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
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.
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.
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
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...
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.
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!
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