Double-check valve assembly and reduced pressure zone valve assembly. If you are at all involved in construction design, you’ve undoubtedly heard these terms before.and you feel pretty sure that you know what they mean. If these are new phrases to you, or if you just need a refresher, here’s a quick rundown on what they are and how they differ.
A designer may specify one of two types of backflow prevention devices. First, the double-check valve assembly (DC). A DC is typically considered the appropriate solution for low-hazard conditions. Secondly, the reduced pressure zone valve assembly (RPZ), is typically thought of as an appropriate solution for high-hazard conditions.          

What is a hazard? 

The simple answer is that the purveyor determines it, but every jurisdiction is different. Every municipality has its own list of specific examples for what meets the hazard threshold. They also stipulate that if the anticipated use, as you know it, is not named explicitly, then they reserve the right to make the decision during plans review. It’s also important to understand that you cannot, as a designer, over-protect incoming water service. This means there is no penalty for providing a higher degree of protection.

The difference between double-check and reduced pressure zone assembly

The double-check assembly (Figure 1) was developed in the 1950s for the fire industry. Any time the pressure on the property (downstream) side exceeds the pressure on the city (public) side, the two redundant check valves close, stopping backwards water flow. 

There are two problems with the double-check backflow preventer. First, no remedy exists in the event of a malfunction of the valve closures or if debris in the water line causes the valves to not close completely. Second, and this is the big problem, there is no way to know when such a failure has occurred without conducting a full test by a qualified professional. A DC valve assembly is a closed system. It has no method of revealing whether the internal check valves are functioning properly and no way to detect the presence of debris that is impeding full closure.

On the other hand, the RPZ valve assembly (Figure 2) consists of two independently operating check valves just like the DC, as well as a hydraulically-operated differential relief valve located below the first check valve.

This hydraulic valve and its placement makes the RPZ virtually failsafe, but it comes at a cost to the area around the device.

Where DC valves can fail, RPZ valves succeed: An example 

When a flow stop occurs, both check valves close. At that moment, the relief valve opens and evacuates the water between the valves. (Figure 3) Some think this event defines the limit of how much water can ever flow from the RPZ into a drain. 

This is not the case!

Consider a flow-stop situation, like one that might naturally occur at the end of the day (Figure 4). If you look closely, you can see that a small pebble has lodged in the #2 check valve. Now, consider some sort of back siphon event overnight. Perhaps a nearby building fire leaches water back with the draw of a fire hydrant, or a pump station fails, or a water main breaks. Because the #2 check valve won’t close, the water that has been delivered to the building will continue to flow out the relief valve until the private lines are cleared. 

If this is a four-story building, that’s a lot of water!

Watch the video below for an example of RPZ flooding and to see just how much water is discharged when a small amount of debris becomes lodged in the device.

Now consider a full failure of the #2 check valve, like one that might occur if the device is knocked out of round or has a mechanical failure. 

It’s essentially the same event, with the exception that with a larger valve opening, there is actually a higher flood rate. Now water is flowing through the relief valve at full head pressure. Again, if this is a large or multi-story building, that’s a lot of water very fast.

RPZ is the preferred solution for backflow prevention

Both backflow assemblies are designed to perform the same function, but the RPZ is also designed to protect the public water supply by disposing of any backwards-flowing water if either the check valves or the relief valve fails. What matters most is that, regardless of your backflow solution, it is regularly inspected and maintained.

“The RPZ is the preferred backflow preventer in high hazard applications because it will show you when something is wrong in the system and you can address it immediately,” Sara Hiers, Regional Channel Manager at Watts Water Technology, told Safe-T-Cover. “The DC is a testable assembly that prevents all types of backflow, but the only way you know it is working properly is to test it on a regular basis.”

The advantage is clear. Both can do the job, but only one gives immediate indication of any issues while not needing the added time and investment of testing. The RPZ is the preferred choice for backflow prevention.