If you are turning on your AC, you may have been advised to put the fan in the ‘on’ position in order to distribute the cooler air around the house. Most HVACs have an option for the fan in ‘auto’ or ‘on’ positions.
Your HVAC has a component that creates cool or heated air and another that moves that conditioned air. You can adjust the temperature by changing the setting on the thermostat and the air movement by setting the fan to ‘auto’ or ‘on’.
When you select the auto setting, the fan will only run when the thermostat activates the cooling/heating component of your HVAC. Selecting the ‘on’ position will mean the fan is always on.
Most homes are fitted with oversized HVAC units. ACs cool and dehumidify your home in the warmer months. Moisture from the air will condense on the evaporator coil when the AC is running because the evaporator is so much cooler. Oversized evaporators will shut off after a short time, which leaves the condensed water on the coil. When you run the fan continuously, it causes the water to evaporate and puts it right back into your home. This means your home will not be effectively dehumidified, which makes it less comfortable.
So, putting the fan in the “on” position compounds the error of having an oversized cooling system. Your house will cool down quickly without dehumidifying well. Running the fan continuously makes the humidity worse and your home less comfortable.
Running the 300-watt fan continuously in the “on” position will increase your monthly energy bill. If you have leaky ducts outside the building envelope, either higher infiltration (air leaking in) or higher exfiltration (air leaking out) will add to your energy bill and make your home less comfortable.