A double check valve assembly is two independent spring-loaded check valves in series, with a shutoff on each end and four test cocks so it can be certified.
Two checks rather than one is the whole idea. Each is an independent barrier, and the assembly is only considered failed when both have stopped sealing — which is why it is trusted on connections a single check would not be.
What it does
Unlike the vacuum breakers, a DCVA protects against both backsiphonage and backpressure. There is no air inlet and no height requirement relative to what it protects, so it can be installed in a vault below grade, which matters on a commercial service line where there is nowhere good to put an above-ground assembly.
What it does not do
It is a low hazard assembly. It is approved for connections where backflow would be objectionable — a nuisance, a taste, a discoloration — not dangerous. Anything classified as a health hazard goes to a reduced pressure assembly instead, and that line is not negotiable with a water provider.
The practical failure here is a building whose hazard classification changed. A chemical step added to a process, a treatment program started on a boiler loop, an injector added to an irrigation system — any of those can move a connection out of what a DCVA is allowed to protect, and nothing about the assembly announces it.
Where you find one
Most often on fire sprinkler systems, frequently as a detector check — the same assembly with a metered bypass, so the provider can see whether water is moving through a system that should be sitting still. Also on domestic service lines for buildings classified low hazard, and on some irrigation systems depending on what the provider requires.
Testing
Testable, certified once a year, report filed directly to your provider. A below-grade installation adds a step — the vault has to be pumped down and safe to work in before the assembly can be reached, which is worth knowing when the test is scheduled rather than when the truck arrives.
Where one sits inside the building, we test and repair it but do not replace it — that line is explained here.
Schedule a backflow test or call 970-835-9661