Anonymous asked in Home & GardenMaintenance & Repairs · 4 weeks ago

Does leaving the fan set "on" instead of "auto" on the thermostat really make the house hotter, even if the thermostat is set to "cool"?

For the past week it's been unbearably hot in the house and we have not been able to get the temperature indoors below 80°F.

My father-in-law called a guy he knew to come take a look at it. He went out back and looked at the unit outside, then came inside and looked at the thermostat, and then he said that he didn't see any problems with the unit. But he also said that the reason why it was hot was because the fan was set to "on" instead of "auto," which didn't seem right to me. Before he came, he said that it was probably just the Saharan dust clogging up the air filter, which we had replaced only a month ago. None of it seemed to be right or make a lot of sense.

He was over for about twenty minutes and charged us $160, but it didn't look like he actually did anything. He didn't even take a look at the unit in the attic.

In the twenty-eight years that my father-in-law has lived in this house, he has not had the air ducts cleaned once. There is a lot of dust and mold inside them and the airflow from the vents has been very weak for at least a few years. The unit outside was last replaced in 2008, after Hurricane Ike. I don't know how old the unit upstairs is, but it's been leaking a lot from a hole in its side that's due to years of corrosion. I don't think he has ever had this unit service. In general, he just doesn't do anything as far as preventative maintenance is concerned.


Another thing that could be affecting the air conditioner's ability to cool the house is that the builders did not put enough insulation in the attic floor to meet the building code's requirements, which still fall short of what's recommended, and the roof itself is not insulated. It gets extremely hot in the attic.

8 Answers

  • Anonymous
    3 weeks ago

    A constantly running motor adds heat to the air that it's circulating.  If the a/c is on, that heat gets removed by the process.  If the a/c is off, the heat just keeps being added to the interior of the house. 

    This is also true for a fan in a room.  It's heat is spread throughout the room. 

    Air conditioners wear internally and lose efficiency over time. 

    Central air needs the condensing unit coil (outside) cleaned regularly and the evaporator filter (inside) changed regularly.  Being clogged with dirt and leaves, etc can reduce the cooling efficiency of the system or even injure the system.

  • 3 weeks ago

    Just having the fan on means that it is simply circulating the air in the house. If that air is hot you will just get hot air moving around.  The aircon is not working, just the fan.

    It needs to be set in your case to "auto", then the aircon will take heed of the thermostat and adjust the temperature accordingly.  If it can. Yours is clearly in a poor state and others have given suggestions on how to improve it.

  • 3 weeks ago

    Any system that is 28 year old and has not been maintained can have all kinds of this wrong with it. If you are really trying to fix the problem then you need to hire a HVAC technician who is not an old friend of your father-in-law to fully access the system including the duct work and the insulation in the attic.  One simple test he should do it check the temperature of the air coming out of the air registers and compare it to the temperature in the house. The cool air should be at least 15 degrees or so cooler than the air in the house. 


    Leaving the fan in the "on: setting helps keep the temperature a bit more even but it does nothing to make the house cooler or warmer.

  • 3 weeks ago

    No. Leaving the an on circulates the air. It doesn't change the temperature enough to notice. The MINUSCULE amount of heat created by the motor doesn't matter. In any case, unless the temperature is low enough for the A/C to shut off, the fan is on in either setting.

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  • y
    Lv 7
    3 weeks ago

    Air handler in the attic, which means keeping the fan on you are picking up the heat in the attic, and bringing it on down into the home. Insulation, insulating the attic floor, those ducts, and yes, even the air handler. Don't forget to seal the ducts before insulating them, in your situation. It can make a huge difference and this is also something, you can do. Proper insulation gets you more bang for your buck.  Age of ducts and system doesn't really matter. Duct cleaning may not even be needed if the ducts were sealed and proper filters have been being used. Filters may need to be changed more often depending on the situation. My outside unit, the condensing unit, was installed in 82, and still works fine. Of course, I maintain it the best I can and such. I used to work as an HVAC tech once upon a lifetime ago. Airflow is actually subjective, you need to actually have it tested to know what it is doing, you also need to balance the supply ducts and adjust them on occasion. The tech said everything is working properly. One of the easiest ways to check is the TD between the supply and the return, takes a couple of minutes  to get it, and no, you don't even need to see either unit.  You did say you had mold inside your ducts, to me, that is a concern if it is actually mold, not sure how you determined it was. It has nothing to do with your airflow but if true, it does need to be addressed.

  • elhigh
    Lv 7
    4 weeks ago

    I am an HVAC pro.  I think about heating and cooling all the time.

    The indoor unit being in the attic is a disaster, but you can't really do anything about it now.  It's hotter than hell up there and a lot of the unit's capacity is being lost to the fact that the attic is so hot.  The air conditioner is wasting a lot of energy cooling space that you don't even use - the heat leaks into the ducts that run through the attic, using up a lot of the cooling capacity.

    Consider cleaning the ducts, but don't get hung up on that.  I've never cleaned mine in 20 years and since I keep up with the filters, they're fine.  But do add insulaton to them.  Then add more.  Then add more.  In a really hot region you probably need a full 12" of insulation above your ceilings, and if there's any way to put that much more insulation around your ducts, do it.  You don't want to waste energy cooling the attic.  Insulate the air handler, too, as much as possible.

    Paint your roof white.  I'm not joking.  Anything you can do to reduce how much energy (heat) gets absorbed by your house, is energy (heat) you don't have to try to remove again with your underperforming HVAC.

    Change the filter again.  In really dusty regions, change the filter more often, maybe even as frequently as every month.  Filters are cheap, and keeping them fresh means plenty of clean air through your indoor coils (the cold ones).

    Your outdoor coils don't have a filter however, and all the heat from inside your house is getting pumped through them.  If the outdoor coils are dirty then the system can't dump the heat into the outside air, and it will slowly overheat and maybe destroy the compressor.

    Clean the coils.  Lots of tutorials on YouTube how to do it, so I won't describe it here.

    The fan being ON all the time normally wouldn't make a big difference, except with the ducts running through your hot attic, it really is making your place hotter, running cool house air through the ducts where it can pick up attic heat, then bringing that back inside to heat your place up.

    Turn the fan to AUTO, and add lots of insulation.

  • Anonymous
    4 weeks ago

    Replace the filter in the evaporator.  Set the thermostat to the coldest setting.  Check if air is coming out of the vents.  Go outside and check if the outside unit is running. 

    If air is coming out of the vents really good and the outside unit is running (there should be a compressor buzzing sound and the fan should be running), the system could be just low on refrigerant.  Turn off the AC and clean the fins in the outside unit with the garden hose and see if it fixes the problem.  No luck, it is time to call a competent AC technician.  System could be low on refrigerant or there is not much air flow.  Evaporator might need manual cleaning and the refrigerant charge needs to be checked.

  • 4 weeks ago

    Putting the fan ON keeps the air moving thru the house and you will feel cooler due to the constant motion of the air.   Same idea as a ceiling fan but it does not work as well.   I would do it if I have a basement that stays cool all day long.  I would set the return to the furnace to come from the basement and where it blows out to the upper floors.  

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