AprilAire 4004 In-Line Strainer for Humidifier Models 220, 224, 350, 360, 400, 400A, 400M, 500 Series, 600 Series, and 700 Series
The Aprilaire 4004 In-Line Strainer is a tiny strainer that fits in the inlet fitting of the water solenoid valve which allows water to enter your humidifier. It is designed to keep debris from entering the solenoid valve and should be checked periodically. See the owner's manual section labeled "Periodic Preventative Maintenance" for more information.