HVAC Question about running fan continuously VS setting on Auto so it runs when AC or furnace runs

I get a lot of confusion from homeowners who are told by their HVAC contractor to run the fan "on" all the time.  I realize that these high-efficiency furnaces now have the DC motors that take less electric cost but should they run all the time? 

I want to give my customer the correct answer and not which is correct or if there is a correct answer.

I know the older alternating current motors take a lot of electricity to run for a year.

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Many of our Wisconsin residents do not yet have the ECM (electrically commutated motors) though others do. It appears ECMs will become more standard soon. This is background for those answering your question.https://www.achrnews.com/articles/136459-fer-standards-require-elec...
There is no Pat answer. It depends.

In a well performing home with proplerly sized equipment and duct work (very rare) the answer is "YES"

Eliminating continuous furnace fan operation is #3 on list of top ways to cut energy use, in this recent study in Minnesota. So tell homeowners not to run furnace fan all the time unless there's a special reason. My understanding is that spring and fall "shoulder" months have more potential for stale air, lack of air movement, a good time to run fan more if you're worried about getting enough ventilation for good health. https://www.cards.commerce.state.mn.us/CARDS/security/search.do?met...

In regions with _high_ humidity, running the fan continuously will be put moisture that the evaporator coil has collected back into the air. It will cause higher indoor humidity and the homeowner will be less comfortable at the same temperature.

** On single system unzoned multi-story structures, running the fan continuously will in most cases remove the temperature gradient between the floors. As long as the home has upper floor and lower floor returns, then the gradient will be averaged away by constant air flow.

Jerry, as Chris Heenan said above, running the fan continuously will raise the indoor humidity in humid climates.  Here's a graph of data from the experiment I ran at my condo a few years ago:

You can read the article here:

This Thermostat Setting Can Cost You Money and Make You Sick

Looking at that graph, Allison, the high RH period of five days looks like a vacation period perhaps? With AC set-point at a higher temperature?

My thinking is that, aside from contributed fan heat, running the fan is enthalpy-neutral. There may be a rise in RH when the compressor is off due to normal diffusion/infiltration and internal latent loads, but also re-evaporation from the drain pan, if not well-drained. 

But the diffusion/infiltration/internal latent loads remain essentially constant for discussion's sake and so would occur regardless of the fan operation mode.

But if in "Auto", sure the cooling and fan coincide. In "Fan On" the cooling-side would function the same, assuming identical set-points, would it not?

At Comfort Institute we get asked this question all the time.  The argument for leaving the fan on is better air stratisfication, more even temperatures and constant air filtration.  While this is very true the cons can out weigh the benefits.  One issue is that in the summer when the fan is left on and the homeowner has an air conditioner, the moisture that is still in the coil is transported back into the home and the  homeowner can feel more humid.  Many contractors have had complaints especially with the newer system and larger coils. Duct leakage is huge problem when the fan is left on.  Now the ducts are leaking 24/7. Dust, allergens, humidity and hot air now is being brought into the home.  Homes with pressure imbalances caused by closing doors to rooms with supplies and no returns again are also a big problem when fans left on as they are also a large source for impurities being brought in all day.  And if the system has high static pressure, the energy savings could be used up to get proper air flow or in some cases the energy it takes to move the air could result in a complaint of higher energy costs.  Of course this could lead to the premature damage of motors and or control boards. So the short answer is we are not in favor unless all problems are addressed and repaired and only when the AC is not running.  Feel free to reach out is you have any questions.


This is a great response.  Lots of arguments against leaving the fan on. THANKS

If an E/HRV system is installed in unison with the furnace ducting -- you might consider making the ventilation system operate in unison with the furnace fan. Many clients choose to run both systems on a timer set to activate for 20 minutes each hour. The E/HRV will introduce the fresh air and the furnace fan will distribute the fresh air to prevent staleness. This is a common strategy we see requested.

Jerry, I am a whole house fan expert but not an HVAC expert. However, I have no idea why any HVAC contractor would advise that. I know it can be done but you are correct  - the new brushless electronically commutated DC motors (ECMs) are incredibly economical and have multiple speeds - but one cannot predict the speed and they might run high so would cost too much to run 24/7. I have been using 1/2 HP EC motors in my whole house fans for some time and I am the only one who offers them with infinitely variable speed. So mine can be run at 10% of full speed and they take just 50 watts when they do. You can see them at http://www.ThorwaldsonWholeHouseFans.com


Thanks for the information on the whole house fan and the advise about leaving the fan on all the time during the AC season.  I have been in the energy consulting business for about 10 years and have had my own whole house fan in my new home for 13 years.  Love it.

The one thing I would love to see is do you have any videos on your systems and installs?




I don't have videos posted yet but I have some nice pictures from customers. See them here - 


As an energy expert you might be aware of the latest shakeup in the whole house fan space - a requirement that all suppliers use HVI-916 specs now. QuietCool is the only holdout - still displaying higher CFMs alongside their HVI-916 results. In fact, Neil at Airscape just came out with a new CFM spec - "Q-CFM' which he defines as "Quackery CFM". 

As a result, all supplier's performances are reduced to where they should be.

Another breakthrough is that I invented the world's first high performance rooftop! 


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