This GeneralAire 81-4 water pan is a replacement part specifically for the model number 81 humidifier. The pan sits inside of the humidifier, and is filled with water. A drum covered in an evaporator sleeve slowly spins inside the pan, collecting water to circulate into the air and add the humidity that keeps your home comfortable. The water pan is made of sturdy plastic, but may eventually crack or experience mineral build-up, and need to be replaced.