Aprilaire, like any other humidifier manufacturer, designs their humidifiers to maintain an indoor relative humidity of 35% at an indoor temperature of 70F, anytime the outdoor temperature is 20F or above. Some homes may be able to attain a higher indoor humidity, however, factors such as the age, cubic footage (square footage times ceiling height) and tightness of the home, as well as the operating time of the HVAC heating system will affect the actual level achieved. Humidifiers require heat, water and air movement in order to add moisture to the air. Moisture in the 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. If your HVAC system has a short run time for a heat call, this will result in a short run time for the humidifier and may result in not reaching the desired humidity set point.
Shown below are the recommended setting for an Aprilaire Humidifier.