I'm not complaining about my attic fan working as well as it does. I
just don't know a lot about home construction and I don't know why the
fan is so effective.
We have a ranch-style home with an unfinished attic. There are gable
vents at each end of the attic. The fan is in one of the vents, the
other one is open.
The attic floor is covered with deep insulation, fiberglass, I think.
I understand that the purpose of the attic fan is to exhaust the air
that's been heated by the sun shining on the roof, and replacing it
with the relatively cooler air from outside.
With the central air conditioning running, turning on the attic fan
can cause the house temperature to drop two degrees (maybe more; we've
never tried it because the two degree drop is fine).
My question is how does the fan make this much difference with all
that insulation between the attic floor and the house ceiling? Is the
fan somehow removing warm air from the living quarters? Is there a
stack or vent or something that leads from the house into the attic
through which the fan might be exhausting warm air from the house?
Any comment will be appreciated.
The A/C should be maintaining the inside temp at the set point, unless
it's set so low that it can't reach it, so how are you measuring that
turning on the fan causes the temp in the whole house to drop 2
degrees? A more typical scenario would be to notice that with the
attic fan on, the house cools down quicker, the A/C runs less, or a
particularly hot area of the house, ie, an upstairs bedroom is cooler.
That's the case in my house, where the attic fan helps keep the
upstairs bedrooms cooler, but not the whole house.
As others have pointed out, it works by reducing the temps in the attic
from say 140 degrees to a more manageable 110. The amount of heat
entering the house from the attic is proportional to the temp
difference. So by going from 140 to 110, the fan can cut the temp
difference from 140-70p to 110-70@, that's about a 40% decrease.
On 20 Aug 2005 05:39:11 -0700, firstname.lastname@example.org wrote:
I should have explained more clearly how we know the temperature is
dropping in the house when the fan is on.
Let's say we've set the thermostat for 77F degrees, but the
thermometer on the thermostat is showing that currently the air is
79F. There have been times when the weather is very hot when the A/C
doesn't ever get the temperature down to 77F, or sometimes not even
down to 78F.
When we run the fan, however, the temperature does drop to where we've
set it, usually in about an hour, and we actually can feel the
We have experimented with turning off the fan after the air has
dropped to the set temperature. In every case, the indicated
temperature climbed again. Turning the fan back on again results in a
temperature drop. Not very scientific, but it did convince us.
Thanks for the response.
Not actually taking heat from the house, but preventing it from entering.
First, there is "heat". There is no such thing as "cold", but merely the
absence of heat. If the attic is very hot, the heat energy is going to go
to any place that is cooler. That is your air-conditioned living space.
In spite of all the insulation, heat will still seek cooler places. (with
less insulation it would be worse) Removing the heat from the attic, it
goes and heats the outside air and thus, the AC has less work to do.
Look at how a solar panel works. Your attic is just a big solar panel, not
as efficient as one designed for that use though, but still the same
In our case the fan blows out. I can tell because there are shutters
that open outward on the fan exhaust vent.
Maybe I'm not using the term "gables" properly. Our house has a peaked
roof which runs the length of the house. The opening for fresh air is
at one end beneath the point of the peak, and the fan and exhaust are
at the other end.
Thanks for the response.
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