Should My HVAC Systems Fan Be Set To Auto Or On?

In general, you should leave your HVAC system fan on the AUTO setting since it is more energy efficient. However, there are some cases when you should set it to ON. You can find the FAN setting on your thermostat, but don’t switch it until you read this article and know what you are doing.

a woman programming the fan setting

What Do The Different Settings Mean?

You’ll find two settings on your thermostat: auto and on. The “ON” setting means that the fan will continuously blow air, regardless of whether your HVAC system is actively heating or cooling air. 

On the other hand, “AUTO” mode will automatically turn on and off the fan only when your air conditioner or furnace is on. So, it only blows when your HVAC system is producing warm or cool air. Once the system heats or cools your home to the thermostat temperature setting, the entire system, including the fan, turns off until the next cycle. 

Should The Setting Change Between Summer And Winter?

woman changing the fan setting to AUTO

For best results, the AUTO setting is ideal for summer and winter. However, the best setting for your home ultimately depends on your personal preference.

For instance, if there are hotspots in your home during summer, keeping the fan running continuously can be more beneficial. This is because it constantly moves air through the ducts in your home. However, if your HVAC system is adequately sized and properly balanced, you should never need to switch it to “ON.”

The AUTO setting is also usually your best option in the colder winter months because the fan will only come on when your HVAC system is actively producing warm air. This means you can save energy and money while maintaining a consistent temperature. Moreover, you don’t have cooler air circulating through your home when the system isn’t on.

What Is The Most Efficient Setting?

AUTO is the more energy-efficient option because the fan only runs when needed. Moreover, running your fan on AUTO can prevent issues associated with running the fan on “ON” during the summer or winter.

If you leave the fan set to ON in the summertime, it can allow warm air from the higher parts of your home (attic) to enter the ducts. Since the fan is constantly running, it can push this warm air down into the rest of your home, raising the temperature. This can make your air conditioner work harder to compensate for the extra heat, leading to higher energy costs.

A similar scenario can happen in the winter when the fan is set to ON. The fan constantly blows air through the ductwork, even if your system isn’t actively heating the air. So, since the air moving through the ducts can cool it when the furnace is off, it can feel somewhat chilly.

Should I Run The Fan On My Furnace In The Winter?

woman setting the auto fan mode on the thermostat

Yes, you should run the fan on your furnace in the water. However, instead of setting the fan to ON, it’s best to leave it on AUTO. By switching the setting to AUTO, you can avoid “chilly” air from exiting the vents when the system isn’t actively producing warmed air.

The AUTO setting in the winter also helps save energy and money while keeping your home at an even, comfortable temperature. This way, you won’t have cool air blowing from the vents throughout your home when the system is off. The fan will only push warm air when the thermostat tells the furnace or heat pump to begin another heating cycle.

Pros And Cons Of AUTO vs. ON

As you decide between keeping your fan on AUTO or ON, consider the following pros and cons.

Advantages of AUTO

man adjusting the thermostat while looking at his electricity bill

Like most things, keeping your fan on AUTO has advantages and disadvantages. Here are a few upsides to keeping your fan in this setting:

  • Energy efficient: AUTO is the most energy-efficient setting for your HVAC system’s fan, as it only runs when it needs to, not constantly.
  • Superior dehumidification: In the summer months, the AUTO fan setting helps keep humidity at bay. When your system is set to AUTO, the moisture from the cooling coils can drip into your condensate pan and drain outside. However, when the fan is constantly running, moisture doesn’t have an opportunity to drip and drain outside. So, it blows back into your home, raising the humidity levels.

Disadvantages of AUTO

On the flip side, a few downsides to running your fan in AUTO may include:

  • Worn-out parts: The blower fan in your system might wear out faster when set to AUTO, as it starts and stops more frequently.
  • Uneven distribution: When your fan is set to AUTO, you might notice the air distribution is less even, as the fan stops moving air throughout your home when the thermostat reaches its correct temperature.

Advantages of ON

woman pressing the ON fan setting on the thermostat

If you decide to keep your HVAC fan set to ON, here are a few benefits and drawbacks to consider. Notable upsides to this setting include:

  • Even distribution: When the fan runs constantly, you might notice a more even distribution of warm or cool air throughout your home. The constant air movement in your home will help minimize hot or cold spots in certain areas.
  • Reduced stress: Since the fan doesn’t frequently start and stop, this could lead to reduced stress on the fan, potentially leading to a longer lifespan.

Disadvantages of ON

On the other hand, a few potential drawbacks of this include:

  • Opposite effect: In the cooler winter months, you might notice cool air exiting from your vents. This can happen because the air temperature is similar to the space temperature, making your home feel chilly. In the summer, ducts that run through your attic can fill with warm air, pulling it down into your home and making it warmer. This can make your home warmer and cause your AC unit to run more, leading to equipment strain and higher energy costs.
  • Pricier: Running the fan nonstop can cost you more money, especially if your system’s blower motor isn’t rated for continuous use (but some motors are designed to run constantly).
  • Frequent replacements: Since air is constantly flowing through your system, you’ll likely need to replace your furnace filter more frequently.

The Bottom Line for ON and AUTO

Most of the time, you should just leave your HVAC fan set to AUTO. Only turn it to “ON” if you have hot/cold spots or need to quickly ventilate your home. For example, you burnt popcorn in the microwave and want the smell to dissipate faster. Or you spilled water on your carpet and need extra airflow to dry it out.

Your HVAC system is optimized for cooling and heating with the AUTO setting. If your home is not cool or warm enough unless you turn the fan to ON, contact your local HVAC technician. You likely need some maintenance on your HVAC system to make your home more comfortable.

About The Author

Jonathon is a mechanical engineer with over ten years of experience in the HVAC industry. He has hands-on experience with all types of HVAC systems.

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