Choosing the Right Whole House
There are two basic styles of whole house exhaust fans. The gable mounted exhaust fan is mounted on an outside attic wall (see the topmost graphic in this article). The floor mounted exhaust fan is mounted directly on the attic floor above a set of self-closing louvers. There are advantages to each type of installation. The floor mounted fan is the most common do-it-yourselfer fan because installation is usually very simple, requiring little carpentry skill. Gable mounted fans, on the other hand, require the homeowner to cut through the outside wall of the attic and install a second set of self-closing louvers. Gable mounted fans, for all their extra installation work, are much less noisy because they are further from the living space. And because you are installing louvers that match the fan's size, gable mounted fans have less flow restriction and are often more efficient than floor mounted fans.
To complicate your choice further there are two different methods for transferring power from the motor to the fan blade. Direct drive fans feature a fan blade that is attached directly to the spinning shaft of the motor. Belt driven fans turn the blade by utilizing a fibrous rubber belt (similar to the belts on your car's engine) between a pulley on the motor shaft and the fan pulley. The belt drive units are less noisy because the belt absorbs motor vibration that can be transferred to the fan blade. The extra workmanship and parts required for belt driven fans tends to make them more expensive than direct drive fans, but they also tend to be both more powerful, longer lasting, and more easily maintained and repaired.
Regardless of which type of fan you purchase, a special set of self-closing louvers must be mounted on a ceiling beneath the attic. These louvers open to allow the fan to suck stale are from the living area, and close to prevent hot attic air and dust and vermin from filtering down. After installing many of these units, I must warn you that most homes have at most one or two possible locations for the louvers that make any sense from either a decorating or functional viewpoint. Obviously, you don't want them in a bedroom or bathroom so most installations are done in hallways or the ceilings of high stairwells.
Look in the attic for obstructions. Most homes have some utility-type stuff in the attic. Electrical wires are almost universal as well as HVAC ducting... if you have central air or forced air heating. You will find plumbing vent pipes, miscellaneous wood supports, flooring and even the occasional hot water heater or air conditioning unit! So it is imperative you look into the attic above your chosen location. Electrical and plumbing connections can be changed without too much problem. Ducting, AC units and other appliances may be difficult or impossible to relocate without great effort and expense.
If you don't have a nice set of attic stairs or trap door to the attic area, you will have to improvise. For a quick access hole a large closet is the cat's meow. Frankly, this location will give you the freedom to make a mistake with only small home repair consequences. By confining the hole to a small area the repairs are relatively easy. For example, you can put opaque plastic over the hole in the ceiling and stop looking up. Hey, looking up is bad for the neck!
NOTE: If you have decided to install a gable mounted fan, you can pass up this section. They provide themselves with more than enough venting via their wall mounted self-closing louvers.
Once the whole house fan pulls air from your living space into the attic, it has to go somewhere. If you have a floor mounted fan, the air sucked into that attic will exit through all of your existing vents... soffit vents, ridge vents and/or gable vents depending on the style of your home.
You can compute the square feet of venting required by using the following formula:
The volume of air in a room is obtained by multiplying the length by the width by the ceiling height. Add up the volumes of all rooms you want to ventilate. Be sure to include hallways and stairwells in your calculations.
As an example, say you house has six rooms, 10ft by 10ft, with 8ft ceilings. Each room has an air volume of 10x10x8=800 cubic feet. So the volume of all six rooms is 6 x800 = 4800 cubic feet.
Add another 400 cubic feet for hallways, and you have a total volume of 5200 cubic feet of air in your home.
Divide that by 750, and you reach an optimal attic vent area of just under 7 square feet. Go into the attic and measure the size of your vents. If you have two 2ft x 2ft gable vents, you have a total of 8 feet of venting, which should be adequate. Of course, screens on vents decrease the overall vent area by up to 25%, so you have an actual usable venting area of 6 square feet. Slightly less than optimal but, in my opinion, adequate for good air flow... and sure better than nothing!
If you have a home with a ridge vent... the type of vent that runs along the peak of the roof, you don't even have to measure. A 40ft long ridge vent is equivalent to 20 sq. feet of gable vent... more than enough venting for even the largest whole house fans. Soffit vents... the vents located under roof overhangs that work together with ridge vents to circulate air through the attic year round... add significantly to the total vent area.
If you find that you don't meet the minimum requirements, you have a few options. You could get out the circular saw and install larger gable vents. You could install a ridge vent along the peak of the roof. Or you may instead install moveable exhaust louvers on the outside of the house, such as those used for gable mounted fans. Because they only open when the fan is on, they do not need screening... unless you have supersonic pterodactyls flying around in your neighborhood. If so, don't ask me to dinner... at least after dark.
To summarize, even if you don't meet an optimal venting figure, you can still install a fan. It will just be less effective at moving the volume of air it is designed to move. If you are confident an attic fan is right for you, it's time to choose the right size!
The rule is simple... purchase a whole house fan large enough to change the air in your house completely in around three minutes. This is a tremendous air flow, but necessary to cool down the house and produce a pleasant breeze even when many windows are open. A fan that is too small will cause so little air movement that you will hardly notice it! Fans are rated based on their CFM, or how many cubic-feet-per-minute of air they can move.
THE VOLUME OF AIR in your
house DIVIDED BY 3 =
Again, be sure to include hallways and stairwells in your calculations, or you may underestimate the fan size you need. Exclude from your calculations closets, storage rooms, separate walk-in attics and other rooms you don't wish to ventilate.
If you have a finished basement, only include it in your measurements if you plan to ventilate it. The reason for mentioning this separately may not be obvious, but important. Basements are usually cooler than the rest of the house, so it would be self-defeating to suck the relatively warm, humid air outside into the cool, dry basement. If you have a humidifier running to keep the basement dry, all the more reason to not use the whole house fan for ventilating the basement... unless, of course, the basement is warmer and more humid than the outside! Then, by all means, let 'er rip. The fan, that is!
Fan kits can be purchased with single or multiple speeds. There are also timers available for these fans. My personal recommendation is to purchase a fan with two speeds and a 12 hour timer. The lower speed allows for quieter operation during the evening and night; the timer turns the fan off unattended.
Written by Jerry Alonzy
Jerry Alonzy, a.k.a. the Natural Handyman, has been an active handyman for over 30 years with experience in most areas of home repair and renovation.
As a do-it-yourself author and web developer since 1995, he has been featured in USA Today, the Today Show and on radio shows, magazines, newspapers and websites. His material appears widely on the web, but primarily on his website... The Natural Handyman. You can also find him on Google+.