What Is a Backflow Prevention Device and How Does It Work?

Posted by
John Woodard on April 10, 2024

In 1933, Chicago hosted the World’s Fair and, unintentionally, a backflow disaster. The sewage and plumbing systems in Chicago at the time were undersized, and the influx of visitors led to disease caused by a contaminated water supply. Around 100 deaths and over 1000 cases of amoebic dysentery later, people realized that backflow was not something that should be taken lightly. Since 1933, plumbing codes have been created that require backflow prevention devices to be installed in water applications. Today’s plumbing codes essentially eliminate the risk of backflow, but proper understanding of how backflow prevention systems work is key to understanding if the systems are protecting your home and business. In this article, you can find information on what backflow is, types of backflow prevention devices, how these devices work, and how to install them in your home.

What is backflow?

Backflow is a plumbing term that refers to the pollution of a clean water line by contaminated water flowing in the reverse direction. Backflow occurs in cross-connections, such as in dishwashers and reverse osmosis systems, between potable and non-potable water lines and results in water that is unsafe to consume. For example, dirty dish water can siphon into your clean water line and dirty your drinking water. Two types of changes in water pressure, back pressure and back siphonage, are the primary causes of backflow.

What is back pressure?

Back pressure results from wastewater systems with higher pressure than the incoming fresh water supply. Problems within the sewer line can cause pressure to increase and lead to back pressure, but the most common source of back pressure is a pressure decrease in the supply line. Back pressure most often occurs in pressurized systems, such as elevated tanks, pumps, or boilers. Sprinkler systems and power washers can also fall victim to backflow via back pressure.

What is back siphonage?

Back siphonage is caused by negative pressure in the supply system. An unexpected pressure decrease within a system can cause water from the sewer line to be pulled back into the clean water line. Back siphonage most commonly results from a breakage or repairs on a water main or from heavy demand such as firefighting.

Is backflow testing required?

Backflow testing is required for businesses on a yearly basis. Some states, such as Texas and California, require homes to be tested annually for backflow as well. Even if your local legislation does not require backflow testing, it is recommended that you get a licensed plumber to check on your home's plumbing. To test for backflow, plumbers shut off your water and use gate and relief valves to check for leaks, unwanted movement, and the functionality of backflow preventers. If backflow testing is not required by law where you live, you can test for it yourself. However, backflow testing tools are expensive and testing can be difficult, so it is recommended that a professional checks your system for potential harm.

What is a backflow prevention device?

A backflow prevention device is a product that ensures water can only flow in one direction. There are multiple types of backflow prevention devices, and they all work best in specific applications. All backflow preventers, no matter the type, protect clean water lines from wastewater contaminants. The following are three common types of backflow preventers:

Backflow preventers should be tested yearly for wear and defects. All businesses are required by law to have backflow prevention devices tested yearly, and homes are subject to testing based on local legislation.

Types of backflow prevention devices

A backflow prevention device works by prohibiting wastewater from reversing into the supply water line. How a specific backflow prevention device works depends on the application it is used for, as they each employ unique mechanics to protect the clean water from contamination. Below you can find how air gaps, vacuum breakers, and check valves operate to eliminate backflow.

Air Gaps

Air gaps are used in sinks to prevent debris from entering drinking water or a dishwasher. They are a literal gap of air between the water outlet and flood level of a dishwasher. Air gap devices are used to prevent backflow with air gaps. They are generally inexpensive and come in a variety of materials and finishes. Air gaps eliminate contamination risk by separating the hose that runs dirty water and the hose that runs to the drain. Air gap devices separate these two hoses, making contamination impossible because these hoses never intersect.

Dual inlet air gap devices can connect two dishwashers to one drain line or drain both a dishwasher and a water filtration system simultaneously. These devices are particularly useful if you have an under-sink reverse osmosis system and want to avoid contaminating other appliances.

Air gaps and air gap devices are essential if you wish to avoid your dishwasher flooding with contaminated water. Plumbing codes in many states require air gaps to be included upon dishwasher installation, so air gaps may be essential in keeping your house up to code. States that do not require an air gap typically require a "high loop" drain connection that uses gravity to prevent backflow.

Learn More: What are dishwasher air gaps and are they necessary?

Vacuum Breakers

Atmospheric vacuum breakers (AVB) prevent backflow in hose, faucet, and spigot applications. AVBs contain a check valve that opens an air vent when the system loses pressure. When open, the air vent breaks the vacuum, and the water supply line is sealed off. This process prevents back siphonage from occurring when pressure is lost in the supply line.

AVBs must be installed at least six inches above ground to be effective in preventing backflow. If air contaminants are present, they can enter the AVB and contaminate the pipes. AVBs are not effective in a consistent pressure water system and should not be placed upstream of a shutoff valve.

Check Valves

Check valves are devices that ensure water flows in only one direction. They open and shut based on the pressure on both sides of the valve. When pressure is higher on the incoming side than the outgoing side, the valve is open, and water can flow like normal. When pressure is higher on the output than the input, the valve shuts, eliminating the risk of water being pulled into the clean water line.

Check valves can be installed at the service entrance or at individual water outlets. Unlike atmospheric vacuum breakers, check valves are designed for continuous pressure systems. Some valves can be installed either horizontally or vertically, and others must be installed in one orientation or the other. Swing check valves, for example, cannot be installed vertically. Horizontal installation is preferable because gravity will not affect a check valve’s performance. However, you should install your check valve in the orientation it was designed to work in.

Do I need a backflow prevention device?

You must have a backflow preventer installed at the water meter if you have an irrigation system or get your water from a well. If you have recently noticed discoloration, sulfuric smells, or bad taste in your water, you will want to get your system checked by a plumber. Even if your water does not show any of these signs, backflow prevention devices are recommended anywhere that potable and non-potable water lines connect. If you do not have backflow prevention devices installed, the purity of your drinking, cooking, and cleaning water is put at risk. If pressure changes in your system and you do not have a preventer, you are at risk of backflow.

Modern plumbing codes require backflow prevention to be installed wherever a cross-connection exists. If the plumbing in your home is old, you are at greater risk of having no backflow prevention systems installed.

How to install a backflow prevention device

You can install some backflow prevention devices yourself, and others must be installed by a licensed plumber. The ease of installation depends on the type of device and where it will be installed.

Air Gap Installation

Air gap installation is a straightforward process you can do without the assistance of a plumber.

  1. Locate or drill a hole for the air gap located where water can spill from the air gap back into the sink if it should have back flow.
  2. Assemble the air gap in the hole following the manufacturer’s instructions.
  3. Attach the drain line from the dishwasher to the air gap inlet – typically a 5/8” ID hose.
  4. Attach the air gap outlet to the garbage disposer inlet or drain line connection – typically a 7/8” ID hose. 
  5. Run the dishwasher and inspect for leaks.

Vacuum Breaker Installation

Vacuum breaker installation is extremely straightforward. To install an AVB, simply screw the breaker onto your hose, faucet, or spigot. Ensure that the size of the breaker you purchase matches the size of the system you wish to connect to. Most vacuum breakers come with a locking feature that prevents the connection from separating.

Check Valve Installation

Check valves come in many types, and installation will vary based on what you buy. The Watts LF7RU2-2, for example, can be installed both horizontally and vertically. Many valves can be installed yourself, and more complex units will likely require the help of a plumber. You should check the documentation of your check valve for specific installation instructions. Here are some general check valve installation guidelines.

  • Ensure check valves are installed in the correct flow direction. If installed backward, increased pressure can cause damage.
  • Install check valves in accessible locations. If your valve requires servicing or testing, it will need to be reached easily.
  • Flush pipelines before installing your check valve. Foreign debris can interfere with a valve’s functionality.
  • Avoid installing check valves in areas where freezing is possible.
  • Do not overtighten your check valve. Many incorporate O-rings that help seal the unit.

What causes backflow prevention device leaks?

Immediately after installation, you can expect a small amount of water to come out of your backflow prevention device. If your backflow preventer drips constantly, there is a leak that needs to be addressed.

Causes of backflow leaks

Backflow prevention leaks can be caused by many factors, and some are more preventable than others. Here are some common causes of backflow prevention device leaks.

  • Debris
  • Improper installation
  • High pressure
  • Old parts

How to fix backflow preventer leaks

Many factors can cause a backflow prevention device to leak, so you must first identify the cause before you attempt to fix your preventer. If an air gap leaks, it may be doing its job. It also may have been installed incorrectly, or there could be an issue with the drain line.

Debris in your device can be removed by cleaning its seal. Over time, dirt and grime can build up, reducing the effectiveness of the device’s seal. To eliminate debris from your preventer, remove the spring assembly and thoroughly rinse it until the debris is gone.

Improper installation of a backflow prevention device can also cause leakage. Preventers are easy to install incorrectly, and small mistakes can cause your device to malfunction. A common mistake when installing a backflow preventer is overtightening the head assembly. Always tighten the head assembly by hand so that it does not overtighten. Overtightening can damage the seal of the device and necessitate the washer being replaced. You must also ensure that the device is installed in the correct direction. Since backflow preventers force water to flow in only one direction, installing it backwards eliminates its usefulness.

High pressure in a backflow prevention device can trigger a pressure relief valve. When opened, the valve will leak until the pressure returns to normal levels. High pressure indicates a problem in your water system, not your backflow preventer.

Old parts in a backflow preventer must be replaced over time to avoid leakage. If your device is leaking, check the washer in the seal. If the washer is deteriorated, simply replace it to repair the seal.


If you have any further questions about backflow prevention, please don’t hesitate to contact our experts.

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