A backflow preventer is a plumbing device that stops contaminated water from reversing into your clean drinking supply. It’s a crucial safeguard that protects your home from bacteria, pesticides, and other pollutants that could enter through irrigation systems, burst pipes, or sudden drops in water pressure.
Most homes with sprinkler systems, wells, or fire suppression systems are required by law to have a backflow preventer. Installation typically costs $150 to $1,000, depending on the type and complexity of the system.
If you’re unsure whether your home needs one, local plumbing codes or a licensed plumber can provide guidance. In this guide, we’ll break down how backflow preventers work, the three main types, and when professional installation is required.
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What Is Backflow and What Causes It?
Backflow occurs when water flows backward through your plumbing, potentially contaminating your home’s drinking supply. Your plumbing system is designed to deliver clean water in one direction, but under certain conditions, non-potable water can reverse course and mix with your drinking water.
Several factors can cause backflow, including:
- Improper Pipe Sizing: If your pipes are too large or too small for your water demand, pressure imbalances can force water to flow backward.
- Reverse Pressure: Malfunctioning boilers, water heaters, or pumps can create excess pressure, pushing water in the wrong direction.
- Sudden Drops in Water Pressure: Water main breaks, firefighting hydrant use, or high neighborhood demand can cause pressure drops, triggering back-siphonage, where water from your home is pulled back into the main supply.
- Temperature Changes: Frozen pipes can create pressure build-ups, leading to backflow when they thaw.
Why You’ll Want to Prevent Backflow
Backflow isn’t just a plumbing issue—it’s a serious health risk. Contaminated water can introduce bacteria, pesticides, and other harmful substances into your home’s supply—or even your entire city’s water system.
Because of this, backflow prevention is legally required in all states, though specific regulations vary by location. Failing to comply can lead to fines and costly upgrades.
On a personal level, backflow can introduce harmful chemicals from irrigation systems into your drinking water or even damage your plumbing. Pressure fluctuations can cause water hammer, a forceful shockwave that can weaken pipes and lead to leaks or bursts.
Installing a backflow preventer is the best way to protect your home and family from contamination and costly water damage.
Learn more about how to stop water hammering from the video below.
What Is a Backflow Preventer?
A backflow preventer is a device that ensures water flows in only one direction, preventing contaminated water from reversing into your clean supply. Installed directly in your plumbing line, it uses one-way valves or vacuums to block backflow. Different types of backflow preventers exist based on the level of risk associated with a particular water line.
How Backflow Preventers Work
Backflow preventers rely on check valves and a relief valve to regulate pressure. Inside the device, a low-pressure chamber is sealed by two check valves. If pressure builds up beyond safe levels—normally a condition that would cause backflow—the system releases water through a relief valve instead of allowing it to contaminate your supply.
Since backflow preventers can discharge large amounts of water, they are typically installed in locations with proper drainage to prevent water damage.
3 Types of Backflow Preventers
Backflow preventers come in three main types, each designed for different risk levels:
- Atmospheric Vacuum Breakers (AVBs) – Best for low-risk applications like garden hoses and sprinkler systems.
- Pressure Vacuum Breakers (PVBs) – Common in irrigation systems where moderate backflow risk exists.
- Reduced Pressure Zone (RPZ) Assemblies – Required for high-risk situations where contaminants could pose serious health hazards.
Below, we’ll break down how each type works and when to use them.
Atmospheric Vacuum Breakers (AVBs)
AVBs use a check valve and an air inlet valve to stop backflow. When water pressure drops, the air inlet opens, breaking the vacuum inside the pipe and preventing contaminated water from flowing backward.
Image: Courtesy of Watts
AVBs are best for low-risk applications like garden hoses and in-ground sprinkler systems.
How To Install It
Installing an AVB is a simple DIY job that most homeowners can handle. In many areas, permits aren’t required, but always check local regulations before installation.
Step-by-Step Installation Guide:
- Choose a Location: Pick a convenient spot on your irrigation line. If your sprinkler system is already installed, you’ll need to cut into the existing PVC pipe.
- Measure and Cut: Determine the correct length for your backflow preventer and cut a section of PVC pipe accordingly.
- Prepare the Pipes: Sand the ends of the cut PVC pipe and the inside of the AVB couplings. Clean all surfaces thoroughly.
- Attach the AVB: Apply PVC cement or two-part glue to the connections. Quickly insert the pipe into the coupling, press firmly, and hold for about 15 seconds while it sets.
- Check the Flow Direction: Make sure the AVB is installed correctly, allowing water to flow away from your home.
Pressure Vacuum Breakers (PVBs)
PVBs function similarly to AVBs but add an extra layer of protection. They feature:
- A design that prevents sudden pressure spikes from reversing water flow
- A check valve and air inlet valve to prevent backflow.
- A high-pressure chamber that ensures water only flows in one direction.
Image: Courtesy of Watts
PVBs and DCVAs are best for medium-risk applications like residential sprinkler systems, commercial irrigation, and some fire sprinkler systems.
How To Install It
Because PVBs protect against more serious contamination risks, most states require professional installation and permits. DIY installation is typically not allowed.
A licensed plumber should install the device, ensuring it meets local plumbing codes and has proper drainage.
Reduced Pressure Zone Assemblies (RPZs)
RPZs are the most advanced and effective backflow preventers, designed for high-risk situations. They feature:
- Two check valves for redundancy.
- A low-pressure chamber between them.
- A pressure relief valve that automatically drains water if backflow occurs.
If pressure builds inside the chamber—indicating backflow—the system releases water through the relief valve until the pressure normalizes.
Image: Courtesy of Watts
RPZs are best used for high-risk applications like commercial buildings, hotels, municipal water systems, and industrial fire sprinkler systems.
How To Install It
RPZs must be installed by a certified plumber and require a permit in most areas. They are typically placed outdoors or in designated drainage areas since they discharge water when backflow occurs.
Where To Get Help With Installing a Backflow Preventer
Installing a basic backflow preventer on a sprinkler system is a manageable DIY project for most homeowners. However, installing one on a water main or high-volume supply line often requires a licensed plumber—and in many areas, it’s legally mandated.
If you’re unsure about the installation process or need a certified professional, use the tool below to find reputable, licensed plumbers in your area. Proper installation ensures your system is up to code and fully protects your water supply.
FAQs About Backflow Preventers
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.
What does a backflow preventer do?
A backflow preventer stops water from flowing backward through your pipes, preventing contamination of your drinking water. In a sprinkler system, for example, it blocks fertilizers, pesticides, and bacteria from getting into your home’s water supply—or even the public water system.
How do I know if my home has a backflow preventer?
If you have an in-ground sprinkler system, you likely have a backflow preventer installed. It’s typically located above ground, between two vertical pipes that make a 90-degree turn toward each other. You’ll often see a ball valve or shut-off handle on the device.
If you’re unsure whether you have one, check near your irrigation system or ask a plumber to inspect your setup.
What’s the difference between a check valve and a backflow preventer?
A check valve is a simple, one-way valve that prevents small amounts of water from flowing backward. You’ll find them in toilets, dishwashers, and faucets to stop minor backflow.
A backflow preventer is a more advanced device designed for high-risk situations like irrigation systems, fire sprinklers, and main water lines. It often includes multiple check valves, pressure chambers, and relief valves to prevent contamination even under extreme pressure changes.
