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Attic Space and Sizing for Whole House Fans

Whole house fans are gaining popularity. These energy efficient appliances can be excellent for increasing home comfort and creating a better space for you and your family, but choosing the right one can be difficult.

Each whole house fan has different features and characteristics, so selecting the right one can be tough. One of the most important factors that will guide your choice is the size of your home. To select the right fan, you have to know the overall air volume of your house.

Fortunately, measuring the size, and then using that number to make a purchase, is quite simple. After you know the size, you can also look at other features that make the fan more effective.

attic space

What is a Whole House Fan?

A whole house fan is a ventilation system that pulls air from the living space to the attic, creating air circulation throughout the home.

The system pulls air through open windows and doors, draws it through the home, and expels it through vents in the attic. By moving cooler air, the fan creates ventilation in the home and in the attic, which is beneficial for many different reasons.

The fan is installed on the ceiling between the living space and the attic, and it has louvers that close when the homeowner is not using the fan.

Whole house fans serve as an alternate choice to air conditioning. Under the right circumstances, they can increase efficiency over using an air conditioner while keeping the home comfortable. Compared to air conditioners, operating a whole house fan generally means less energy use, which is why they are considered a better choice for home efficiency.

However, whole house fans should not be used at the same time as air conditioners. When a whole-house fan is in operation, it pulls air from the living space and expels it outside. If the air conditioning is running, this means the whole house-fan is expelling conditioned air. In other words, you’re pushing out air that you paid to cool.

How are Whole House Fans and Attic Fans Different?

It can be easy to confuse the two, so we’d like to briefly explain the difference between whole-house fans and attic fans.

The biggest difference is their overall purpose and use. A whole house fan will circulate air throughout the entire home (or the majority of the home) including the living space. An attic fan, however, is intended only to circulate air in the attic, helping to remove hot air.

A whole house fan is installed in hole that is cut in the ceiling between the living space and the attic. The fan rests on the ceiling gables and sucks air up out of the attic.

An attic fan, on the other hand, is installed on the roof of the house, pushing hot air directly to the outside. Attic power fans can also be installed on the side walls of attics, but the result is the same: hot air is moved through the attic and then expelled.

The purpose of an attic fan is ventilation and hot air movement strictly in the attic and nowhere else. This can enhance cooling in the home and can also help with air quality, and if you choose a solar-powered attic fan, you can also have energy-efficiency.

A whole house fan, however, moves air all through the home. It serves much of the same purpose, but it essentially serves as a replacement for the air conditioner and not a supplement.

whole house fan blades

Whole House Fan Installation

The installation process of a whole-house fan can be difficult, and in almost all cases it should be done by a trained professional or experienced DIY’er. A knowledgeable technician can take your attic measurements and install the right system, including proper wiring and new attic vents if they are required.

Additional attic ventilation may be needed to push the air outward. If there is not enough free ventilation in the attic, the system will struggle to move enough air. Generally, you’ll need at least twice the normal area for attic vents, with roughly one square foot of free area for every 750 cubic feet per minute of fan capacity. The area of the vent uses the resistance created by louvers and screens. The more vent space you have, the better your whole house fan will operate.

If you live in a cold-weather area, you’ll need to have a winter cover or insulation for your whole-house fan. This cover should be able to seal tightly, giving you superior insulation. When using the whole house fans, be sure to exercise caution. Open windows throughout the house to prevent powerful suction in one area. Not having enough inward ventilation can cause a backdraft in your furnace, which creates potential safety problems.

Determining Attic Space for a Whole House Fan

A whole-house fan will work best when it is properly sized to your home, so you need to measure both the space of your attic and the living space, both of which will need to be calculated in cubic feet.

First, you’ll need to find the air volume in your home. You can do this by taking the square footage of your home and then multiply this by the average height of your ceilings and you will have the air volume for your home, which will be stated in cubic feet.

For example, if your home has a total floor area of 2,000 square feet with eight-foot ceilings, you would have a volume total of 16,000 cubic feet. If the ceilings are 10 feet high then the area is 20,000 cubic feet (or 25% more).

The general rule of thumb is that you will want a fan that can move from 2 to 3 CFM (cubic feet per minute) per square foot of space. This is based on a standard 8 ft ceiling. So, if your home is 2,000 sq ft then multiply this by 2 to 3 CFM to get to a suggested range of 4,000 – 6000 CFM system.

To get to the suggested range you can use one or more fan systems.

Second, before you decide on a fan you will also need to consider the total size of the vent openings in your attic. To estimate the size of your attic vents you’ll need at least one square foot of space for each 750 CFM.

Therefore, if you choose a 4,000 CFM system, you should divide 4,000 by 750, which means you need at least 5.3 square feet of attic venting. If you go with a 6000 CFM system you will need 8 sq ft of attic venting.

Now you can select the whole-house fan that meets your specific needs for ventilation and home comfort. Fans labels often show the diameter of the fan or the amount of air they move. (The CFM).

For example, you might find a fan that has a 36-inch diameter that will move roughly 7,000 CFM, while 30-inches in diameter will move somewhere around 5,500 CFM and a 24-inch fan will move about 4,500. Many products will have both the diameter and the CMF listed for the products.

In almost all cases, you can get by with purchasing a fan that is larger than you need. However, you should avoid buying a fan that is too small.

Other Factors to Consider in Choosing a Fan

Now that we have general idea for the size of whole-house fan you should purchase, let’s take a quick look at some of the other features and qualities you should have for your system. If you look for whole house fans with these qualities, you will have much better results.

First, you should look for a quiet product. Whole house fans are running most of the time, so it’s essential that you have a product that doesn’t make a lot of noise. Many fans are rated by “sones,” and the typical fan you purchase should have a rating around 1 sone; any louder and you will likely find the fan annoying. In many cases, belt-drive house fans that use a pulley reduce friction and remain quieter than direct-drive fans. Also, the more blades a fan has, the quieter it will be. Make sure the unit has a welded frame so it will not become loose and begin to squeak.

You will also need a fan that has quality shutters, especially if you live in a cold climate. Quality shutters can also reduce thumping when the unit is turned off. Most energy-saving fans have self-sealing shutters that automatically close when the fan is not in use. This prevents warm air from escaping into the attic.

The whole-house fan that you select should also have a timer control or, at the very least, an easy-to-access manual control. This is often recommended over a thermostat because a thermostat will turn on the fan even when no one is in the home.

Before you purchase, be sure to research the installation method for the fan. Most whole house fans have a simple installation process, but you need to do your research and inspect the home to make sure the product will work for your needs without significant home modification. In most cases, attic joists will not need to be cut.

Install Attic Air Vents

One of the most important elements of installing a whole-house fan is to have the right vents in place. If you don’t take this step, which is sometimes overlooked, you will have significant problems with your home ventilation. After all, whole-house fans push air from your house into the attic, and while most attics are already ventilated, they often don’t have the right amount of ventilation. If you don’t have enough ventilation, pressure can build up, which will harm circulation rates and air could be pushed back into your living space.

While we gave some general guidelines for the square footage of air vents that you need, you will find the ventilation requirements listed for most products. Your home will have many different vents, including ridge vents, louvers, soffit vents, round soffit vents, and turbine vents. You need to understand exactly how much ventilation you currently have and add more if necessary.

For the Best Results, Seal the Home for Leaks

No matter what the size, it’s also essential that you check the home for leaks before or after you install a whole-house fan. Finding leaks and controlling the flow of air will increase the effectiveness of the fan, creating better comfort and circulation for you home.

Finding leaks may seem like a challenge, but the Department of Energy has provided a useful guide for detecting air leaks. They recommend hiring a certified Home Energy Professional (HEP) to perform an audit on your home. You may already know where a few leaks are (such as window frames), but this professional can perform a blower test for the best results. A blower test depressurizes the home, revealing leak locations. With a full energy assessment, you know which areas of your home are allowing air inside.

If you don’t have a blower test, the process can be more complex. You’ll have to start with a visual inspection of the home. Inspect all the areas where two building materials meet, such as window frames, the top of the foundation, and edges of flooring trim. Check all exterior corners and outdoor water faucets, as well as siding and chimney edges. Inside the home, there is a long list of areas you need to inspect. This includes outlets, doors, windows, attic hatches, cable lines, dryer vents, and baseboards.

If you have old windows, you’ll likely discover leaks. When installing a whole house fan, it may be best to replace these old windows with energy-efficient products.

You can also conduct your own pressure test. Although less effective than a professional test, it can help you target specific leaks. It’s best to conduct this test on a windy day.

Start by turning off all combustion appliances such as water heaters and furnaces. Then shut all windows and doors tightly and turn on any fans that exhaust air outwards, including bathroom fans, clothes dryers, and stove vents. Now light an incense stick and pass it over areas where leaks may occur, such as windows and baseboards. Where there is a leak, you’ll likely see the smoke start to ripple as air is being pulled inward. You’ll now understand the areas that need to be patched.

Helping You Have Healthy Indoor Air

If you want to have excellent air for your family, contact the team at Oransi today. We’ll help you choose leading HEPA air purifiers that trap contaminants and allergens, allowing you to have healthy air for the entire home.

When used with a whole house fan, our air purifiers can help you reduce pollen, dander, and other allergens that may come into the home. They can also reduce smoke particles if you live in a high-smog area.

From medical-grade purifiers to small units for a child’s bedroom, we have the right products for your specific needs!
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