What Does a Backflow Preventer Do?
If you’ve ever wondered why cities require a backflow preventer in your plumbing system—or what one even looks like—you’re not alone. In this article, we’ll explain what a backflow preventer does, why it’s so important, and where you’re most likely to find one in your home or business.
What Is a Backflow Preventer?
A backflow preventer is a plumbing device that keeps contaminated water from flowing backward into your clean water supply. Without it, dirty water—often containing fertilizers, chemicals, or waste—can reverse direction and pollute your drinking water.
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Why Is a Backflow Preventer Important?
When water flows in the wrong direction (called backflow), it can introduce serious health risks. This can happen when there's a sudden drop in water pressure—like when a fire hydrant is opened nearby. A backflow preventer ensures water only flows one way, keeping your clean supply separate from possible pollutants.
Where Is the Backflow Preventer Located?
In residential homes, backflow preventers are typically installed:
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On irrigation lines (like your lawn sprinkler system)
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Near your outdoor hose bibs
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Sometimes right after the water meter
In commercial buildings, you’ll find them:
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Near water mains
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On fire suppression systems
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On boiler feeds and other high-risk water connections
Tip: Look for a metal or brass valve with test ports—it’s often labeled or tagged by your city.
Is a Backflow Preventer Required by Code?
Yes. Most local plumbing codes require a backflow preventer for:
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Irrigation systems
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Commercial buildings
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Fire sprinkler systems
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Certain appliances or devices that connect to your water supply
According to the International Plumbing Code (IPC 608.16), backflow prevention is legally required to prevent contamination.
From quick test adapters to replacement backflow fittings, we stock the tools professionals trust for accurate testing and dependable performance. Browse all backflow testing solutions available from NASCO Supply.
Residential Backflow Preventers
In homes, backflow preventers are often used on:
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Sprinkler systems (the most common)
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Boiler systems for heating
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Swimming pools or decorative fountains
Some systems use a simple atmospheric vacuum breaker (AVB), while others may use a more advanced reduced pressure zone (RPZ) assembly.
How Does a Backflow Preventer Work?
Backflow preventers typically use:
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Check valves that close automatically if water tries to reverse
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Air gaps or mechanical relief valves to release pressure safely
These devices only allow water to flow one way—from the municipal water supply to your home—not the other way around.
Does a Backflow Preventer Reduce Water Pressure?
Yes, slightly.
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Double check valve assemblies may reduce pressure by 6–9 PSI
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RPZ assemblies might reduce pressure by 9–14 PSI
This is usually not a problem, but low pressure can affect irrigation and fire systems, so proper sizing is essential.
What Does a Backflow Preventer Look Like?
A typical backflow preventer:
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Is made of brass or stainless steel
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Has two or more test cocks
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Often includes shut-off valves on either end
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Can be large (commercial) or small (residential irrigation)
Common Questions Answered
Do I need a backflow preventer if I have a well?
Yes, especially if you also use municipal water or irrigation.
How do I know if I already have one?
Look near your main water shutoff, irrigation system, or ask your water utility.
Why is water leaking from my backflow preventer?
That could be a failed relief valve or improper installation. Contact a licensed plumber for inspection.
Is a backflow preventer worth it?
Absolutely. It protects you, your family, and your neighborhood from water contamination.
Need to Buy a Backflow Preventer?
We stock a wide range of residential and commercial backflow preventers, including:
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Double Check Valve Assemblies
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RPZ Backflow Preventers
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Irrigation Vacuum Breakers
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Test Kits & Accessories
Whether you’re maintaining, testing, or upgrading a backflow system, having the right fittings makes all the difference. Shop our complete selection of backflow testing tools & items today.
A backflow preventer is a small device with a massive impact. Whether you’re installing a sprinkler system, maintaining a fire safety system, or updating your plumbing, this is one device you can’t afford to skip.
Need help choosing the right valve? Contact our team today for expert guidance!
Sources
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City of Durango: Why is a Backflow Preventer Required on Lawn Sprinkler Systems?
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Fairfax Water: Where in my home or business are backflow prevention devices required?
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Fire Systems Inc: What is a Backflow Preventer and What Maintenance Does it Require?
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Contra Costa Water District: Backflow FAQ
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ValveMan.com: Types of Backflow Preventers
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QRFS: Understanding RPZ vs. DCVA Backflow Devices
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[International Plumbing Code (IPC) 2006, Section 608.16]