Once the fan shuts off in between cooling cycles, that moisture has time to slide off the coils and drain away outdoors
You might be inviting humidity into your home without even knowing it by setting your AC thermostat fan to On instead of Auto. On and Auto are the two settings for your fan. The difference between ON and AUTO boils down to when the fan blows. In the Auto position, the fan only blows during an AC cooling cycle. In the On position, the fan blows constantly, even when the AC isn’t cooling. This also means that it uses more energy and costs more. To better understand how a fan affects indoor humidity, first you should know what the AC fan does and where it is located. If you have a split system AC, you actually have two fans in your AC unit. One fan for your outdoor unit and one fan for your indoor unit. Your indoor unit fan sits directly behind the cooling coils and is responsible for pushing air through the ducts and into your home. Basically, the On setting is not allowing your AC to properly dehumidify your home. Your AC has two jobs. It cools your home and dehumidifies it. When warm air from inside your home is pulled into your indoor unit, it passes over very cold refrigerant coils. Those cooling coils absorb the warmth and moisture from the air. Then, that cold dry air is pushed back into your home. If the fan is set to Auto, that moisture collects and sits on the refrigerant coils. Once the fan shuts off in between cooling cycles, that moisture has time to slide off the coils and drain away outdoors. But, if the fan is set to On, the moisture on the refrigerant coils does not get the chance to drain away. Because the fan blows constantly, most of the moisture evaporates and gets blown right back into your house.
Commercial AC