How should DPVs be positioned when installed in parallel in a combined DPV system?

When double check valve assemblies (DPVs) are installed in parallel within a combined DPV system, it is essential to check them against one another. This approach ensures that both valves function correctly and are balanced in terms of flow and pressure. When DPVs are positioned in parallel, they share the fluid flow, and any discrepancy in performance can lead to issues such as backflow or unequal pressure distribution, which could compromise the system's integrity.

By checking the DPVs against one another, a fitter can ensure that they are performing as intended. This practice not only aids in system performance monitoring but also contributes to the overall safety and reliability of the sprinkler system, as it allows for early detection of potential malfunctions.

The other options, while they may seem plausible, do not address the critical need for performance verification among parallel-installed DPVs. Disconnected setups would be impractical and render the system inoperative, while horizontal alignment or vertical stacking doesn't directly relate to the necessary functionality and safety checks that are pivotal in parallel configurations.