Multiport valve
Also called: multiport, multi-port valve, spider gasket, backwash valve.
A multiport valve is the multi-position valve on a sand or DE pool filter that sends water to FILTER, BACKWASH, RINSE, WASTE or recirculate. A cartridge filter has no multiport valve. Always turn the pump off before changing the valve position.
Why it matters
Moving a multiport with the pump running is the fastest way to destroy its spider gasket, and a blown gasket shows up as water running continuously to waste. Hayward's multiport manual prints it in capitals: turn off pump before changing valve position. The WASTE setting is what lets you vacuum settled flocculant out of the pool, which is why flocculant is not a tool for a standard cartridge filter. Setting the valve to recirculate cuts resistance while a pump primes. For a freezing winter, Pentair's sand filters get set to WINTERIZE, which takes pressure off the valve's rubber seal all season.