Whole House Humidifiers are available for Under Duct Humidifier installation. The Under Duct Humidifier mounts on the under side of the supply side air duct. The Under Duct Humidifier is an Evaporative Humidifier which collects water on rotating bronze discs. Air circulates through the rotating discs - evaporating the water - humidifying the air. Under Duct Humidifiers are excellent for hard water applications and furnace mounting restrictions. Under Duct Humidifiers are high capacity humidifiers due to their large evaporative (bronze discs) surface area.