Choosing the Right Whole House
Exhaust Fan For Your Home
Part 1 of 2

Whole House Fans Can Really Keep You Cool!

The fact is, everyone can't afford air conditioning. And another fact is that some people who can afford it prefer to... gasp... not use it "24-7" during the hotter months!!

So what's a mother to do? Well, since the advent of electricity, fans have cooled more people than most anything else... and more economically, too. But whole house fans are fans nonpareil. Turned on as the sun meets the horizon, they produce almost sinful pleasure.

Care to join us near the window?

Whole house exhaust fans aren't for everyone!

If you live in the more tropical climes and air condition your home, the LAST thing you want to do is pump all of your expensive conditioned air outside, so skip this article and go fix your toilet!

However, if you live in the northern half of the country or the midwest and have relatively cool evenings, a whole house fan can make them sheer bliss! Let's explore the world of whole house fans and find out if one should be in your future!

The pros and cons...

1)  Attic fans are economical to run when compared to central air conditioning.

They can cool a house to outside temperature in a few minutes. Even in homes with central air conditioning, using the fan in the cool of the evening can cut your electric bill substantially plus refresh the air in the house.

2)  Attic fans are not suitable for all climates or all people.

If you live in a very dusty area or if there are high levels of pollen in the air, the attic fan will bring this pollution into your home as it exchanges the air. This is not good for folk with dust sensitivities, allergies, or even those who would prefer not to have to dust their homes often!

3)  If your area is very humid, turning on the attic fan will bring this humidity into your home.

This is not an issue unless your central air system has been de-humidifying your home all day. Under this circumstance, the "apparent" cooling effect is lost to the increase in air-borne moisture inside your home. In other words, even if the outside temperature is lower than the inside temperature, raising the humidity by turning on the fan will make it "feel" warmer inside!

As a personal example, I do not use my attic fan in the early spring because of the dense pollen in our wooded area. All the furniture takes on a greenish hue and dusting must be done with a vacuum cleaner! However, the rest of the summer, the fan cools my home so well I have no need to own an air conditioner!

4)  Electrical skills are required for installation.

Some fans have built-in pull switches, so you only need run one electrical wire to the fan. Since these fans draw a lot of power, especially when they are first turned on, they should be wired into their own circuit. This is an advanced electrical installation and should only be done by a qualified person. If you plan on doing this electrical work yourself, check the local electrical code requirements first and follow them 100%.

5)  Possibility of back-flow of carbon monoxide from chimneys!

If there are not enough windows open in the home, an attic fan may draw furnace exhaust back into the home right down the chimney, presenting a carbon monoxide hazardBe sure windows are open whenever the fan is running and be sure there are carbon monoxide detectors throughout your home, especially in bedroom and near the furnace.

A comparison of the types of whole house exhaust fans...

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.

Deciding where to locate the ceiling louvers...

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!

Check your available attic venting and compute your needs...

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:

Square feet of vent needed = TOTAL VOLUME of air in your house DIVIDED BY 750

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!

Choosing the right size fan for your home...

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 =
The ideal CFM (cubic feet per minute) of your dream fan!

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.

Click here to continue to Part 2 -  Install Your Own Whole House Exhaust Fan

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