If you’re interested in keeping your cooling costs down, use the auto setting so the fan only runs when needed to maintain the set temperature. However, if you prefer a more consistent temperature, use the on setting for uninterrupted airflow. This option may also be beneficial if you have allergies or asthma because air will constantly be filtered as it flows through the system. A variable speed furnace is made to run 24/7 and will cause pennies monthly to run 24/7. Older furnaces with old motor technology may cost a substantial amount of money to run 24/7 and may not be worth it until exploring options for the newer blower technology.