The Safe-T-Cover Blog

How Does an RPZ Work?

Posted by Craig Carmon on April 28, 2021

It's important to understand how the Reduced Pressure Zone (RPZ) backflow preventer works. Most mechanical/plumbing engineers don't spend much time thinking about the backflow preventer during the design of the project, and in many cities across the country, the backflow preventer is installed inside the building.  

Reduced pressure zone backflow assemblies (RPZ valves) are extremely important. They’re used to keep contamination or pollution out of the public drinking water and provide the highest level of protection.

That’s why there is a growing trend across the country where water jurisdictions are requiring the use of the RPZ backflow prevention assembly. In fact, many areas are now requiring the RPZ on all domestic and fire service lines. So, even if you’re in an area where single checks or double checks were once acceptable, that could change.

The Mechanics: How Does It Work?

RPZ backflow assemblies consist of two independent check valves. They work like a double-check (DC) backflow preventer, but also have an intermediate relief valve that opens to the atmosphere if both check valves should fail.

Backflow preventers work by letting water flow through them in one direction, but prevent water from flowing back through them in a reverse direction. This is how the water supply is protected. A commercial building or industrial plant can use all the water it wants, but once the water has passed through the meter and the backflow preventer, it will not be allowed back into the water supply. An RPZ provides the best level of protection because it has a built-in relief valve, which will open up and actually dump the backflowing water out of the valve, in order to prevent any chance of contaminated water re-entering the water supply. For more details about the differences between DC and RPZ backflow preventers, check this out.

To see how the RPZ works in various situations, watch this less than 6-minute video.  The video includes illustrations and scenarios of when and why the RPZ dumps water.

Why is the RPZ Important?

The RPZ indicates whether the valve is working properly or if service is needed. If no water is dumping out of the relief valve, the backflow preventer is working properly. If the relief valve is dumping out water or spitting out small or large amounts of water, then something is not right and maintenance on the valve is required. This adds a level of safety to protect the public drinking water and is why more and more municipalities across the country are requiring the RPZ for both domestic and fire lines.

Where Should the RPZ be Installed?

Now that the RPZ is being required in many jurisdictions, thinking about where to install it becomes important. There are two options: inside in a mechanical room or in an RPZ backflow cover.

Because an RPZ is designed to dump water, the surrounding area is going to get wet. An RPZ that is dumping water is working exactly how it’s supposed to. Unfortunately, this is not a good situation if the RPZ is installed inside a mechanical room or in a utility closet as the surrounding area will be damaged by the water pouring out of the backflow preventer. rpz valveIn this picture, the RPZ was installed in a mechanical room. Notice the electrical panels! The plant moved the enclosure outdoors when someone determined this was not a wise location for the RPZ.  Read more about this installation here.

An RPZ will dump water. This short video shows an RPZ inside an enclosure in action. Additionally, the relief valve discharge rate charts for a popular backflow preventer manufacturer, show that an RPZ can discharge an incredible amount of water in a very short amount of time. Check out how much water dumps from this small diameter RPZ. Now that you’ve seen these videos and discharge rates, an RPZ should be installed outside the building in an insulated backflow cover.

Backflow covers were designed with RPZ installation in mind. ASSE 1060 regulations ensure that backflow enclosure manufacturers are designing their products properly. The ASSE 1060 testing requires that backflow covers provide a drain for when the RPZ inevitably dumps water and the drain requirements are strict. For backflow preventers sized 4" and larger, the drain must be able to move 710 gallons per minute. Backflow covers made to these standards provide easy access for maintenance and repairs. The materials used for RPZ enclosures are designed to withstand the elements and are tested for structural strength as well as security.

A backflow cover provides excellent protection against theft and vandalism, not only because of the durability of the enclosure but also because the device is hidden from view. If an RPZ was to discharge water, backflow covers provide visible means to detect it. If the RPZ were dumping water inside the building, water would rise until someone entered the mechanical room, or until the damage was so great it surpassed the mechanical room. A heated backflow cover is a perfect place to install the RPZ, providing the public drinking water supply with the highest level of protection, and protecting the building from water damage.

Backflow Cover – The Right Decision for RPZ Placement

The RPZ will absolutely protect the public drinking water supply, but if installed in the wrong space, it can potentially cause flood damage to the building. It is important to use best practices when installing an RPZ, and the best place for installation is outside in an RPZ backflow cover.

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Topics: RPZ Flooding