The Water Panels for your Aprilaire humidifier (sometimes called a humidifier filter) are the real heart of your system. They are designed to deliver the appropriate humidity with Aprilaire systems, and are original OEM parts. You should replace your water panel annually for best performance. Water Panel Evaporators For Models 110, 220, 500, 550, 550A and 558. Fits Models 550, 550A, 500, 110 and 220. Water Panel Evaporators are the heart of your humidifier and the component which generates the important humidity to keep you comfortable. The dimensions of the water panel are 9 3/4" x 10" x 1 3/4"