GermGuardian True HEPA Replacement Filter (Filter Size B, Model FLT4825) box. [1] (https://www.ebay.com/itm/235608161949), [2] (https://poshmark.com/listing/Germ-Guardian-True-Hepa-Replacement-Filter-Size-B-Model-FLT4825PC-AC4300-AC4800-6a3a9951b2780ca1a7c0adcd)Filter DetailsCompatible Models: AC4300, AC4800, and AC4900 series air purifiers.Replacement Schedule: Replace every 6 to 8 months for best performance. [1] (https://www.ebay.com/itm/335274671787), [2] (https://www.ebay.com/itm/405461007287)How to Replace the FilterPress the lock and release button to remove the rear cover.Remove the old combination filter and throw it away.Insert the new filter system so the arrow points in the correct installation direction.Reset the filter replacement indicator if your model has one.If you need help finding where to purchase a replacement or resetting your specific air purifier model's indicator light, let me know!