Aprilaire 10 Replacement Water Panel for Aprilaire Whole House Humidifier Models 110, 220, 500, 500A, 500M, 550, 558 (Pack of 4) BUY WITH CONFIDENCE This genuine replacement water panel was designed and manufactured in the U.S.A. by Aprilaire  the leader in indoor air quality solutions, to optimize the performance of your Aprilaire Humidifier Model: 110, 220, 500, 500A, 550, 558 PERFECT APRILAIRE FIT Water panel dimensions: 10" H x 9.75" L x 1.75" W FOR OPTIMAL PERFORMANCE replace water panel annually HUMIDITY FOR HEALTH Aprilaire Humidifiers can help you maintain optimal humidity in your home of 35%  45% which has been shown to reduce the incidence of respiratory infections and symptoms related to allergies and asthma