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The Furnace Fan Showdown: On v. Auto

Which is best? "On" vs "Auto" settings

By: | November 17, 2014 3:58 pm

Here’s a great question from one of our readers…

Q: Is it more efficient to keep your furnace fan turned to the “on” setting rather than “auto”? I’ve heard that if you keep the fan going, it will circulate the air in your home and keep the temp more even… and thus the furnace/air conditioner won’t have to turn on as often.

A: It depends on several factors we’ve listed here:

The type of motor you have in your furnace. Furnaces generally come with two basic types of blower motors; PSC (permanent split capacitor) and ECM (electronically commuted motor). The PSC, which has been used in forced air furnace since the invention of the forced air furnaces is less efficient than an ECM and so running this type of motor may not be more efficient. However, you also consider temperature stability, which is a comfort issue. It is true that running a motor continuously will result in more even temperature distribution.

How well your duct-work is designed. Properly designed duct-work allows air to travel through it with the least amount of resistance. Duct-work that’s properly designed will be sized and laid out to handle the volume of air required to serve the needs of the house. It also needs to be constructed of the proper material. A fan that is run continuously with improperly designed ducts will have to work harder to push air which will result on increased electricity usage, regardless of the type of motor being used.

How well your house is insulated. A house that can’t hold on to its heat will require the furnace to run longer and more often. An argument could be made that very leaky houses would do worse if a blower motor is set to the ON position. The theory is that by turning on the blower motor, a house is put under pressure which would increase the rate of heat loss to the outside.

Our suggestion. Test the savings advantage by setting your thermostat to AUTO for one month and to ON for the next month (ideally months that have similar weather) and compare the energy usage. This would also provide an opportunity to see if there is any noticeable difference in comfort.

Going forward.

Consider using our HomeSmart program in Colorado or Minnesota to assure the performance of your appliances. HomeSmart provides your home with customized appliance repair plans. With one monthly payment, your appliances (which can include your furnace) will be covered in the occurrence of an unexpected breakdown. No cost for labor, no cost for parts.

Another smart option is Home Energy Audit. You can choose from several audit options including a basic, standard and a premium level audit that includes an infrared scope with a blower test that pinpoints insulation gaps and air leaks. Learn more how Home Energy Audits can provide you a comprehensive profile of your home’s energy performance and specific areas for improvement.


A self-professed energy nerd, Paul loves to explore how technology trends are bringing convenience and surprising benefits to our lives.

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