If you've ever watched a reduced pressure zone backflow preventer open its relief valve and start dumping water, your first instinct was probably to assume something failed. It's a reasonable assumption: water is flowing out of a valve that's supposed to stop water from going in the wrong direction. That looks like a failure!
It isn't. And understanding the difference between what an RPZ is designed to do and what actually constitutes a problem is one of the most important things a design engineer, facility manager or water authority can know — because it changes how you respond, how you specify, and how you protect the installation around the valve.
The Misconception: A Discharging RPZ Is a Broken RPZ
That assumption of failure or malfunctioning equipment leads to hasty, reactive responses: emergency service calls, premature valve replacements and, in poorly designed installations, significant water damage to the surrounding space.
The reality is different. An RPZ that opens its relief valve is doing exactly what it was engineered to do.
The Reality: RPZ Hydraulics Are Meant To Discharge
Sean Perry, national sales manager at Apollo Valves, explains how it works: "60psi of water comes into the [RP] valve, [and] hits a 5psi minimum differential spring in the first check. [The] path of least resistance is to go through a sensing line down to the 2psi differential relief valve spring and close the relief valve. Now water can go past the first check, losing 5psi into the zone, and with zone pressure of 55psi plus the 2psi relief spring, [it's] 57psi against 60psi. [The] 60psi wins that fight every single time. Now the water can satisfy the demand downstream by going past the second check with a minimum 1psi differential spring. Super easy, and the relief valve is happy."
When the relief valve is happy — meaning it stays closed — the pressure zone is intact and the water supply is protected. The moment that balance is disrupted, the relief valve opens. That's not a malfunction. That's the alert system working.
The Two Most Common RPZ Failure Scenarios
When an RPZ "fails," it almost always comes down to one of two conditions. Here's how to read what the valve is telling you.
| Scenario | What's Happening | What the Relief Valve Does | First Step |
|---|---|---|---|
|
Debris in the first check
|
First check can't fully close; zone pressure drops | Opens and discharges water (dripping to flowing) | Inspect and clean the first check |
|
Debris in the second check
|
Second check can't fully close; zone pressure is compromised | Opens and discharges water | Inspect and clean the second check |
In roughly nine out of 10 cases, debris in the first check is the cause. Always start there.
What To Do When Your RPZ Relief Valve Opens
When a relief valve activates, the sequence matters. Here's the correct response:
- Do not assume the valve is defective. A discharging relief valve is a diagnostic signal, not a condemnation of the assembly.
- Note whether water is dripping or flowing freely. A drip suggests minor pressure imbalance; sustained flow indicates a more significant blockage or check failure.
- Shut off the downstream demand if possible to isolate whether flow continues with no draw on the system.
- Inspect the first check for debris. This is the cause in the large majority of cases. Remove, clean and reseat before testing further.
- If the relief valve continues to discharge after clearing the first check, inspect the second check.
- Have the assembly tested by a certified backflow tester to confirm proper differential pressure across both checks and the relief valve before returning the system to service.
- Document the event. Water authorities and facility managers should log relief valve activations, including date, duration and resolution, as part of ongoing cross-connection control records.
Where You Install Your RPZ Backflow Preventer Matters
An RPZ doing its job — opening its relief valve and discharging water — is manageable when the installation is designed for it. It becomes a crisis when it isn't.
RPZ assemblies can discharge at rates of hundreds of gallons per minute. In a below-grade vault, that water has nowhere to go. Vaults flood, creating cross-connection risks from submerged test cocks and valves.
In a mechanical room or building interior, that discharge can destroy flooring, equipment and building systems and trigger liability claims, as seen in the photo.
Above-ground installations in ASSE 1060-certified enclosures are designed to handle this. ASSE 1060 Table 3 specifies drainage requirements based on valve size, and compliant enclosures provide both the drainage capacity and the clearance — typically 12 inches below the relief valve — needed to manage discharge safely. When the valve does what it's supposed to do, the enclosure handles it without incident.
This is why the installation environment isn't an afterthought in RPZ specification. It's part of the protection.
Best Installation for an RPZ Backflow Preventer
Understanding why an RPZ fails is the foundation. Designing an installation for your backflow prevention device that handles failure safely is what you need to do next. Improper installation can lead to cross-connection, contaminated water in the potable water supply.
The free Best Practices in Backflow Protection Guide can show you how. It outlines RPZ backflow assemblies and ASSE 1060 enclosure requirements, plus it offers real-world design recommendations for above-ground installations. Download yours to build backflow preventer installations that function properly.
RPZ Valves FAQ
Does an RPZ failing mean it needs to be replaced?
Not necessarily. In most cases, an RPZ that opens its relief valve has detected a debris-related pressure imbalance, not a mechanical failure. Once the debris is cleared and the assembly passes a certified test, it can return to service. Replacement is warranted only if internal components are damaged or if the assembly repeatedly fails testing.
How much water does an RPZ discharge when the relief valve opens?
Discharge rates vary by valve size and system pressure, but large-diameter RPZ assemblies can release hundreds of gallons per minute. This is why drainage capacity in the surrounding installation — and the enclosure specification — matters as much as the valve specification itself.
What's the difference between a relief valve dripping and flowing freely?
A drip typically indicates a minor pressure imbalance, often from small debris partially obstructing the first check. Free-flowing discharge indicates a more significant condition: a check that cannot seat at all or a more substantial blockage. Both require inspection, but sustained flow should be treated with greater urgency.
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