Duct mounted Flow-Thru humidifiers have water trickling thru a water panel and then out the bottom to a drain. Powered models use a fan where as By-Pass models use the path of least resistance to create the air flow.
The steam humidifier simmers water to create humidity. They do not need warm air and can be wired to turn on the blower when there is a call for humidity. Not waiting for the hot air allows for more precise RH control.
Self contained humidifiers have a fan and do not need to be duct mounted. The atomizing humidifiers blow a mist in to the room or duct. The centrifugal atomizing is often used for tobacco humidors where it is shelve mounted.
Water quality is a major problem causing corrosion, mineral build-up, and slime clogs. Purified water will eliminate mineral dust that falls from the evaporating air. As well as cut health issues with mold and mildew.