Thermostat Fan, On or Auto?



HVAC Thermostat Switch, On or Auto – Answered!


thermostat fan switch auto



Where should I set my thermostat fan, on or auto? I just answered this question for a friend, and it reminded me a lot of people don’t know. However, you’ll now know which is right for your home & A/C unit after reading this article: Thermostat Fan, ON or Auto?



Thermostat Fan Switch – ON vs AUTO


Having been an HVAC Technician for over 30-years, I’ve met many homeowners who were unsure how to set their thermostat. The most common misunderstanding is what the “Fan” switch even does. Before we cover where to set it for your home, let’s review what it does.


The fan switch on your air conditioning & heating system only controls the “indoor” fan – often called the blower motor.


The indoor fan is one of two fan motors in an A/C & Heating System. The other fan is the “outdoor” fan – often called the condenser fan.


The indoor fan is the one that blows the air you feel coming out of your vents. This fan also “sucks” air through your air filter, known as “return air”.



Fan Switch – ON 

  • When your thermostat fan is set to the ON position, the indoor motor runs nonstop 24/7. The fan will continue to run regardless of your temperature setting or heat / cool settings.


Fan Switch – AUTO

  • When your thermostat fan is set to AUTO, the indoor fan runs only when the system is heating or cooling.




When Should You Set Your Fan to ON?


Running the indoor fan motor constantly on your air conditioner or furnace has its benefits. However, setting your thermostat fan to the ON position with the wrong unit can be a costly mistake – literally. I’ve had customers who wondered why their power bill suddenly skyrocketed, only to find someone had turned the fan switch on. “Someone” usually turns out to be one of the kids by the way!


Set your fan to the ON position if:


  • Your A/C or Furnace is equipped with a variable speed blower motor*.
  • You have a whole-home electronic air cleaner or air scrubber connected to your ductwork.
  • Indoor Air Quality is a top priority (outweighing the additional power consumption).
  • You would like more even temperatures from room to room.

*If you’re unsure what type of blower motor you have, ask your HVAC Contractor at your next maintenance visit.


Set your fan to the AUTO position if:


  • You have an older, inefficient A/C or furnace.
  • Indoor Air Quality and good filtration is not important to you.
  • Lower energy bills are very important to you.
  • You have an older (leaky or poorly insulated) duct system.




thermostat program setting



If you live in the Greater Phoenix, AZ area, just call us for guaranteed 5-Star service. Magic Touch Mechanical provides HVAC & Energy Auditing services anywhere within 35-miles of Mesa, Arizona.