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The Reality of Choice

It is a reality of the human mind that as soon as we are faced with a set of options, we start pondering the pros and cons of each choice and even panicking that we have picked the wrong option. All good AC systems have a setting to switch the fan to either AUTO or permanently ON. Which one is the best option? Which one is the most energy-efficient? Should the AC fan be on AUTO or ON? Let’s investigate this conundrum.

The Functional Purpose of the Two Settings

The functionality of the two AC fan settings is pretty apparent, but may be expounded on for broader clarity:

ON. Set to this option, the fans of the AC system are always hard at work to circulate air through the ducts and into the rooms of the house, irrespective of whether the HVAC is busy heating or cooling the air. It functions like a desk fan but, as will be explained below, is less efficient.

AUTO. Set to this option, the fans are automatically controlled by the electronics of the HVAC system and only switch on when the air is being actively heated or cooled. Once the thermostat senses that the desired temperature has been achieved, the fans are switch off until the next cycle of the system.

The Advantages and Disadvantages of the ON Setting

People who swear by the ON setting cite two factors that make the AUTO setting less energy efficient. The first is that when a fan is switched on, the motor draws more power to overcome the inertia of the blades. Although true, the amount of extra energy required by a modern residential-grade fan is negligible. The second is the analogy of the desk fan that cools the room by circulating the hot air around. For the obvious reason that the HVAC system does not create a paper blowing breeze in a room, this principle does not apply.

But let’s look at the actual advantages and disadvantages of the ON setting:

  • A fan that is constantly running will also always be drawing power. Perhaps through precise measurements, it might be determined if the startup power spike is more than several minutes of operational power.

  • Running the fan constantly set to ON largely depends on the type of fan installed and the HVAC system. Most fans are designed for stop-start cycles, so running this type of fan continuously will significantly reduce its operational life. The HVAC system might also be programmed to run the fan at a slower speed, which only increases when the AC unit switches on, meaning ON doesn’t really mean ON.

  • The constant airflow will normalize the air temperature within the house, but not necessarily at a comfortable level. The air might also be more humid, as an ON fan will not allow condensate to drain away from the AC unit between operational cycles.

  • The constant airflow will also necessitate more frequent filter changes and duct cleanings, but on the flip side, this is a great way to only have filtered air inside the house.

The Advantages and Disadvantages of the AUTO Setting

The AUTO setting is the one with which the engineers designed the HVAC system. Should we not assume that they know best? However, setting the fan to AUTO does have some disadvantages, as everything in life and reality must be balanced. Here are a few of the top points to ponder:

  • AUTO is designed and tested to be more energy-efficient than the ON setting.

  • Good quality fans are designed for the start-stop cycle and variable speeds, so it might be a good idea to check with the service technician if the best fan is installed for your preference.

  • You might notice an initial blast of either hot or cold air when the HVAC switches on, as the air in the ducts will have normalized to the general temperature of the environment or undergone structural heating or cooling. How much depends on the design principles followed when the HVAC system was installed, as a good engineer will have minimized this effect with careful routing and sufficient insulation.

Should the AC fan be on AUTO or ON?

Should the AC fan be on AUTO or ON? The choice of setting is a matter of personal preference for many people, even after explaining the facts. The AUTO setting is the best choice, as it is energy efficient and contributes to the longevity of the HVAC system. However, if someone in the family suffers from pollen allergies, it should be tested whether the ON setting alleviates the symptoms by constantly filtering the air.

If your AC fans must be set to continuously ON to actually work, then it is high time to call a qualified HVAC service engineer and have your system thoroughly checked and serviced.