Aprilaire #35 Humidifier Water Panel – (2-Pack) The “heart” of your whole house humidifier, the water panel (also known as humidifier filter, water pad, evaporator pad), should be changed at least once per heating season. The panel is constructed of an expanded aluminum honeycomb mesh, which is dipped in a clay coating. This coating is able to absorb water. Warm, dry air is then forced through the wet panel to introduce water vapor into the home. Product Features: Factory original OEM part. Dimensions are approximately – 10″ x 13″ x 1-1/2″. Instructions for changing your water panel are printed on the product carton. Fits the Following Humidifiers: Aprilaire 350 Aprilaire 360 Aprilaire 560 Aprilaire 560A Aprilaire 568 Aprilaire 600 Aprilaire 700 Aprilaire 760 Aprilaire 760A Aprilaire 768 Humidifier water panels (sometimes referred to as a filter) are a regular replacement item in whole house humidifiers. We recommend you replace your water panel once a year for the best performance. This can be done before the start of the heating season or at the end of the heating season. If you have especially hard water, then you may need to replace the water panel twice in a heating season.