• U.S. EPA, Cross-Connection Control Manual (EPA 816-R-03-002): https://www.epa.gov/sites/default/files/2015-09/documents/epa816r030020.pdf
• U.S. EPA, Protecting Water Quality Through Cross-Connection Control and Backflow Prevention (fact sheet): https://www.epa.gov/system/files/documents/2021-12/ds-toolbox-fact-sheetsccc.pdf
• USC Foundation for Cross-Connection Control and Hydraulic Research, Training Tools: https://fccchr.usc.edu/tools.html
• American Water (Virginia American Water), Common Methods of Backflow Prevention: https://www.amwater.com/vaaw/Methods+of+Backflow+Prevention.pdf
• Montana DEQ, Atmospheric Vacuum Breaker (cross-connection fact sheet): https://deq.mt.gov/files/Water/Forms/Cross%20Connection/Cross_Connection-AVB-2020.pdf
• ASSE International, Product Standards (ASSE 1013, 1015, 1020): https://asse-plumbing.org/standards/product-standards
• Minnesota Department of Health, High Hazard Cross Connections and Responsible Authorities: https://www.health.state.mn.us/communities/environment/water/com/ccrespauth.html