There are 2 settings for the fan on my thermostat: “automatic” and “on”. Which setting should I use?

These two positions are usually found if you have a central air cleaning system. The “automatic” setting means the fan’s cycle will be controlled by the temperature in the room, however, if your home is equipped with an air cleaner or you wish to keep a continuous flow of air, switch the setting to “on.” Central air cleaning devices only work when the furnace is circulating air, so if you wish to get the most from your air cleaner, you should keep the setting to “on.”