How RPZ Backflow Preventers Protect the Drinking Water Supply

Reduced pressure zone (RPZ) backflow preventers are the best protection you can buy, especially for high-hazard applications like in irrigation. In general, irrigation systems require an RPZ device because they can come into contact with fertilizer and pesticides.

Let's take a closer look at reduced pressure zone assemblies, what they do, and why they're so important.

About Reduced Pressure Zone Assemblies

RPZ backflow preventers consist of two independently operating check valves. They work like a double-check valve, but they also have an intermediate relief valve spring that opens to the atmosphere if either check valves should fail.

RPZ assemblies also include four test cocks (or test ports) for routine testing and maintenance, along with shutoff valves to isolate the assembly.

For these reasons, RPZs are increasingly being required across the country. Many municipal, state and national plumbing codes mandate RPZ assemblies for high-hazard applications. An RPZ provides the best level of protection: the built-in relief valve responds to downstream changes in supply pressure and opens during a backflow event if a check valve fails. The water only dumps if there is a problem with one of the check valves.

Rusty Atha, of Able Too Environmental, sent us this video of an RPZ dumping water inside a mechanical room in an arts building in Atlanta, Ga. This shows just how much water an RPZ valve can dump per its design.

How To Test a RPZ Valve

Visual checks of a RPZ backflow assembly are not a substitute for formal testing through the assembly’s integrated test cocks, because proper operation depends on how water flows under test conditions. If the pressure has failed, the relief valve will engage. When that happens, water discharges to the drain as the relief valve opens during a failure condition or other exception.

Using RPZ Backflow Preventers for Better Backflow Prevention

Municipalities are updating their standard details and local plumbing codes to conform with recommended best practices for clean municipal water supplies, and many municipal, state and national plumbing codes mandate RPZ assemblies for high-hazard applications. Some notable examples around the country include:

• Fort Worth, TX
• Lynchburg, VA
• Delaware, OH

RPZ assemblies are also commonly specified in fire protection systems, especially where stagnant water or antifreeze can increase hazard concerns.

These new standard details typically require the use of reduced pressure zone assemblies, and mandate that the device must be tested once each year for code compliance and ongoing maintenance. Once the RPZ is tested, the certification paperwork is forwarded to the city.

Because RPZ backflow preventers discharge water (and lots of it) when they're working properly — they can cause flooding and serious damage depending on where the devices are installed. That's why more and more RPZ backflow preventers are being installed outside and above ground in backflow enclosures. Historically, RPZs have been installed in a basement or somewhere else inside the building (like next to the mechanical room) and when RPZ releases the water, everything near it will get saturated.

Where To Install an RPZ Backflow Preventer

There are basically two options for where to install an RPZ backflow preventer. The key difference from a double check valve assembly is that an RPZ provides higher-hazard protection. Those options include inside in a mechanical room or in an RPZ enclosure.

An RPZ that is dumping water is working exactly as designed.

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, so the installation method must also account for a safe drain.

An air gap is the most effective backflow prevention method, but when a mechanical device is required, RPZ protection still needs safe drainage.

We strongly recommend installing outdoors and above ground inside an ASSE 1060 compliant RPZ enclosure.

Move the RPZ Outside

If you're an architect or design engineer, consider the evidence when looking at different types of backflow prevention devices. As the designer, select the right option for the hazard level and location. Why take the risk of installing the RPZ inside the building? There's nothing to gain and several hidden costs.

Don't to go with the status quo on your next project's RPZ backflow device design. Keep your client's best interests in mind and move it outside and above ground. A figure or specification detail can also show how water supply pressure and discharge may affect downstream property conditions.