GeneralAire Humidifier Unit 81 GeneralAire Split Drum Humidifer A high-capacity model that delivers 18 GPD. Split drum design allows easy removal of evaporator sleeve for replacement (replacement sleeve part number 81-15). Drum-type humidifier with capacity for most homes. Installs on vertical warm air supply or cold air return (recommended) plenum. Heated air passes through a water-laden evaporator sleeve, absorbs moisture and then returns to the heating system for distribution throughout your home. An M-2 low voltage humidistat is included.