With the furnace fan set to on, air is circulated constantly, which can result in a more even temperature throughout the home. However, the air doesn’t always feel warm. As the thermostat cycles on and off to maintain the selected setting, the temperature of the air coming from the vents will vary.
Air is filtered as it passes through the furnace, which can be advantageous to allergy and asthma sufferers if the furnace is equipped with a high-grade filter that screens out allergens and other particles. Wisconsin Focus on Energy suggests setting the furnace fan to on when your allergies are most active and leaving it on auto the rest of the time.
Running the furnace blower constantly means you'll need to replace or clean the filter more often due to the continuous movement of air through it. The fan will experience more wear and tear than it would if the furnace were set to auto and will need to be maintained more often.
Fan Speed and Efficiency
Older furnaces were produced with standard fans that operated at one speed. Newer, more energy-efficient furnaces are equipped with variable-speed fans that provide better airflow, reduced energy use and quieter operation.
A standard furnace fan will use from 300 to 875 watts, depending on the horsepower. If you were to keep the furnace fan on constantly for 365 days per year at 875 watts, the kWh would total 7,665 (875 watts x 24 hours x 365 days/1,000). To calculate the annual cost, multiply the kWh times the rate your electric company charges. At $0.06 per kWh, the cost would be $459.90 per year (7,665 kWh x $0.06). If your furnace has a variable-speed fan rated at 100 watts, the cost for constant operation at $0.06 per kWh would be $52.56.