Running the fan of a residential air conditioning unit does consume electricity, but the amount varies depending on factors such as the fan speed setting and the efficiency of the fan motor. Generally, the fan itself accounts for a relatively small portion of the total energy consumption of an air conditioning system.
When the thermostat is set to "auto," the fan operates only when the cooling or heating system is actively running. In this mode, the fan cycles on and off along with the compressor or heating elements, providing airflow only when needed. This can help save energy because the fan isn't running continuously.
On the other hand, when the thermostat is set to "on," the fan runs continuously, even when the cooling or heating system is not actively cooling or heating the space. While this constant airflow can help with air circulation and filtration, it does consume more energy since the fan is continuously operating.
However, the actual impact on energy consumption will depend on several factors, including the efficiency of the fan motor, the duration for which the fan runs, and the local climate. In some cases, running the fan continuously may be desired for specific reasons, such as maintaining more consistent temperatures throughout the home or improving indoor air quality.
To optimize energy efficiency, it is generally recommended to set the thermostat to "auto" mode, allowing the fan to cycle on and off along with the cooling or heating system. This way, the fan operates when necessary, providing comfort while minimizing energy consumption. If improved air circulation is desired, using ceiling fans or standalone fans in occupied rooms can be a more energy-efficient option compared to running the HVAC fan continuously.