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HVAC thermostat fan - Auto or on?

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  • Jul 4th, 2021 9:45 am
Nov 16, 2017
64 posts

HVAC thermostat fan - Auto or on?

Hi Guys,
I was wondering what most of you guys leave your fan setting on your HVAC to - Auto or on?
I've just recently been leaving it "ON" and the house has been feeling cooler because it recirculates the air from the basement which is cooler, but I have noticed my daily energy consumption has gone up.
I'm guessing it should be around $30-$40 higher a month from leaving the fan in ON mode.

So my question is
1. what do you guys leave your fan on - Auto or On?
2. Does it really make a $30-40 difference? or do you think mine was just coincidental (I only measured for a few days).
3. Any pros or cons to either?

8 replies
Jun 4, 2020
1634 posts
Clarington, ON
I leave mine on.

Does your furnace have a DC ECM motor?
Nov 16, 2017
64 posts
no idea about the kind of motor, how do I find out?
Deal Fanatic
Jul 7, 2017
9166 posts
SW corner of the cou…
kkil4life wrote: no idea about the kind of motor, how do I find out?
Furnace model number would help. Picture of motor label even better.
I smile when I see container ships sailing past my house laden with stuff made in China
Deal Fanatic
User avatar
Sep 27, 2006
5337 posts
Not so easy there Ma…
Leaving the fan running could certainly add $30 to $40. As indicated DC motors use less than AC. If your furnace is an older model or an original builder installed furnace it's more likely to be an A.C. motor.
Deal Addict
Feb 10, 2006
2620 posts
Fan is set to on 24hrs a day 365. Been running none stop for several years.

I'd only recommend switch to auto if the furnace is old (ac motor) or for elderly customers when they feel cold all the time.
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Deal Addict
Jun 16, 2009
3994 posts
Having the Fan run continuously will provide even distribution of hot or cool air. This will reduce hot or cold blind spots in the home. Continuously running Fan also reduced the stress on the fan from frequent starts and stops, which can potentially help extends its lifespan.
Most modern furnaces are equipped with ECM motors and the operating cost is minimal compared to the benefits it provides.
Deal Addict
Oct 19, 2020
1047 posts
Running the fan continuously re-evaporates moisture off the indoor coil when the call for cooling ends.
See here: https://www.energyvanguard.com/blog/thi ... -you-sick/
The larger the unit is relative to what it should be, the shorter the cycles which makes this problem worse.

Many furnaces out there still have inefficient fan motors which draw 300w+ on the lower speeds (usually heat/continuous fan), more on high so energy use can add up fast. The extra energy used ends up in the form of heat which needs to be removed by the a/c.
ECM motors start gradually so the stress on the motor due to cycling is greatly minimized.

Yes, can be helpful for balancing, but for most cases "auto" is the best setting at least during humid weather.
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