The function of a furnace humidifier is to introduce moisture into the hot or warm air coming from a furnace and evaporating it into steam. As of 2015, there are three types of furnace humidifiers, including flow-through, reservoir and steam humidifiers.
A flow-through furnace humidifier is designed to let water through the attachment machine prior to draining out. A reservoir humidifier has an internal rotating drum and a water reservoir that work together to create the necessary moisture to humidify air. A reservoir furnace humidifier generally carries a slight risk of bacteria cultivation in the standing water inside it, causing people breathing in the humidified air to become sick or contract humidifier fever.