A double check valve backflow preventer is a testable plumbing assembly that uses two independently acting check valves to stop contaminated water from flowing backward into a clean supply line.
Clean drinking water should move one direction only. A sudden pressure drop—a water main break, a fire hydrant in heavy use—can reverse the flow and pull dirty water back into the supply. The two internal check valves are the barrier that stops it.
This article covers what the assembly is, how its parts work, where it fits in real systems, and what most people get wrong about it.
How Does A Double Check Valve Backflow Preventer Work?
Both check valves stay closed until normal pressure pushes them open during regular flow. The moment downstream pressure reverses, both snap shut and block backward travel.
The two valves work as a pair, not a backup. Each acts independently, so if the first fails to seat fully, the second still stands between the clean supply and what’s on the other side. That redundancy is the point of the design.
During normal demand, water pushes through the first valve, then the second, then out to the building. That simplicity is why the assembly is so common on commercial and irrigation lines.
What Is Inside A Double Check Valve Assembly?
An approved assembly ships as a single integrated unit with specific parts that let it be tested and shut down as a complete device:
- Two single, independently acting check valves — the core that stops reverse flow.
- Shutoff valves on each end — upstream and downstream, so the unit can be isolated for testing and service.
- Four test cocks — ports positioned so a certified tester can check each check valve individually.
That testability separates a real assembly from two ordinary check valves placed in series. A pair of check valves looks similar but cannot be tested as an approved device, so it won’t pass inspection or meet a water authority’s requirement.
Some assemblies also call for a strainer ahead of the unit to keep debris from fouling the check valve discs—a fouled disc is a common reason these devices fail a test.
Where Can A Double Check Valve Assembly Be Used?
This assembly is generally approved for non-hazard situations—lines exposed to a pollutant but not a hazardous contaminant. When risk climbs into hazardous territory, a reduced-pressure principle assembly replaces it.
It handles both failure modes: backsiphonage, where a drop in supply pressure pulls water backward, and backpressure, where downstream pressure pushes it that way.
The valve body has an arrow cast into it that must point in the direction of flow. Install it backward and the device does nothing. In most jurisdictions the assembly needs approval from the Public Water System before installation—some state manuals say this plainly.
If you’re sizing a unit, this rounded up double check valve preventer options page compares real models against these limits.
Which Brands And Models Are Common?
These assemblies come in a wide range of sizes and pressure ratings, so the model matters less than matching the assembly to your line size, flow demand, and local code. Zurn Wilkins, Watts, FEBCO, and Apollo Conbraco all make versions:
| Model / Source | Typical Form | Price Signal |
|---|---|---|
| Zurn Wilkins 950XL | Small residential-scale unit | Roughly $210–$283 depending on size and seller |
| Watts LF007 / 757-NRS | Lead-free, larger commercial builds | Varies widely by size |
| Apollo Conbraco 4A / 4ALF | Multiple sizes, lead-free variants | Mid-range for the class |
| FEBCO MasterSeries MM | Commercial-grade assembly | Higher tier |
| Backflow Direct grooved units | Large grooved-connection builds | MSRP around $2,969–$3,909 |
That range is genuine: a small 3/4-inch residential unit and a large grooved commercial assembly are not the same product in anything but name. Prices also swing by seller.
The EPA’s cross-connection control guidance shapes how and when these devices get required. The EPA cross-connection control manual covers the regulatory logic behind the requirement.
The Bottom Line
Match the assembly to your line size, flow demand, and hazard level, and confirm your water authority approves the model before installation. A double check valve backflow preventer is not a high-hazard device, so don’t swap it in where a reduced-pressure principle assembly is required.
Install with the body arrow pointing downstream, add a strainer upstream if the design calls for it, and have the unit tested by a certified tester after installation and on whatever schedule your water system sets.
FAQs
What is the difference between a double check and a reduced pressure assembly?
A double check assembly handles non-hazard situations where the risk is a pollutant. A reduced pressure principle assembly is required when hazardous contaminants could enter the line. The two are not interchangeable, and choosing the wrong one can fail an inspection.
How often should the assembly be tested?
Test frequency is set by local rules, but annual testing is the common standard for commercial and irrigation assemblies. A certified tester handles the check and files the result with the water provider.
Can two regular check valves replace a double check assembly?
No. Two ordinary check valves in series will not pass as an approved device. They lack shutoff valves and test cocks, so a tester cannot verify each valve separately. Assemblies must be testable to count as backflow prevention.
References & Sources
- U.S. Environmental Protection Agency. “Cross-Connection Control Manual” Supports the regulatory basis for backflow prevention and device approval.
- West Virginia Department of Health and Human Resources. “Cross-Connections and Backflow Prevention Manual” Supports pollutant-vs-hazard use limits and Public Water System approval language.
- Sage Water. “Backflow Preventers” Supports operating principle, assembly construction, and test cock use.
