This Aprilaire 4004 Humidifier Inline Strainer replacement is used in used in all Aprilaire humidifier models except for commercial units and residential models Aprilaire 110, Aprilaire 112, Aprilaire 445, Aprilaire 445A, Aprilaire 448, and Aprilaire 330. This Aprilaire 4004 Inline Strainer attaches to the inlet fitting of the water solenoid valve of your Aprilaire humidifier to keep debris from clogging it.