The Double Check (DC) assembly uses two check valves in series to stop backflow on low- to medium-hazard connections. It's the assembly you'll most often find on fire sprinkler systems and straightforward irrigation lines.
Lower hazard doesn't mean lower obligation: DC assemblies still require an annual certified test under Title 17, and your water purveyor still expects the filed result on time.
Fire sprinkler systems (without chemical additives), non-chemical irrigation, and many multi-family and commercial low-hazard connections.
California Title 17 (and Sacramento-region purveyors) require this assembly to be tested and certified by a registered tester once a year, with the result filed with your water district.
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