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A backflow preventer is designed to stop water from flowing backward through your pipes. In the case of a sprinkler system, installing one can prevent harmful contaminants like fertilizers, pesticides, and bacteria from getting into your drinking water. They can also keep public water supplies from getting contaminated with dirty water.

AVBs use a simple check valve and an air inlet valve to prevent water from flowing backward. The air inlet valve is designed to open and allow air in when the water pressure on the check valves gets too high. This introduces air into the sealed pipe, breaking the vacuum inside and preventing back-pressure and back-siphoning.

RPZs are the most complex and effective types of backflow preventers available. They use two check valves for redundancy, plus a chamber between them that sits at a lower pressure. If the pressure inside that chamber ever gets too high, which would only be caused by backflow, there’s also a pressure relief valve that will empty the chamber until the pressure on the receiving end of the cross-connection has subsided.

RPZs are the most effective kinds of backflow preventers, and they’re also the most expensive. They’re used in cases where the risk of backflow is high and the potential consequences are very serious. Some use cases include commercial buildings, high-occupancy buildings like apartment buildings or hotels, municipal water systems, and some fire sprinkler systems — specifically those in industrial buildings.

Check valves are usually one component inside of a backflow preventer, which is designed to resist higher pressure differences, as in the case of an irrigation system, fire sprinkler system, or main water supply. Backflow preventers can also contain high- or low-pressure chambers and relief valves to help prevent backflow even in high-risk situations.

Generally speaking, there are three different kinds of backflow preventers, each designed for a different severity level. The three kinds of backflow prevention assemblies include the following:

While most homeowners can handle installing a backflow preventer on a sprinkler system, most shouldn’t or can’t legally tackle installation if it’s done on a water main or a higher-volume supply line.

A double-check valve assembly (DCVA) works similarly, but it use two check valves for redundancy and greater protection.

A check valve is a simple one-way valve that prevents water from flowing backward through a pipe. These are usually installed on faucets, toilets, dishwashers, and other fixtures to prevent backflow where only a small amount of water could work its way back into your supply.

PVBs generally require permits and need to be installed by licensed and certified plumbers. Since the risk and consequences of backflow in cases where you’d use PVBs are high and severe, most cities and states don’t allow for legal DIY installation.

A backflow preventer is designed to stop water from traveling in both directions. It gets installed in-line and uses one-way valves or vacuums to prevent backflow. Backflow preventers come in a few designs based on how severe backflow through a particular water line would be considered.

Backflow preventers can also help prevent damage to your plumbing system. Sudden changes in pressure inside your pipes — which backflow preventers stop — can cause a sudden jolt to your pipes caused by pressure changes, commonly called a water hammer. Water hammer, caused by something as simple as de-winterizing your sprinkler system, can damage your plumbing, potentially leaving you with expensive repairs and costly water damage.

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Preventing backflow is mandatory in some capacity in all states — although the specifics regarding the lengths to which you need to go to keep public water supplies safe varies based on state, county, and city. Aside from health issues, you could face monetary assessments and fines if your backflow prevention system isn’t present or isn’t up to plumbing code.

RPZs should never be installed by homeowners. These backflow preventers require a permit and a certified plumber to ensure they’re installed correctly and that adequate drainage is possible in the event of purging during backflow.

In most cases, homes will have a backflow preventer if they have an in-ground sprinkler system. It’s usually installed above ground for easy access. It will typically have a ball valve or shut-off valve handle on it and sits between two pipe sections coming up out of the ground and turning toward each other at a 90-degree angle.

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Backflow can cause very serious health concerns for your home and, potentially, anyone connected to the same water supply. If you’re on a water well, that could mean only your immediate neighbors, but it might mean anyone in your city if you’re connected to a municipal water supply. In the case of bacterial infiltration, you could end up infecting all the water delivered to your entire city.

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Backflow is a term used to describe when water flows backward through your plumbing pipes. Your water supply is designed to flow in one direction only, bringing clean water to your fixtures. When water flows backward — especially in situations where whatever is flowing backward could be contaminated — you could get potentially harmful water in your city water supply or home.

This guide will discuss what a backflow preventer is, what it does, the three main kinds of backflow preventers, and why you might need one in your home.

Like AVBs, PVBs are backflow prevention assemblies with a check valve and an air inlet valve to prevent backflow. However, they also use a high-pressure chamber between the two to prevent water from flowing backward. The differential is designed only to be overcome by water from the supply side and should prevent sudden spikes in pressure on the opposite end of the cross-connection.

AVBs are super simple to install, so most homeowners should be able to handle it themselves. Plus, since there’s minimal risk of backflow from a garden hose, there often aren’t any permits required or guidelines to follow for the installation. This can vary, though, so be sure to check local guidelines.

On a more personal level, backflow from in-ground sprinklers could introduce pesticides, fertilizers, harmful bacteria, fungi, and other contaminants into your drinking water. Keeping your drinking water clean for your family should be your top priority, so preventing backflow into your own water is crucial.

PVBs and DCVAs are typically used in cases where backflow is a medium risk. These include some residential sprinkler systems, commercial irrigation systems, and fire sprinkler systems.

The pressure relief valve is designed to eliminate water in the plumbing pipes until the pressure is normal again — so, during backflow, it will spit water out. As such, RPZs are often installed outdoors where drainage is possible, and the risk of flooding is minimal.

Backflow preventers can purge large volumes of water to prevent backflow, so the relief valve is usually installed where water damage wouldn’t be an issue and drainage is available.

Backflow preventers work by maintaining a low-pressure vacuum sealed by two check valves. If the pressure inside the low-pressure chamber gets too high — which would normally cause backflow to occur — the water gets expelled through a relief valve. The relief valve will remain open until the pressure regulates itself once again.

We get a decent number of questions about backflow prevention devices from homeowners across the country. We’ll answer some of the most common ones we see below.

AVBs are best for situations where the risk of backflow is low, and the consequences are minimal. Common uses typically include garden hoses and in-ground sprinkler systems.

Dan Simms worked in real estate management for five years before using his experience to help property owners maintain their own homes. He got his master’s degree in English Literature and Creative Writing, and he now enjoys sharing his knowledge about homeownership and DIY projects with others on Today’s Homeowner. When he’s not writing, he’s usually outdoors with his wife and his dog, enjoying mountain biking, skiing, and hiking.

Roxanne Downer is a commerce editor at Today’s Homeowner, where she tackles everything from foundation repair to solar panel installation. She brings more than 15 years of writing and editing experience to bear in her meticulous approach to ensuring accurate, up-to-date, and engaging content. She’s previously edited for outlets including MSN, Architectural Digest, and Better Homes & Gardens. An alumna of the University of Pennsylvania, Roxanne is now an Oklahoma homeowner, DIY enthusiast, and the proud parent of a playful pug.

When it comes to drinking water, the last thing you want is a potentially harmful contaminant that could cause harm to you or a family member. Backflow preventers are key to maintaining safe, clean drinking water — especially in areas where non-potable water could mix with your potable water supply.