DEAR TIM: What is the difference between a whole-house fan and an attic fan? Which is the best one to have in a home? What is the best way to cool a home using these fans? Can either of these fans cause problems? Renee H., Littleton, CO
DEAR RENEE: Whole-house fans and attic fans are as different as apples and watermelons. Both fans move air just as apples and watermelons are both fruit, but the similarities end there.
A whole-house fan is a fixture that is often located in the ceiling of a home. In rare occasions, they can be located in a wall. This fan might be in a hallway or some other central location of the home. The fan is always located on the highest ceiling or wall and oriented so that it blows air directly into an attic space.
These fans are designed to pull air through a house and exhaust the air into an attic space. Since you live in a area that has low humidity, they are superb and highly efficient cooling devices. They work best when they are operated in the early evening or night hours as outdoor temperatures start to nosedive and the inside air temperature of a house is still high. As outdoor humidity rises, the cooling effect from whole-house fans drops.
An attic fan is completely different. These are smaller fans designed to move hundreds of cubic feet of air per minute out of your attic area only. They are not supposed to move air through your home. The cooling principle here is simple: If you replace the extreme hot air inside an attic that might be 140-160F with air that is 85-95F, the inside of your home should also be cooler. The heat gain from extremely hot ceilings adjacent to hot attics can stress air-conditioning systems, and push them to the limits of their ability to cool a home.
For both fans to work properly, they need to easily exhaust the air to the outdoors. The attic fans do this by default, as you typically have to cut an opening in the roof or a side wall of an attic to mount the fan. The attic fans also need lots of intake air vents along the lower edges of the roof. Continuous venting is the best and the air from the outdoors needs to be able to easily enter the lower reaches of the attic where the roof passes over the outside walls of the house. Since most attic fans are located high up on a roof, this design allows for excellent cross ventilation of the entire attic space.
If you do not provide enough intake air vents for attic fans, they can draw air from inside your home. This happens because the moving blades of the fan create a partial vacuum. It is not a good idea to send cool air from the inside of your home up into an attic, since you are paying good money to cool inside air with an air conditioner.
Whole-house fans also need plenty of exhaust air vents through the roof or through side gable walls of attics. Each whole-house fan manufacturer tells you for each size fan how much net-free area of exhaust ventilation the fan requires to work properly. If you do not provide this exhaust ventilation area, the fan blades will spin, but not move the maximum amount of air.
Whole-house fans serve double duty as they remove hot air from the attic as they work. But you do not necessarily want the whole-house fan operating in the middle of the day as your attic heats up. For this reason, it makes sense to have both types of fans in a house. They operate at different times of day and night, since they are designed for two different purposes.
Whole-house fans work so well, they are often equipped with timers. If you operate one all night long, you might need to sleep with blankets. They often can cool a house to a comfortable temperature in just several hours.
You can get concentrated cooling with a whole-house fan by opening the windows in selected rooms. As the fan operates, it creates a comfortable breeze in the rooms with open windows. Be sure you do open windows before you turn on a whole-house fan. If you do not and you have a fireplace with ashes in it, you will create an ash storm as air is sucked down the chimney and rushes across the dry fireplace ashes. The dirty ashes are then broadcast through the house as they are sucked towards the whole-house fan. How do I know this? It is the voice of experience talking.
