In the South Bay Peninsula and Santa Cruz area, where we tend to run our air conditioners daily in the summer, the right thermostat setting can bring you noticeable savings.
Using the "On" Setting
The "on" thermostat setting runs your fan constantly. Keeping your air circulating like this helps redistribute the conditioned air, reducing hot and cold spots. In winter, however, it can create uncomfortable drafts.
If you have a whole-house humidifier or dehumidifier, keeping the thermostat set to "on" helps balance your humidity because the device is always running.
Your home's air passes through the air filter and, if you have one, the air cleaner more often. While your filter will get dirty faster, your indoor air quality will be higher. When the fan runs longer, it sustains less wear from stopping and starting. On the down side, the fan is constantly using electricity, which runs up your bills.
All this means this setting is best used when you need to clean up your indoor air, such as while you're dusting and vacuuming.
Using the "Auto" Setting
The "auto" thermostat setting turns the fan on only when the heating or cooling kicks on. While the frequent stops and starts place some additional wear on the motor, you'll use less energy and enjoy lower bills. With an older furnace, you could end up with cold and hot spots due to poor air circulation. This is less of an issue with modern two-stage and variable-speed motors. With these models, the "auto" or "constant on" setting is usually the best choice.
If none of these options quite meet your needs, check your thermostat for a "circulate" setting. This lets you set the fan to run for a limited amount of time per hour. You'll enjoy better air circulation and air quality for less than it would cost you to run the fan all the time.
Our goal is to help educate our customers in the South Bay Peninsula and Santa Cruz areas of California about energy and home comfort issues (specific to HVAC systems).