Cartridge Filter Housings The housing catalog number is made up of several variable characters i.e. length, material, port size, and O-ring material. For example: Natural Polypropylene Filter Housing , for 20” Filter Cartridge Length, 2-222 Filter Connection, 3/4” FNPT Inlet/Outlet, Viton (or FKM) O-Ring, With Accessory Kit = CHPN20B7V2