Abstract: A humidifier for use with a forced air heating system. The humidifier comprises a water reservoir in the form of a sealed container having a water inlet connection and suitable control means for maintaining the level of the water at a predetermined depth within the reservoir. An electric immersion type heater is also provided within the sealed container at a level which is normally below the water level and, when energized, heats the water to produce steam. The reservoir can be mounted completely external to a duct of the forced air heating system, but an inverted U-shaped tube having preparations along the base thereof is disposed within the duct and has its parallel legs passing through the duct and reservoir so as to communicate with the portion of the reservoir above the level of the water therein. As such, the steam passes through the U-shaped tube and out its perforations to thereby add moisture to the air passing through the duct.