Types of Backflow Prevention Devices

What is Backflow?

Backflow is the unwanted reverse flow of substances into a potable water distribution system. There are two main types, back siphonage and backpressure. According to the Florida Department of Environmental Protection, backsiphonage is caused by negative pressure in a potable water supply, whereas backpressure occurs when the downstream pressure is greater than the upstream or supply pressure.

Suppose an irrigation system installed in a residential area experienced an increase in water demand on a hot summer day when many residents were using water simultaneously. This high demand could cause the pressure in the main water supply to drop below the pressure in the irrigation system, leading to backsiphonage. Now let’s imagine that there is an irrigation system installed in a hilly area where the water supply comes from a higher elevation. If the sprinklers used to distribute water were not properly regulated and set to produce a higher water pressure than the water supply could handle, backpressure could occur.

What is Backflow Prevention?
Backflow prevention refers to the set of measures and devices designed to prevent the reverse flow of water or other substances from entering the main water supply system. Complete prevention programs consist of two types of protection. One is containment, which protects public water supply at the meter, and the other is isolation, which ensures point of use protection. Backflow preventers must meet specific standards and regulations established by local authorities, so it’s important to consult them before proceeding with any installations.

Why is Backflow Prevention Important?
Backflow can introduce harmful substances into the water supply, posing significant health risks to the public. Implementing proper prevention measures minimizes the risk of these contaminants being consumed or used for purposes like cooking, bathing, or irrigation, helping to maintain public health and safety. Backflow incidents can also cause damage to your clients’ properties’ plumbing systems. Taking steps to protect their property reduces the chances that they’ll have to undergo costly repairs.

Types of Prevention Devices

There are various types of backflow prevention devices that can be employed depending on the level of protection you require and your specific circumstances. Local building codes, the size of the landscape, and the installation location all are factors that must be taken into consideration when choosing a backflow prevention device. Always check with local codes to ensure proper installation as they vary between municipalities.

Atmospheric Vacuum Breakers (AVBs)
AVBs are typically made of brass or plastic and consist of a housing with an inlet connection, an outlet connection, and an air inlet valve. These devices should be installed vertically to allow for proper air intake and to prevent water from entering and obstructing the air inlet. According to Colorado State University, they should also be installed 6 inches above the highest point of the downstream water outlet.

AVBs rely on the physical barrier of an air space to prevent backflow and are not affected by pressure differentials and can therefore be used in situations where continuous pressure isn’t guaranteed. The devices exclusively prevent back siphonage and should not be installed anywhere that protection against backpressure is required. They are non-testable and occasionally emit water discharge. AVBs are commonly used in low hazard applications, such as residential irrigation systems and some commercial plumbing.

Pressure Vacuum Breakers (PVBs)
Pressure vacuum breakers are the most common and inexpensive type of backflow preventer. They consist of a spring-loaded check valve that ensures water flows in one direction, coupled with an air inlet valve that opens to allow air to enter the system when downstream pressure exceeds supply pressure. This design prevents the formation of a vacuum effect and subsequent backsiphonage. PVBs require a continuous stream of pressure to function properly; if the supply pressure becomes intermittent, the air inlet valve may open, compromising the device’s backflow prevention capabilities. The devices should be installed 12’’ above all downstream piping and outlets but may have shut off valves installed downstream, and should be positioned vertically to facilitate the release of air and to prevent water from accumulating.

PVBs only provide protection against backsiphonage, and the installment of these devices is prohibited anywhere backpressure poses a potential threat. PVBs are commonly used in residential and commercial plumbing systems and are frequently installed on irrigation systems and outdoor faucets. They will not provide adequate protection in high-hazard situations, as in any facility dealing with hazardous chemicals or toxic materials.

Double Check Valves (DCVs)
Double check valves consist of two independently operating check valves. These valves are typically made of brass or bronze and have a spring-loaded mechanism that ensures water flows in only one direction, which is key in preventing backflow. Unlike the other prevention devices, DVCs offer protection against both backpressure and backsiphonage, making them the ideal device for irrigation, fire sprinkler, and other plumbing systems. PVBs should be installed in a vertical position to allow for proper functioning. Their built-in testing ports allow for convenient testing and maintenance, both of which are essential to ensuring proper functioning of the device. Testing involves checking the closure of each check valve and verifying that each valve can operate independently and efficiently.

Reduced Pressure Zones (RPZs)
According to experts at Colorado State University, RPZs are designed to provide the highest level of protection against backsiphonage and backpressure. The device operates similarly to double check valves, but also contain an independently acting pressure relief valve between the two check valves. The check valve ensures water flows in one direction while the relief valve opens to release any excessive pressure that may build up between the check valves. Colorado State University also states that RPZs should be installed vertically to allow for proper drainage of the relief valve and a minimum of 12” of access between the lowest portion of the device and grade is required when installing the device. When selecting the installation location, refer to local regulations and system requirements.

RPZs rely on continuous pressure to maintain proper closure of the check values. If pressure on either side of the device becomes equal or reversed, the check values may fail to close, and you run the risk of potential contamination. The devices typically include built in testing ports that provide easy access to each check valve and the relief value. Regular testing is crucial to ensure proper functioning and reliability and faulty components should be promptly addressed. RPZs are commonly used in high hazard situations where the risk of contamination is significant and are often required for industrial applications, commercial buildings, ad large-scale plumbing systems.

Choose SiteOne:
SiteOne associates are available to answer any questions you may have regarding backflow prevention device selection and installation. For more educational articles, refer to the irrigation portion of our Learn section. For more information on check valves, get a rundown on the basics here.