GeneralAire’s 990-13 evaporator pad, sometimes called a humidifier/water filter, is an indispensible part of the humidifier. The pad soaks up water as it passes through the humidifier, and then circulates humidity into the air. This keeps your home or business nice and comfortable during the dry season. This pad is made of aluminum encased in a heavy wire frame, and is coated with a special absorbent material which evenly draws water across the pad, and increases the amount of humidity produced. While these pads work hard to provide clean and comfortable air, as time passes, pollutants and minerals build up on the pad and diminish performance. It’s important to note that the pad cannot be cleaned, and must be replaced at least once a year to ensure superior performance and air quality. Fits models 709, 709L, 709LH, 1040, 1040L, 1040LH, 1040R, 1042, 1042L, 1042LH, 1042R, 1137, 1137R and SL-16 Series.