The two positions are usually used in conjunction with a central air cleaning system. The normal setting is on ‘automatic’ and the fan`s cycle will be controlled by the temperature in the room. However, if your home is equipped with an air cleaner (media or electronic) or you wish to keep a continuous flow of air, switch the setting to “”on””. Remember, central air cleaning devices only work when the furnace is circulating air. If you wish to get the most from your air cleaner, you should keep the setting to ‘on’.