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AUTO or ON: Choosing the Right Fan Setting on your Heating System

When we start up our heat in the fall, most of us routinely flip the thermostat to ON. But is that the best setting? What about the AUTO option? How are they different? In this month’s blog, the home comfort specialists at Main Stream Mechanical explain how each works, and the benefits and disadvantages of both. Learn everything you should know to make an informed decision on what’s best for your home and family.

How does the AUTO setting work?

The AUTO function engages the fan to automatically start when your system is active. Once the thermostat has detected that the air in your home has reached the designated temperature, the fan will turn itself off with the furnace. The AUTO setting repeats this cycle, turning the fan and furnace on together once the air temperature falls below or rises above the temperature setpoint.


How is the ON setting different?

The ON setting keeps your HVAC system’s fan actively engaged, even when the system isn’t heating (or cooling) air. This means the fan will constantly blow, even when the thermostat reaches the desired temperature and the furnace stops.


What advantages does the AUTO setting offer?

Think of the AUTO setting as on demand, it engages the fan only when needed, saving you energy and money on your utility bills since your HVAC system is ONLY using energy while the fan is running. Another plus, your air filter will last longer. The filter only traps dust and debris while the fan is moving air, so by using the fan intermittently, you can go longer between replacements.


And the disadvantages?

While using the AUTO setting increases the longevity of your filter, the repetitive stops and starts can cause wear and tear on the fan and its components. This could lead to costly repairs. Additionally, you may notice that your home’s temperatures vary in between the fan’s engagement cycles. This is because the system stops blowing air throughout your home based on the temperature of the air closest to your thermostat—as opposed to running continuously and balancing the air temperature throughout your home.


What about the ON setting?

The ON setting keeps the fan running without multiple starts and stops, preventing stress on your system and preserving the life of your blower’s motor. Running the fan continuously keeps the air circulating through the filter, reducing the amount of dust and other allergens within your home and improving your indoor air quality. Your home can be heated or cooled more evenly. Running the fan continuously can help better distribute comfortable air in those areas that are harder to heat or cool.


What are the drawbacks?

The circulated air will be less comfortable over time. After a while, the air coming from your vents will feel less cozy from your heater or less cool and crisp from your AC. Since the air in your home is being circulated at a greater rate, setting your thermostat fan to ON can clog the filter sooner than the AUTO setting, and require more frequent filter changes. The constant forced air blowing also puts stress on air ducts, making them susceptible to leakage. Depending on how energy-efficient your system is, running your fan constantly could be less eco-friendly and increase your utility bill.

HVAC Installation, Service, Maintenance, and Repair

Understanding the advantages and disadvantages of your HVAC settings can help you choose the best option based on your home heating needs. If you have questions about the right settings for your heating and cooling system, contact our home comfort specialists. We’re ready to help.


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Main Stream Mechanical

25 Macy Street, Suite C

Amesbury, MA 01913

978-834-0849   |   877-800-HVAC

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About Main Stream

Main Stream Mechanical, Inc. is a full-service plumbing, heating, and air conditioning company. We service a large portion of Massachusetts and Southern New Hampshire.