Backflow Basics: An Overview of Different Backflow Prevention Devices

Part two of this four-part series looks at both common and uncommon backflow preventers that are available

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In the first article of this four-part series, I covered common questions about why backflow prevention devices are necessary. Here I’ll cover some basic product information.

How does a backflow prevention device prevent backflow?

A testable backflow prevention device has two check valves — basically two layers of protection. The device allows water to push the check valves open when water is coming into the building, but closes them shut when water tries to go the wrong way. There are two different types of testable backflow prevention devices that are commonly specified — a reduced pressure zone backflow device and a double check backflow device. There are a few others that I will mention but do not get hung up on them. I mention them only because they will be on a certification test.

Double Check Backflow Device

A double check backflow device (DC) is the simplest and most common type of device. It has two check valves in a completely closed body — two layers of backflow protection. No matter how large the valve is, all DCs are equipped with 1-pound springs that hold the check valves closed during a backflow situation. Those 1-pound springs are what you are testing when you ultimately test a backflow preventer. The device passes if each check valve holds a differential of 1 pound or more on your backflow gauge. That means that the check valve is holding shut with at least 1 pound of force. The downside of a DC is that if the first line of defense fails and the second line of defense fails (both check valves fail to hold), a backflow situation could occur. That is why this type of valve is not specified for what a water company considers “high risk” establishments.

Reduced Pressure Zone Backflow Device

A reduced pressure zone backflow device (RPZ) is a testable type of backflow prevention device that is installed in “high risk” applications. The RPZ is similar to the DC. It has two spring checks, but also has a third check valve that is actually a vent open to atmosphere on the bottom of the valve (kept closed by the water pressure). The reason it is used in high-risk situations is because if the first check valve ever malfunctions, the bottom of the valve then opens and drains. This relief valve offers an additional layer of protection compared to the DC, which will allow backflow if both check valves malfunction. This is the reason that when you test an RPZ, you force water to backflow to make sure the RPZ discharges.

Residential Dual Check Valve

Residential dual check valves are different than the regular spring check you are used to installing that only has one spring check. They are a small device that has two check valves in line. However, it is not a testable device. Some water purveyors will specify this as a method of backflow prevention device even though it is not a testable device. This device has no test ports and is very small and easy to install on existing services, but it is not commonly specified by water purveyors. Do not confuse the residential dual check and the DC. The residential dual check is small and not testable while the DC is larger and testable, which is why it is called a “device.”

Atmospheric Vacuum Breaker

An atmospheric vacuum breaker (AVB) is not a testable backflow preventer and is not commonly specified by water purveyors. It contains an air inlet valve, check seat, and air inlet port. In basic operational use, water pressure pushes the seat up and closes off the air inlet. When the water flow stops, it falls back down to open the air inlet port and breaks the vacuum, allowing the device to drain as well as preventing back siphonage. The word “siphonage” is very important here. This device only breaks the vacuum to drain water and will not protect against back pressure. Think of this device as a frost-proof hose bibb with a vacuum breaker. When you shut the water off, the vacuum breaker opens and allows the water to drain. But as we all know, if a customer leaves the hose full of water with winter approaching, the vacuum breaker remains closed and everything freezes and breaks. Same thing goes for an AVB, which is why it is illegal to install a downstream shutoff valve of any type. It also has to be located at least 6 inches above any overflow rim or downstream piping.

Pressure Vacuum Breaker

A pressure vacuum breaker (PVB) is considered a testable backflow preventer, but it is also very uncommon. They were manufactured so as to give the plumber a testable vacuum breaker device. As for the risk, this device covers hazardous as well as non-hazardous situations. But just as with the AVB it cannot handle back pressure, only back siphonage.

In part three of this series, I will cover test equipment, valve location, orientation, and clearances.

About the Author

Anthony Pacilla is a registered master plumber for McVehil Plumbing in Washington, Pennsylvania. He has 22 years' experience in the plumbing and HVAC trades, and has a bachelor’s in business and economics from Thiel College. 


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