Bypass Type Humidifiers
Bypass humidifiers use the heat of the furnace to evaporate water into moisture and they use the pressure differential of the supply and return to pull the moisture into your heating and cooling system and to all your conditioned rooms.
This type of humidifier has no pan of standing water, but rather when humidity is needed a water valve opens up and water is sent down a ceramic coated evaporative pad that dries out when the heat turns off.
This feature keeps mold and bacteria from growing in a pan of standing water.
The bypass humidifier is controlled by a humidistat that senses how dry your home is and automatically turns the device on when needed.
Reliable sells a number of brands of this type of humidifier with the most popular ones built by Aprilaire and Honeywell.