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.
Quick answer: A backflow preventer is a device that lets water flow only one way, so dirty water can’t get pulled or pushed back into your clean water supply. The main types are air gaps, vacuum breakers (AVB and PVB), double check valve assemblies (DCVA) and reduced pressure zone assemblies (RPZ). Which one you need depends on the hazard: RPZs are used for high-hazard connections like boilers and chemical systems, while PVBs and DCVAs are common on irrigation and fire lines. Most testable assemblies need a yearly test by a certified tester.
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 happens when the pressure on the downstream side of a system is higher than the incoming fresh water supply, which pushes water backward. It can happen when downstream pressure rises or when supply pressure drops. 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 a connected fixture, hose or system 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?In most areas, yes, for testable backflow assemblies like RPZs, double check valves and pressure vacuum breakers. Rules are set by your state and local water utility, not one national law. Most utilities require these assemblies to be tested when they’re installed and then once a year, which commonly applies to businesses and to homes with irrigation systems, fire sprinklers or boilers. Simple devices like air gaps and hose bibb vacuum breakers usually don’t need testing.
Testing is done by a certified backflow tester, who connects a differential pressure test kit to the test cocks on the assembly and checks that each check valve and relief valve is working. Many utilities only accept results from a certified tester, so check with your water provider before scheduling one. Even if your home doesn’t require testing, it’s a good idea to have a licensed plumber check your plumbing for cross-connections.
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 devicesA 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 at kitchen sinks to keep dirty drain water from backing up into a dishwasher or reverse osmosis system. They are a literal gap of air between the water outlet and flood level of a dishwasher. 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 float 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 the highest downstream outlet or piping to be effective in preventing backflow. If air contaminants are present, they can enter the AVB and contaminate the pipes. AVBs can’t be under constant pressure, so there should never be a shutoff valve downstream of one. For lines that stay pressurized, use a pressure vacuum breaker (PVB) instead.
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, should only be installed horizontally or in vertical lines with upward flow. 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.
Testable backflow assemblies: PVB, DCVA and RPZ
Homes and small fixtures often use simple devices like air gaps and hose bibb vacuum breakers. Irrigation systems, fire sprinklers, boilers and commercial buildings usually need a testable assembly. These have shutoff valves and test cocks so a certified tester can check them.
- Pressure vacuum breaker (PVB): A spring-loaded check valve plus an air inlet. Unlike an AVB, it can stay under constant pressure. Protects against back siphonage only, so it’s common on lawn irrigation.
- Double check valve assembly (DCVA): Two independent check valves in a row. Protects against back pressure and back siphonage for low-hazard connections, like many fire sprinkler lines.
- Reduced pressure zone assembly (RPZ): Two check valves with a relief valve between them that dumps water if either check fails. It’s the highest level of mechanical protection and is used for high-hazard connections like boilers, chemical feeds and medical equipment. RPZs must be installed above ground with room to drain.
Backflow preventer comparison
| Type | Back siphonage | Back pressure | Hazard level | Testable | Common uses |
|---|---|---|---|---|---|
| Air gap | Yes | Yes | High | No (visual check) | Dishwashers, RO drains, sinks |
| Hose bibb vacuum breaker | Yes | No | Low to high | No | Outdoor faucets, utility sinks |
| AVB | Yes | No | Low to high | No | Irrigation with no downstream valves |
| PVB | Yes | No | Low to high | Yes | Lawn irrigation |
| DCVA | Yes | Yes | Low | Yes | Fire sprinklers, low-hazard lines |
| RPZ | Yes | Yes | High | Yes | Boilers, chemical feeds, commercial buildings |
Exact requirements vary by state and water utility, so always check local code before choosing a device.
Do I need a backflow prevention device?Most local codes require a backflow preventer on irrigation systems, fire sprinkler systems, boilers and other connections that could push or pull contaminated water into your supply. Many water utilities also require one at the meter for commercial buildings. 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 deviceYou 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.
- Locate or drill a hole for the air gap where water can spill from the air gap back into the sink if it should back up.
- Assemble the air gap in the hole following the manufacturer’s instructions.
- Attach the drain line from the dishwasher to the air gap inlet – typically a 5/8” ID hose.
- Attach the air gap outlet to the garbage disposer inlet or drain line connection – typically a 7/8” ID hose.
- Run the dishwasher and inspect for leaks.
Vacuum Breaker Installation
Installing a hose bibb vacuum breaker is extremely straightforward: simply screw it onto your outdoor faucet or spigot. In-line AVBs and PVBs for irrigation systems are plumbed into the line and often installed by a plumber. Ensure that the size of the breaker you purchase matches the size of the system you wish to connect to. Many hose bibb vacuum breakers have a breakaway set screw that keeps them from being removed.
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.
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.
Backflow prevention FAQsDo homes need a backflow preventer?
Most homes need at least simple protection, like hose bibb vacuum breakers on outdoor faucets and an air gap or high loop on the dishwasher. Homes with irrigation systems, fire sprinklers or boilers often need a testable assembly, depending on local code.
How often does a backflow preventer need to be tested?
Most water utilities require testable assemblies to be tested at installation and once a year after that, by a certified backflow tester.
What is the difference between an RPZ and a double check valve?
Both have two check valves, but an RPZ adds a relief valve between them that drains water if a check fails. That makes an RPZ suitable for high-hazard connections, while a double check is only approved for low-hazard ones.
Why is my backflow preventer leaking?
The most common causes are debris on a seal, worn parts, improper installation or high pressure. An RPZ that drips from its relief valve is often a sign that a check valve needs cleaning or repair.
If you have any further questions about backflow prevention, please don’t hesitate to contact our experts.
