Aprilaire introduced the first evaporative humidifier for forced-air heating systems in 1954. Since then, Aprilaire has remained the industry leader in whole-house humidification systems. Our evaporative humidifiers work by feeding water into the distribution tray at the top of the humidifier. Water is distributed evenly across the width of the tray and through a scientifically designed outlet system. It flows by gravity over the water panel evaporator. Dry, warm air from the HVAC system flows through the moisture-laden water panel evaporator. Natural evaporation takes place, the water turns into steam and the humidified air circulates throughout the house.Included in the box: Installation instructions and template, humidifier with built-in bypass damper, 24 VAC transformer, automatic digital humidifier control and outdoor temperature sensor, saddle valve, solenoid valve, and water panel.