Evaporative humidifiers have an internal fan that draws in air and passes it through a moist wick at the bottom of the humidifier, causing the water on the wick to evaporate into vapor. The vapor is then released into the surrounding air to increase the ambient humidity in the area without generating heat. These units have humidistats that help them maintain optimal humidity.

Steam humidifiers use less water to humidify the air than evaporative humidifiers and bypass humidifiers. They are also easier to maintain than evaporative humidifiers and bypass humidifiers because they don't require humidifier pad changes or routine cleaning. These humidifiers use electrodes to create steam and add moisture to air that is moving through the HVAC system. The HVAC system circulates the humidified air into the surrounding area. Steam duct-mount dehumidifiers require the HVAC system to be running in order to operate, but they do not require the building's furnace to be running.