This Aprilaire 4330 Humidifier Scale Control Insert replacement is used in Aprilaire 600 Humidifier, Aprilaire 600A Humidifier, and Aprilaire 600M Humidifier. This Aprilaire 4330 scale control insert is used with the Aprilaire Water Panel Evaporator and the Aprilaire Water Distribution Tray to control the build-up of harmful mineral deposits in your Aprilaire humidifier.