Should I Set My Thermostat Fan to ON or AUTO?

Thermostat on or auto for fan setting?

An AC thermostat for a home normally has two fan settings: ON and AUTO.  The third, of course, is OFF.  The confusion lies in which setting should be used. What is the difference between the two? Does it even matter?

Recommended Thermostat Setting

Under most circumstances, we highly recommend your AC thermostat be set to AUTO. Not doing so could significantly increase indoor humidity, possibly to the point where it causes serious mold and air quality problems.  This is especially the case in a hot/humid climate, such as Florida, or anywhere when the weather outside is hot and humid.

Before explaining the key difference between ON and AUTO for your thermostat, it’s important to know how your AC system works.

Note:  This explanation describes a simple AC system for a home. It may not fully apply to a more sophisticated setup, which may use various modifications to the basics described here to accomplish specific goals.  Before modifying how you operate your system, especially a more complex one, discuss it with your licensed AC consultant or contractor.

How Your Home AC Works

Your air conditioner generally works this way:

  1. The outside component of the system, the condenser, compresses the cool refrigerant coming from the air handler, the inside component of the system, converting it from a gas to a liquid. The hot liquid refrigerant goes through the outside coils and releases heat to the outside.
  2. The now warm liquid refrigerant is pumped back into the house and into the coils of the air handler. As it enters the low-pressure environment of the coils, the evaporator, it expands back to a gas, its temperature drops and it chills the coils.
  3. Air is blown by the AC blower through the cold coils and their fins, cooling the air and transferring heat to the refrigerant. If the air temperature drops below the dew point temperature (actual, not relative, humidity) of the air, humidity condenses out of the air onto the coils.  This moisture then drains into the condensate pan and eventually to a drain or outside.
  4. The refrigerant is now a cool gas again and is pumped back to the condenser where the process repeats.AC heating and cooling process.

Thermostat ON or AUTO Setting

When thermostat is set to ON, the fan runs continuously, with the benefit of providing constant circulation of the air inside. However, at the same time, the fan continues to blow air through the coils after the condenser shuts off and the coils are no longer cold.  Thereby, it stops removing moisture from the air, but there is still some condensed water on the coils and sometimes in the condensate drain pan.  If the fan continues to run, some, perhaps most, of this moisture will evaporate back into the air and be blown back into the home, raising its humidity.  You would also see an increase in your energy bill as well.

Now, when setting the thermostat to AUTO, the fan shuts off when the condenser shuts off.  Condensed moisture on the coils tends to drain into the pan and outside.  Most of this moisture will have drained away by the time the fan comes on again, which means it’s not going back into the air of the building.  Humidity stays lower.

Over time, the relatively small amount of additional moisture evaporated back into the air each time the condenser cycles off with the Fan in AUTO mode can cause humidity to go higher.  How much higher depends on many peculiarities of the system design, and this explanation is of course quite simplified.  But the basic idea is entirely valid:  Operating an air conditioning system in the Fan ON mode may significantly increase indoor humidity.  If humidity goes high enough, it can greatly impact the indoor environment by allowing mold growth and other negative processes to occur.

Related: Controlling Humidity in Florida Homes Year-Round

Higher humidity (also known as latent heat) will increase the amount of energy needed to cool the area to a given temperature, as well as making the environment less comfortable at that temperature.

 

Conclusion

So, in hot/humid weather, operating the thermostat of your AC system in AUTO rather than ON mode will provide the following benefits:

  • Reduce interior humidity.
  • Make the indoor environment more comfortable for the occupants.
  • Save energy and therefore money.

That’s a win-win-win!

5 comments

  1. David Devol says:

    Nuccio install my A/C Heat pump system, and it’s very efficient, however I still am trying to figure out why the “auto” setting works w/ A/C but the heat setting requires the fan/circulation setting requires “on”.
    I’ll be researching the owner’s manual and appreciate the reminder!

  2. IET says:

    David, I am of course not familiar with your system, and you should certainly make sure you’re using it properly.

    However, the issues described in the article arise when the system is operating in Cool Mode. I don’t know of any environmental concerns created by operating the system in Fan On mode when heating. It will of course use somewhat more electricity, but perhaps the purpose of having the fan running continuously is to circulate air constantly and prevent exterior rooms from chilling too rapidly.

    Sincerely,

    Tim Toburen
    Environmental Consultant

  3. Michele says:

    What about during the winter? Would it be beneficial to have the HVAC in the “On” mode during showers to help humidify the dry air?

    • IET says:

      When the system is heating, there are no humidity problems created by operating the fan in ON position. It helps circulate the air and might help prevent those rooms on exterior walls from cooling off too quickly.

      In really cold climates and with older, less-airtight, homes your question might work. Of course, all it does is help circulate humidity from where it accumulated due to showering to the rest of the building.

      However, I would like to point out that if you are in a climate that gets cool, but not really cold, such as Florida, and/or have a really “tight” home, the air in your home will not necessarily be dry at all in the winter. In fact, some of the worst mold problems we’ve see in Florida homes occur during the winter, as a result of accumulation of internally generated humidity from cooking, washing, showering, etc. If the home is really tight, and the temperature is too low to activate the AC thermostat, that humidity just builds up, potentially high enough to cause real problems. See Controlling Humidity in Florida Homes Year Round. The conditions is by no means unique to Florida.

      The best thing you can do is get a hygrometer (relative humidity meter) and keep an eye on it. If it starts climbing towards or above 60%, regardless of the time of year, take appropriate steps to keep it below 60%.

      Sincerely,

      Tim Toburen
      Environmental Consultant

      • Michele says:

        Thanks Tim! That is very interesting. This week in central Alabama it has been quite cold, and our house has been very dry with humidity readings in the 35% range. We have a pretty airtight home. The summer is a different story, with our central dehumidification system running constantly due to excessive humidity. Thanks again.

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