Run Central A/C fan 24/7?

I have a home that was built in 1986 in South Florida. It is a 2- story with only a single central unit. The upstairs has always been warmer than downstairs. I have always used the "auto" setting for the fan, but found that the house has the roller coaster effect when the unit cycles on and off. BTW, the unit IS correctly matched for the house, so it is not over or under rated for the SQ footage of the house.
I read somewhere that when there is a 2-story home w/out zoned A/C, it is preferable to run the fan 24/7 because it will keep circulating the air from upstairs thus keeping the whole house more consistent with temperature. Does this make sense?
The unit that I have is an Intertherm and it has a variable speed fan, so it is moving 1600 cu.ft. when the compressor is on, and when it is off, the fan slows to 1/2 speed.
Thanks in advance for any help or advice.
SD
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Well it's good that it has a dual-speed fan, it can lessen the impact on your electric bill of the 24/7 operation.
Only other downside is you may get an increase in humidity. As I'm sure you know, when the fan is on "auto" it shuts off when the compressor does. At that time, there is still moisture in the evaporator (the "indoor coil" of the A/C) but it mostly stays there. If the fan continues running after the compressor shuts off, the moisture in the evaporator can get re-absorbed by the air in the house, making the house more humid. Exactly how much this affects things you'll have to find out by trial and error.
Eric

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Eric wrote:

Filters will also need to be changed more frequently if air is moving 24 hours a day.
Pete C.
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I never read that anywhere but my old house had central air and leaving the fan on did always make it more comfortable upstairs. I didn't do it 24 hours only at night when I needed the upstairs to be colder. I also had switches on each duck in the basement so that I could shut off all the rooms that didn't need to be as cold. I always had the upstairs ducks open more then the downstairs in the summer and reversed it in the winter.
Recently I put a window unit in the bedroom to run at night. That way I didn't need to cool the entire house. Since I only used one room and don't have any kids it saved me a lot of money. Paid for the a/c unit with in 2-3 months.

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sophiesdad wrote:

Couple of comments besides the others, so far...
Bottom line is, if it is more comfortable operating w/ fan 'on', than without, go for it--that's the whole point. You may find by keeping the air moving more you can actually raise the thermostat a degree or two and still be comfortable actually lowering overall power demand.
Even if not, since by your description the fan is running all the time anyway, just at half-speed w/ compressor isn't running, I doubt the difference will be noticeable.
I also suspect your humidity control won't be noticeably worse, particularly with the above point of fan on lower speed.
Your other option if the upper floor really needs to be cooler during the day rather than just in evenings/nights is to experiment w/ register settings to see if by closing some of those downstairs you can get a little better/more even circulation.
If the symptoms are really bad, could always get an estimate for the installation of an adjustable damper in the main trunk or see about adding more return duct area in the upstairs...
--
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Been running our heating/air conditioning fan 24/7 for about 20 years. If I don't, we find that the air will stratify, or stagnate with hot layers and cold layers. Keeping the air moving makes it more comfortable, and when heating in winter the furnace runs less because the heat doesn't rise to the ceiling allowing the thermostat to cool off.

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Thanks, all for the suggestions.
I could kick myself in the rear sometimes, because after Hurricane Andrew, my entire upstairs was gutted. I should have takent the opportunity at that time to either have a second unit installed on the 2nd floor, OR have other measures installed to overcome the poor design of the builder - i.e., dampers in the ductwork AND a return duct from the 2nd floor. But, having had the house devasated and being displaced for almost 9 months, I was just focusing on getting the house back to its original condition.
Lesson learned: I will NEVER buy a 2-story home w/out a split A/C system, or at least one that is better designed than is currently in this home. Since I don't intend to live in this house more than 5 years before retiring and moving on to cooler climates, I'll try and see if running the fan 24/7 makes for a little more comfort.
Thank you again for the comments!
SD
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