On 20 Aug 2005 05:39:11 -0700, wrote:

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 140-70=70 to 110-70=40, that's about a 40% decrease.

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
difference.

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.

-Len