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What Is a Backflow Preventer and Why Do You Need One?

Candace Nelson
Written by Candace Nelson
Updated October 20, 2021
Mom watches daughter wash hands at kitchen sink
kate_sept2004/E+ via Getty Images

“Backflow” and “water” are two words that should never go together. This is why we have backflow preventers

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When you turn on your kitchen faucet, you expect to see fresh and clean water. But once in a while, things can go backward (literally) and your dirty water can seep into your fresh water system instead. This is called backflow, and it’s most definitely something you want to avoid. 

That dirty, detergent-infused water draining from your washing machine after a heavy load of whites? Your backflow preventer is what redirects that “used” water and stops it from mingling with the fresh kitchen sink water you were going to use to boil tortellini for dinner. 

Simply put, a backflow preventer is a vital system you should consider for your home. Learn more about how it helps keep your family safe and what type of system is right for you.

What Is a Backflow Preventer?

A backflow preventer is a device that attaches to water supply pipes and keeps the water flowing in one direction only. The preventer can also consist of a combination of valves that connect to sewage pipes and sprinkler systems. 

Most commonly, simple backflow valves are found on your outdoor faucet or somewhere near the outdoor water source. Inside, you might have a more comprehensive system near your home’s water main line on the street side of the lowest level of your house or in your basement floor drain.

Backflow (or back siphonage) occurs when there’s a change in pressure, such as when pipes freeze or someone taps the fire hydrant on the corner. This causes the water pressure to switch direction and push water back toward the source. A backflow preventer stops contaminated water from getting siphoned back into the supply where contaminated water could mix with clean water. 

The good news is that you’ll know right away if your dirty water is mixing with clean because your water will look, taste, or smell funky. Backflow is considered a plumbing emergency and should be reported to the city’s water authority. The city might have to shut down the system for cleaning.

Permits May Be Required

Backflow preventers mitigate the potential for contamination, but not all cities require them. Even if yours doesn’t, it’s a good idea to protect your home against backflow as water contaminated with debris, sentiment, chemical, metal, and even pesticides are an obvious health concern.

Check with your city for backflow preventer requirements and permits. If required, you can request a permit for less than $50. A licensed plumber can identify the best places to install a backflow preventer in your home and set it up for you.

Where Are Backflow Preventers Used?

Requirements for backflow preventers will vary across cities and states. Check your local requirements on your city’s website. For most residential areas, here’s where you will typically need a backwater prevention system:

  • Hoses and irrigation systems (to keep bacteria, sediment, and chemicals used on lawns from seeping into the city’s water supply)

  • The water main into the house

  • Anywhere potable water (drinkable water) connects to non-potable water, such as in HVAC, manufacturing, or outdoor sprinkler systems

What Are the Different Kinds of Backflow Preventers?

The type of backflow preventer you’ll need in your home depends on the hazard level involved with the water use. Most plumbers are familiar with these prevention systems and can help you with a plan to keep your water clean and home dry. They can also help you get a permit if necessary.

For Low Hazards, Use Check Valves

The simplest form of backflow preventer is a check valve, most commonly used in low-hazard situations where backflow wouldn’t pose a danger to health, just a funky color or smell. Your indoor sinks and tubs are examples of low-risk situations since this water doesn’t cross with waste water. 

Check valves only allow water to flow in one direction. If there’s a change in pressure, the valve closes itself before the water can flow in the wrong direction.

  • A single-check valve is about $20 at a hardware store and screws easily onto a faucet. However, these valves do not have a safety feature in case the valve malfunctions.

  • Double-check valves are more common because they offer a backup valve in case one fails. Use them on faucets and appliances that might come into contact with chemicals or cleaning supplies—like a swimming pool, darkroom, or washing machine. These cost about $150.

For Medium Hazards, Use a Pressure Vacuum Breaker

Kids playing in water sprinkler
Ariel Skelley/DigitalVision via Getty Images

Outdoor hoses and sprinklers are considered medium hazards and might have a pressure vacuum breaker to protect against backflow. If you have one, it’s probably located above the highest point of the irrigation system, usually near the water source before the water flows into any sprinklers or hoses.

 A pressure vacuum breaker has a check valve plus an air inlet. When the air inlet is closed, water can flow through. When the pressure changes, the air inlet will open to stop water from flowing through the wrong way. Pressure vacuum breakers cost $200 to $300.

For High Hazards, Use Reduced Pressure Zone Valves

Reduced pressure zone (RPZ) valves offer the highest protection against backflow. Two check valves work independently to keep the water going in one direction. They are required in many commercial settings like restaurants, but some cities require them for irrigation systems as well. 

RPZ valves typically require a certified inspector to sign off on their functionality annually. The cost for installing these systems starts around $1,000.

How to Maintain a Backflow Preventer

Your city might require annual inspections on backflow preventer systems, especially if you plan to relocate it, or if it’s connected to an irrigation system. Even if an inspection isn’t required, it’s still a good idea to get your backflow system checked annually. A licensed plumber will connect a testing kit to make sure it’s working correctly.

 Before the first freeze of the winter, winterize outdoor water systems, including backflow preventers connected to sprinkler or irrigation systems. If you’re unsure how to prevent frozen or cracked pipes, a local lawn irrigation company can help safeguard your system for winter.

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