How Does a Backflow Preventer Work?

It's important to understand how a backflow preventer works. If you’re like most of the plumbing engineers we’ve talked to over the last three years, you probably don't spend much timing thinking about the placement of the RPZ backflow preventer during the design of the project. Your firm has been specifying the valve indoors for many years and why make a change, right? Here’s the problem. That the way the RPZ valve works is that it is designed with a relief valve that opens if either of the check valves fails. What that means is water is dumped in the building until the problem is detected and resolved. In the meantime, your design decision to place the RPZ backflow preventer inside the building has resulted in a flooded building and more than likely your client and their insurer may be asking this question – why?

A Common Occurrence 

This video below shows how an RPZ valve works when installed outside of a building in an ASSE 1060 approved backflow enclosure. While some may think something is wrong with the valve, it is actually doing its job. Here's an in depth explanation of how an RPZ backflow preventer works as opposed to a double check backflow preventer. More than likely construction in the area caused debris to get up in the valve causing the RPZ relief valve to open. We just happened to be driving by and noticed water pouring out of the enclosure. We stopped, removed the access panel and shot this short video.

You may be surprised to learn how often this happens. At Safe-T-Cover, we typically get 2-3 phone calls per month from a worried passerby. All they can see is water gushing out of our boxes so they call us worried our equipment has failed. It hasn't, it's just the RPZ inside doing its just and protecting the water supply from backflow.

Unknown Risk

In the last three years STC has made presentations to hundreds of civil, plumbing, and mechanical engineers across the country. During these presentations we learned most designers just aren’t aware the relief valve could dump large volumes of water into the building. And those that did were incorrectly assuming the drain installed below the valve would be sufficient. This lack of awareness puts the building at risk of flooding if the relief valve opens.

Changing Standards

More and more water jurisdictions across the US are requiring the use of the RPZ instead of the double check. An RPZ valve offers better protection to the water supply. With this added protection comes the additional work of specifying and installing reduced pressure zone valves in a safe place. For those assuming the drain below the RPZ is sufficient, check out what a member of the Chicago ASPE Chapter, David DeBord, said in an article in 2013 “The floor drain capacity required for RPZs 3” diameter and larger are likely to be cost-prohibitive due to necessary pipe diameter and fall rates.” You can read the full article here.

Increase Awareness, Improve Safety

Now that you’ve seen the footage of the RPZ device doing what it is designed to, pouring water, what can you do differently? An option would be to move the backflow design to the scope of the civil engineer on your upcoming projects. If the RPZ valve is installed outside the building, you no longer have the design risk associated with the RPZ backflow preventer flooding the building. See page five of this catalog from Zurn, a backflow preventer manufacturer, for examples of where they expect backflow devices to be installed. Another option is to share this information with your colleagues to raise the awareness. As the RPZ is required by more and more water jurisdictions, the need to understand how it works and the risks associated with its placement become more and more important.


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