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Most plumbing engineers, building owners and property insurers have no idea how much water an RPZ assembly can discharge — or what that means for a mechanical room, a commercial building or a vault.
Watch this video of a 3" RPZ dumping water and you'll understand the risk immediately.
Here's more proof why you shouldn't install an RPZ indoors or in a vault, and what you should specify instead so you don’t end up with a room that looks like this.
Designers need to plan for worst-case conditions, so here's how cascading valve failures lead to catastrophic flooding. This diagram shows the normal flow of an RPZ.
Let's say a small pebble lodges in the No. 2 check valve. If a backsiphonage event occurs nearby in the system while that check valve stays open, all water delivered to the building continues flowing through the relief valve until the private lines clear. In a four-story building, that volume of water is significant.
Under normal operating conditions, this failure goes unnoticed and water continues flowing based on user demand. But with this imbalance in the system, changes in demand tend to rock the remaining valves open and closed sporadically. That instability makes the relief valve more prone to momentary openings, which allows debris to block its closure.
Once the relief valve is blocked open and the No. 1 check valve is inoperative, water pours through the relief valve until the supply is cut off. This reduces water pressure for the user, but delivery will continue. The real damage begins when the user stops using water — typically at the end of the day — causing flooding to accelerate. This is the scenario designers must prevent.
One hospital mechanical room took the full force of an RPZ failure. We have some rare photographs of such an event; these pictures almost never reach the public because no one wants this problem on display.
This first image shows a Wilkins 375 4” RPZ valve in a typical mechanical room setup. Note the 4” floor-drain to the right. During a weather event, this RPZ began dumping huge volumes of water — consistent with its intended design -— to protect the water supply.
According to EngineeringToolbox.com, a four-inch drain system installed with a 1% grade descent will clear as much as 93 gallons of clean water per minute. By dumping 375 GPM and only draining at 93 GPM, this facility could have flooded at a rate of 282 GPM!
These photos display the aftermath. The sudden water flow and volume moved the righthand wall into the next room, which happened to be a telephone and low-voltage wiring room. Damage exceeded $1 million. (Keep in mind that even a smaller 3” reduced pressure zone backflow assembly can fill a 12x12 mechanical room to its ceiling in about one hour...if the walls hold.)
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Through the insurance policy's subrogation clause, the property insurer pursued recovery from all risk holders: the engineer, architect, contractor, subcontractor and the most recent recorded tester. Only the tester was exonerated because he showed his annual test record from three months prior, indicating proper working order.
While the breakdown of payments was not made public, the insurer was made whole by one or more litigants. This is not a situation you want to find yourself in!
The most common approach to managing RPZ relief valve discharge is to route it to floor drains, sinks or spouts. In practice, drainage requirements for large assemblies make this approach difficult and too costly.
Backflow preventer manufacturers publish relief valve discharge rates for all RPZ models and sizes, and they've developed flood control valves designed for use with RPZ assemblies. Using that data, David E. DeBord, past president of the American Society of Plumbing Engineers' Chicago chapter, published flood rate charts for typical relief valve sizes at a constant pressure of 65 PSI.
For indoor RPZ installations, he concluded that “…the floor drain capacities of RPZs of three-inch diameter and higher are likely to be cost-prohibitive due to the necessary pipe diameter and fall rates."
Outdoor, above-ground RPZ enclosures constructed from marine-grade aluminum handle relief valve discharge safely when sized with proper drainage. This is the best choice for protecting RPZs.
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Benefits of aluminum enclosures for RPZs:
All Safe-T-Cover enclosures are ASSE 1060 certified. In many jurisdictions, above-ground enclosures must comply with ASSE 1060 — the national standard for outdoor protective enclosures — which governs internal dimensions, freeze protection, drainage and security requirements. Many local plumbing codes also mandate ASSE 1060-certified enclosures specifically to ensure backflow prevention assemblies remain functional and accessible for annual testing.
Backflow enclosures are manufactured from a range of materials — aluminum, stainless steel, fiberglass, high-density polyethylene and plastic — each with different performance profiles for durability, freeze protection and security. Understanding the tradeoffs matters when specifying for long-term infrastructure.
Properly specified enclosures do more than protect the equipment. Enclosures that meet plumbing code requirements help facility owners and engineers avoid legal liability and fines tied to noncompliant installations — an outcome that matters most when an RPZ failure leads to a flood and insurers begin tracing responsibility.
Backflow Cages
Steel cages are built for physical security but provide no insulation, making them appropriate only in climates where temperatures stay consistently above freezing. Once freezing conditions are a possibility, a cage is the wrong choice: it leaves the assembly fully exposed to ambient temperatures, which means cracked valves and failed components after a cold snap.
Choose instead
Insulated and heated enclosure that actively maintain interior temperatures above freezing, which prevents pipes and valves from cracking during winter — a feature that is built in as standard with ASSE 1060-certified aluminum enclosures, not added on.
Fiberglass, Plastic and Fake Rocks
Fiberglass covers can offer locking systems and some insulation, but fiberglass is susceptible to UV degradation over time and generally does not match the long-term durability of aluminum. Plastic enclosures are available at lower upfront cost and may include insulated bags for basic frost resistance, but they typically lack integrated locks and offer limited protection against tampering or theft.
Fake rock enclosures — molded from plastic or fiberglass — provide a low-profile appearance that can reduce visual temptation for theft, but they lack true locking mechanisms and do not provide reliable freeze protection for assemblies in cold climates.
Choose instead
Aluminum enclosures constructed with lockable panels that deter vandalism and tampering, providing added security over open cages that advertise the equipment inside.
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