Q&A / 

Whole House Fans and Attic Fans

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.

This attic fan can also be called a powered attic ventilator (PAV). They are often visible on the outside of a house either on the roof or on a side wall of an attic gable. Whole-house fans are located inside homes. Both perform entirely different functions. PHOTO CREDIT: Tim Carter

This attic fan can also be called a powered attic ventilator (PAV). They are often visible on the outside of a house either on the roof or on a side wall of an attic gable. Whole-house fans are located inside homes. Both perform entirely different functions. PHOTO CREDIT: Tim Carter

A typical whole-house fan moves vast amounts of air. The blades of the fan can be large, and they can have significant pitch so they can move thousands of cubic feet of air per minute. Turn one of these bad boys on inside an average-sized ranch home that has 2,400 square feet of finished floor space, and you can replace every bit of stale, hot inside air with cooler outside air in less than five minutes.

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.

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7 Responses to Whole House Fans and Attic Fans

  1. We just bought a home in New Brunswick Canada and we are trying to find out what the Aston Atic 2000 Box is on the wall. It looks like a thermostat but says humidity percentages. We are very humid out here and have a dehumidifier going in the finished basement and would like to know if this unit is a humidifier or dehumidifier.

  2. We have a 1 1/2 story house. Half the house has vaulted ceilings, the other half is a two story house. The downstairs has a separate a/c unit from the upstairs. I'd like to install a whole house fan upstairs. Only one upstairs room is used as a bedroom. I have two concerns: First, is the noise a whole house fan makes uncomfortable for sleeping? Second, Is there any reason I can't install it in the ceiling of the adjacent upstairs room? If I did this, would it detract from the cooling of the bedroom, which would defeat the purpose?

    • Charles, your question requires lots of typing, plus I have some questions for you so I can give you the correct answer(s). I only do pithy answers here in the comment section. If you want to protect the investment you have in your house and not waste time or money *hoping* you make the right decision, you should talk to me on the phone for just 15 minutes. It'll be the best investment you've ever made in your home!

  3. I would like to install a whole house exhaust fan to remove the hot summer air from my upstairs, that has cathedral ceilings. Unfortunately, my roof has extremely negligible attic space above the cathedral ceilings. Can I just exhaust the whole house fan air to the outside through a fan panel cut very high on the wall, instead of venting through the attic ?

  4. hi ; would you be able to get &/or give me info for a system that is installed in our home. When we moved in we did not get any info on this system. The system in question is a ASTON ATTIC 2000 RELATIVE HUMIDITY. We have tried to locate company, looked it up on web. With no results.Had no luck at all finding anything on this system. If possible please help.. thank you for your time..
    Carl

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