The parallel installation makes use of two or more smaller size water pressure REDUCING VALVES serving a large size supply pipe main. This approach should be used wherever there is a wide variation of reduced pressure requirements such as an apartment building where demand could be .5 gpm at 1am and 100 gpm at 6am and where you must maintain a continuous water supply. Parallel installations also offer the advantage of providing increased capacity where needed beyond that provided by a single valve. In addition, the parallel configuration improves valve performance for wide variable demands and permits servicing of an individual valve without shutting down water flow to the building completely, thus avoiding costly shutdowns.