One valve instead of a manifold
Running a media filter manually needs five or six hand valves opened and closed in strict sequence for every backwash. A multiport valve replaces that manifold: one rotary valve on the vessel (top-mount screwed straight into the opening, or side-mount plumbed alongside on larger vessels) internally reroutes the flows for every stage of the cycle — driven by hand lever on manual versions, or by a motor plus controller (time-clock or volumetric) on automatic ones. The valve connects to five paths: inlet, outlet, riser tube down through the media bed, drain, and — on softener valves — the brine line.
The positions, and what is actually happening
Service — raw water in, down through the media bed, up the riser, out to supply. The working position; everything else exists to restore this one.
Backwash — flow reversed: down the riser, up through the bed, out to drain. Lifts and fluidises the media, carrying trapped solids away. Duration must let the drain run dirty-then-clear (typically 8 – 12 minutes); the vessel and drain line must handle the medium's backwash flow — the hydraulics warning in the backwashing article applies to the valve's orifice too, which is sized per model with a backwash flow controller.
Rinse / fast rinse — service-direction flow to drain: resettles the bed and flushes fines (or residual brine) before water goes back to supply. Skipping or shortening it sends the first minutes of turbidity — or salt — to the taps.
Softener valves add the brine circuit:
Brine draw & slow rinse — the valve's built-in injector (venturi) educts brine from the salt tank and passes it slowly through the resin, then rinses at the same gentle rate to give the exchange time to complete. No separate brine pump exists on a standard softener; the injector does the work, which is why feed pressure below the injector's minimum (typically ~1.5 bar) is a genuine softener fault.
Brine refill — measured water back to the salt tank, setting the brine (and therefore salt dose) for the next regeneration. On most valves the refill time is the salt-dose setting.
Bypass — many valves offer an internal bypass so raw water remains available while the vessel regenerates or is serviced; separate three-valve bypasses do the same job externally and make valve replacement painless.
Buying and servicing notes
Match the valve to the duty on four axes before any brand talk: filter or softener cycle (they are not interchangeable — the next article), control type (time-clock vs volumetric), flow rating (service and backwash), and fitment (vessel thread, riser pipe diameter, top- vs side-mount). Runxin's five-digit codes encode most of this — the code reference tool decodes them. In service, the valve is the moving part of the whole vessel: seals and spacers wear, injectors block with salt-tank sediment, and impulse lines clog — all replaceable items, worth fixing long before replacing a healthy vessel.
Rule of thumb: service, backwash, rinse — plus brine draw and refill on a softener. If you can name what the valve is doing in each position, most "system faults" resolve into one position not doing its job — usually backwash flow, a blocked injector, or a rinse cut short.