Steam Humidifiers introduce heated water droplets into the ducts to bring the humidity up in the home. This type may be more healthy as the steam is less likely to transfer minerals and microorganisms into the air Bypass Humidifiers use an aluminum pad that is sprayed with water when the hygrostat calls for humidity. Hot air from the furnace then passes over the aluminum pad to introduce water vapor to the home. This type of humidifier is less expensive than a steam humidifier, but requires a yearly change of the humidifier pad.