Why does my attic fan work so well?

[Andy Simms]

The main purpose of the fan is to keep the attic well ventilated - the reasons for this are listed quite well on this web page:

[LenS]

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.

-Len

[Edwin Pawlowski]

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 principal applies.

[FDR]

I'm curious as to wether the fan blows in or out at the gable.

[trader4]

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.

[Don]

The fan should blow out the end and pull thru the other... this way it does not pressurize the attic and force hot air in any openings to the house etc...... Don

[LenS]

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.

-Len

[LenS]

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

[Travis Jordan]

You need more attic insulation. Numerous studies have shown that attics that are adquately insulated (R30 - R40 in most climates) don't benefit from powered attic ventilation.