The Aprilaire 65 digital manual control will control your humidifier to maintain the setting you choose for your home's RH (Relative Humidity) level. Designed humidifier requiring an ON/OFF signal for a 24 volt circuit. The RH is also shown on the control. Your degree of comfort and the quantity of moisture your home can withstand without condensation forming on windows or other surfaces will determine the RH setting you pick. Condensation is more prone to happen on cold surfaces, such windows, when external temperature decreases, thus the RH set point must be changed correspondingly. To help you remember to alter the RH setting when the outside temperature changes, the control comes with a label you can stick on the front.