Both stop water from flowing backward into the clean supply, but they fit different risk levels. A double-check assembly suits low-to-medium hazard, non-health cross-connections. An RPZ adds a relief valve for high-hazard, health connections, making it the strongest protection. The hazard level decides which one code requires.
What each assembly is and how it works
Both devices are testable backflow-prevention assemblies. That means each one has shutoff valves and test cocks built in, so a certified tester can verify it still seals. Beyond that shared frame, the internals differ.
A double-check valve assembly (DCVA) uses two spring-loaded check valves in a row. It has a shutoff valve on each end and test cocks between them. If supply pressure drops or downstream pressure rises, the checks close and stop water from reversing. Two checks give a backup. If one fouls with debris, the other still holds. This is the design rated for low-to-medium hazard, non-health connections. It meets ASSE 1015.
A reduced-pressure-zone assembly (RPZ or RP) takes the two-check design and adds a third part. Between the two checks sits a relief valve that opens to the air. The space between the checks is held at a pressure lower than the supply. If either check starts to leak, the relief valve senses the pressure shift. It then dumps water out of the assembly to the open air instead of letting it move toward the clean supply. That visible discharge is the whole point. It breaks the path so dirty water cannot reach the main, even if both checks fail. The RP meets ASSE 1013. It is the highest level of mechanical protection short of a full air gap.
Backflow happens two ways, and both assemblies stop both. Backpressure is when downstream pressure climbs above the supply. A boiler, a pump, or an elevated tank can cause it. Back-siphonage is when supply pressure goes negative. A water-main break or heavy fire-flow demand nearby can cause it. The relief valve in an RPZ is what lets it cover the worst-case version of either event.
Hazard level decides which one is required
The selection rule is not about pipe size or cost. It is about the degree of hazard at the cross-connection. The EPA Cross-Connection Control Manual is direct about this. It states:
"The type of protection required depends upon the degree of hazard the facility represents to the public potable water system."
In plain terms, ask one question. If the water flowed backward, could it harm health, or would it just be unpleasant? A health hazard is anything that could cause illness or death. Connections to chemicals, sewage, process water, or medical equipment all count. A non-health hazard is sometimes called a pollutant rather than a contaminant. It would affect taste, color, or odor but would not make a person sick.
A health hazard requires an RPZ. A non-health, low-to-medium hazard can use a double-check assembly. You cannot trade down. A double-check is not approved for a health hazard. It has no relief valve and no air break, so a failed check could send a contaminant straight into the supply with nothing to stop it. You can install a higher level of protection than the rules require. You can never install a lower one. The local water provider has the final say on the hazard rating for your site.
Comparison at a glance
| Feature | Double-Check Assembly (DCVA) | Reduced-Pressure-Zone (RPZ / RP) |
|---|---|---|
| Hazard level | Low to medium, non-health | High, health hazard |
| Internal design | Two check valves in series | Two checks plus a relief valve |
| Relief valve to atmosphere | No | Yes |
| Protects against | Backpressure and back-siphonage | Backpressure and back-siphonage |
| Standard | ASSE 1015 | ASSE 1013 |
| Visible discharge | None in normal use | Discharges when a check fails |
| Drainage needed at install | Minimal | Yes, must drain relief discharge |
| Level of protection | Moderate | Highest mechanical level |
The drainage row matters in the field. Because an RPZ can dump water, it has to be installed where that discharge can drain safely and where the assembly will not sit in standing water, which can void the relief function. A double-check has no such discharge, so it has more placement flexibility. That difference shapes where each one gets mounted.
Where you see each one in commercial work
Typical low-hazard, double-check connections include lawn and yard irrigation systems with no chemical injection. They also cover certain fire-sprinkler lines that use plain water with no antifreeze or additives, plus some non-health process connections. The reasoning is simple. The water in the line is not drinkable after it sits in the system, but it would not poison anyone.
Typical high-hazard, RPZ connections cover more dangerous uses. These include irrigation with fertilizer or pesticide injection, boilers that use chemical treatment, and fire systems charged with antifreeze or foam additives. They also cover commercial dishwashers and food equipment, soda-fountain carbonators, and any line tied to chemicals, plating, or wastewater. When in doubt, the assembly that guards against the worst case is the RPZ.
This is also why both devices must be tested at least once a year in Arizona. A check valve can wear, scale up, or catch debris and stop sealing with no outside sign. Arizona Administrative Code R18-4-215 requires that each backflow assembly be tested when installed and at least once a year after that by a certified tester. It must also be retested after any repair or move. The Arizona Department of Environmental Quality runs the state cross-connection control program. For the full schedule and the Phoenix filing rules, see our page on commercial backflow testing requirements in Arizona.
Why installing the wrong one is a code and safety problem
Say a site needs an RPZ and a double-check gets installed instead. The building is now out of compliance. Worse, it is under-protected. A double-check has no relief valve. A failed check on a health-hazard line gives a contaminant a clear path into water people drink. Inspectors and water providers can fail the connection, require replacement, and in some cases shut off service until it is fixed.
The reverse is allowed and safe. Putting an RPZ where only a double-check is required works fine. It just adds cost and needs proper drainage for the relief discharge. The right move is to match the assembly to the verified hazard level set by your water provider. Then size and place it correctly. The USC Foundation for Cross-Connection Control and Hydraulic Research approves the assembly models that meet ASSE standards. Those approved-model lists are what local programs use.
Backflow protection is one piece of a larger plumbing-safety picture. For background on the hazard itself, see what is backflow in plumbing and what is a cross-connection. On the irrigation side, our guide to a backflow preventer for sprinklers in Arizona covers which assembly fits a yard line. Not sure which assembly your building has, or whether it matches your hazard level? A certified backflow tester can confirm the rating and the device in one visit.