The Aprilaire RP #58 automatic digital humidifier control works in with your furnace to control the operation of the humidifier. The control senses indoor and outdoor temperature and automatically adjusts the function time of the humidifier to provide the best humidity level in your home. The Aprilaire #58 can also be used in manual mode when it is not possible to run wire to outdoor sensor. The lighted display includes indoor relative humidity readout, service indicator light and water panel change light.