A bypass humidifier is a device that adds moisture to the air by diverting a portion of the heated air from the supply plenum through a water-soaked pad and back into the return air. This moisture is then distributed throughout the home by the HVAC system's blower. Because it relies on the pressure difference between the supply and return ducts, it doesn't require its own fan motor, making it a simple and reliable design. These are essential for preventing dry skin, static electricity, and woodwork damage during the winter months. They must be maintained annually by replacing the water panel to prevent mineral buildup and mold.