ON vs AUTO Pro and Con 

These two settings control the blower in your heating/air conditioning system. When “ON” is selected the blower runs continuously, even when the system is not heating or cooling. With “AUTO” selected the blower runs only when the thermostat activates the system for heating or cooling. Both selections have advantages and disadvantages.

ON

Advantages: continuous filtration of air, and circulation prevents room air from stratifying—where the warm air collects at the ceiling and cool air settles at the floor. Also, the on/off cycles of the the heating/cooling system are not as noticeable when the blower runs continuously.
Disadvantage: Uses more energy than “AUTO.”

AUTO

Advantage: more energy efficient mode than “ON.”
Disadvantage: Less air filtration, possibility of stratification of air—especially during winter heating months in rooms with cathedral ceilings.

   While it might seem that setting the blower at “ON” to run continuously would reduce the life of the blower motor, that’s not necessarily the case. Motor life is reduced more by the repeated on-off cycles of “AUTO” that the extended time running in “ON.”

   Even if you normally set the blower at “AUTO,” when you have an odor problem, consider putting an activated-charcoal type filter in your filter slot, such as the 3M Filtrete Odor Reduction Filter, then setting the blower at “ON” for several days to assist in getting rid of the smell.  

    Also, see our blog post  What does "AUX HEAT" and "EM HEAT" on my thermostat mean?

• • • • • • • • • • • • • • • • • • • • • • • • • • • • 

  To learn more about heating and air conditioning systems, see these other blog posts:

• How can I find out the SEER of my air conditioner? 

• My air conditioner won't turn on. What's wrong? 

• How can I find out the size of my air conditioner? 

• How can I tell whether the condenser (outdoor unit) is an air conditioner or heat pump? 

• Where is the air filter for my central air conditioner and furnace? I can’t find it? 

• Does an old air conditioner use more electricity as it ages? 

• How did homes stay cool in Florida before air conditioning? 

• What is wrong with an air conditioner when the air flow out of the vents is low?

• Why has the thermostat screen gone blank? 

• Why does it take so long to cool a house when an air conditioner has been off for a while? 

• Why is my air conditioner not cooling enough? 

• What are the most common problems with wall/window air conditioners?  

• Will closing doors reduce my heating and cooling costs? 

   Visit our HEATING AND AIR CONDITIONING page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.