All humidifiers require, heat, water, and air movement in order to add moisture to the air. Moisture in humidifierthe form of vapor is added whenever the humidity level in the home is lower than the desired level and the HVAC system fan is moving air. When the HVAC system is not operating, the humidifier may not be wired  to only operate when the fan is on. Humidifiers are designed to be installed on the hot air supply plenum. If however, there is no space available due to the HVAC air conditioning coil or the layout of the mechanical room, the humidifier can be installed on the return air drop. When installed on the return air duct, the humidifier must be supplied with hot water as the temperature of the air in the cold air return duct is not sufficient for proper evaporation.