What Is a Double Check Valve Assembly (DCVA)?
The double check valve assembly — DC, or DCVA — is the most common testable backflow assembly in Southern California. If your property has one, it is almost certainly on an irrigation service, a fire sprinkler line, or a commercial water service, and your water district has it on a list with an annual due date attached.
Here is what it actually does, and what it does not do.
What is inside the assembly
A DC is a single unit made up of two independently acting, spring-loaded check valves in series, with a shutoff valve at each end and four test cocks. It is approved and installed as a complete assembly — you cannot build one out of two separate check valves and call it a DC.
Water flows from the water main, through the assembly, and on to the property. Each check valve is held open by that normal forward flow. The springs are the important part: they hold each check closed until forward pressure is enough to push it open, so the check is already seated when flow stops rather than waiting for reverse flow to slam it shut.
The two checks act independently. If the first one fouls on a piece of debris and will not seat, the second is still holding. That redundancy is the whole design idea.
What it protects against
Backflow happens in two directions of cause, and the terms get mixed up constantly — including in a lot of what is written online.
| Condition | What is happening | Typical cause |
|---|---|---|
| Backsiphonage | Supply pressure drops below the pressure on your side, creating a partial vacuum that pulls water backward | Water main break, hydrant flowing nearby, supply shut down for repair |
| Backpressure | Pressure on your side rises above supply pressure and pushes water backward | Booster pump, boiler, pressurized loop, tall building or elevated piping |
A double check assembly protects against both, and it is approved for continuous pressure service. That is a real advantage over vacuum breakers, which only handle backsiphonage.
What decides DC versus RP is the hazard, not the pressure. This is the single most common misunderstanding. A double check is approved for non-health hazard cross-connections — pollutants, which affect taste, odor, or color but are not going to make anyone sick. Anything classified as a health hazard — a contaminant that could cause illness — requires a reduced pressure assembly instead, regardless of how much pressure is involved. We cover that comparison in detail in RPZ vs. DCVA explained.
Where you will find them
- Irrigation systems with no chemical injection. Plain landscape irrigation is generally a pollutant-level hazard. Add a fertilizer or pesticide injector and it becomes a health hazard, which means an RP.
- Fire sprinkler lines using ordinary potable water with no antifreeze or additives. On a metered fire service this is usually a double check detector assembly (DCDA), which is a DC with a metered bypass so the district can see leakage or unauthorized use.
- Commercial and multi-family water services where the district requires premises isolation but the hazard survey did not turn up a health hazard.
What the annual test measures
A certified tester closes the downstream shutoff and uses a calibrated differential pressure gauge across the test cocks to check each valve separately. Each check has to hold tight at a minimum of 1.0 psid. Both shutoff valves also have to be verified as holding, because a leaking shutoff makes the test invalid.
The whole thing takes fifteen to thirty minutes on a typical assembly, water is off for part of that, and the result goes to your water purveyor on their form or through their portal. If you want to know what the finished paperwork should look like before you sign it, see what a passing test report looks like.
The limitation worth knowing
A DC has no relief valve. When it fails, nothing leaks, nothing discharges, and nothing outwardly changes — the assembly just quietly stops providing the protection it is there to provide. An RP announces its own failure by dumping water on the ground. A double check does not.
That is exactly why the annual test is not a formality. It is the only routine mechanism that finds a failed double check.
Installation basics
Double checks are installed horizontally unless the specific model is approved for another orientation, with clearance around the body and the test cocks reachable. Unlike a reduced pressure assembly, a DC can sometimes be permitted below grade in a vault, since there is no relief port to submerge — but that is a purveyor-by-purveyor call and plenty of districts in our area will not allow it. Confirm before you build the vault, not after. More on orientation in can a backflow preventer be installed horizontally.
Common questions
Can a double check be repaired, or does it have to be replaced?
Usually repaired. Check valve rubber, springs, and seats are wear items and rebuild kits exist for nearly every common assembly. Replacement makes more sense when the body is cracked, the assembly is obsolete with no available parts, or the repair cost approaches replacement cost. Either way, an assembly that has been opened is out of compliance until a certified tester re-tests it and files a passing report.
My district says my DC has to be replaced with an RP. Why?
Something in their hazard assessment changed — a new tenant, a new process, an added chemical injector, or simply a re-survey that reclassified the connection as a health hazard. The device level follows the hazard, and that determination belongs to the purveyor and the health jurisdiction, not to the property owner.
How long does a double check last?
The body often lasts decades. The internal rubber does not — five to ten years is a normal service interval for a rebuild, sooner in dirty or high-cycle systems. Annual testing is what tells you where yours is in that curve.