8/16/2011

Attic Fans vs. Whole House Fans

Triple digit temperatures are coming back. I hope you enjoyed the reprieve as much as I did!

I did not grow up in homes with attic and whole house fans, so I have only recently learned what that thing in the ceiling was, how it works, and how to use it.

Our house has a whole house fan. I was told this is an attic fan. It is not. The difference is that attic fans are in the attic and they only serve to move hot air from inside the attic to outside of the attic. This is helpful because the attic can reach temperatures of 150 degrees or more, heat your ceiling, and heat your house, so cooling the attic will also cool the house. On the other hand, whole house fans are in the ceiling, connecting the living space to the attic. They serve to draw outside air into the house, air from the house into the attic, and air from the attic to the outdoors. If used correctly, this should cool down both your attic and your living space.

So use common sense here. It won’t be cost effective to use the whole house fan all of the time, since it is using the outdoor air to cool the house. The rule of thumb is that the whole house fan is useful when temperatures outdoors are below 85 degrees and it’s not very humid. Even in Oklahoma there are normally several weeks a year during the spring and fall when I can use only the attic fan for comfort, never turning on the HVAC at all. Then there are weeks like this one. It’s hot during the day, but the temperature should start to reach 85 around 8 or 9 pm, and will probably stay below 85 until noon. This means that in the mornings and late evenings I can switch off my air conditioner and use the whole house fan. (If I was brave I could even leave the whole house fan running while I was sleeping… but I don’t want to leave my windows open at night!) 

In order to use a whole house fan:
1.      Make sure it isn’t humid
2.      Make sure the temperature outside is below 85 degrees
3.      Turn off the air conditioner
4.      Open screened windows
5.      Turn on attic fan

Use an attic fan when:
1.      It is hot outside!

As a side note… you can sometimes break the 85 degree rule based on your needs. For example… last summer, our air conditioner broke during the height of hotness. I turned on the whole house fan and sat smack dab underneath it to cool off. It was still hot, but the breeze it created in combination with cooling the attic kept me cooler. I am glad there are no pictures of me sweaty and half-naked in the hallway, but it was certainly better than the alternative (no a/c and no fan)!

In order to make my whole house fan more energy efficient, I covered it to help prevent air conditioning from leaking into the attic and heat from leaking into the house when it was turned off. I bought a cover for our fan from Battic Door, but I could probably easily have made one with some foam and double-stick Velcro. 

Also – this is Oklahoma! So I must take allergies into consideration. If ragweed is blooming and it makes me wheeze, I can’t use the attic fan.  As an asthmatic with allergies, I have to keep an eye on both allergy and ozone levels to determine if it is safe for me to cool the house using outdoor air.

1 comment:

Kay Hyde said...

Thank you for sharing this! :) This is very informative. Now, I have a clearer understanding about attic and whole house fan.- mxsouth.com