Whole house fan
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A whole-house fan, also known as (and confused with) an attic fan, provides great ventilation, lower indoor temperatures and, most importantly, lower summer electric bills. Note: Attic fans are not the same as whole house fans. Attic fans (as the name implies) are meant to ventilate the attic during the day or night, thereby reducing heat flow into the building envelope. Whole house fans are used to ventilate the house whenever the outdoor air is colder, which is typically after the sun sets.
With the flip of a switch, it pulls cooler outside air through open windows and sends hot indoor air out through the attic. It can lower the temperature in your home by five degrees Fahrenheit (3 °C) in just a few minutes.
One of the biggest advantages of a whole-house fan is the cost—only $150 to $550, compared to thousands of dollars for a central air conditioner. And when used in place of a central air conditioner, it can lower your cooling costs by 30 percent. In addition to the cost of the fan, many homes will need additional ventilation to allow the air blown into the attic to escape back outdoors. This, plus the cost of professional installation, raises the price of the complete fan unit, installed, to about $1000 to $1500 (as of 1/2006).
Installing a whole-house fan can be a do-it-yourself project if you have some home-improvement experience in wiring, and you have adequate space in your attic. If not, you can hire an electrician or handyman to install it for you.
You can use a whole-house fan as a cooling system by itself, or you can combine it with your central air conditioner to help cool the house more quickly.
Factors like outside temperatures and humidity levels can affect how well a whole-house fan works. A good rule of thumb is to use the whole house fan when outside temperatures are below 85 degrees Fahrenheit (30 °C). Remember, however, that moving air feels cooler than its actual temperature, therefore running the fan at higher temperatures may nevertheless increase personal comfort levels.
Houses with basements are able to run the fan mid-day with only the basement windows open. This initially pulls the cooler basement air into the house and then cools the warmer outside air as it flows though the basement, bringing cooler than outside air into the house. During the early afternoon, blowing out the accumulated hot air from the house, especially from the upper levels and from the attic and only then turning on the ac will either minimize or eliminate the need for air conditioning or allow the air conditioner to run more effectively and for less time.
To effectively cool a house, you must try to do one complete air change every two to four minutes. Multiply the living area of the house by the height of the rooms to calculate the volume of air in the house. Divide this by 2, 3, or 4 to determine the air flow rate in cubic feet per minute (CFM) (or cubic meters per second or liters per second) of the fan size most appropriate to the house. Most houses will require a 24" 5000 CFM (2400 L/s), 30" 7500 CFM (3500 L/s) or 36" 10,000 CFM (4700 L/s) fan. Fan size is measured by the blade diameter. It is important to select a fan whose flow rating is associated with low speed. While it is simple to move a lot of air at high speed, this increases noise levels and can be quite irritating. A well engineered, installed and ventilated whole house fan is quiet (under 50 decibels).
Attics must be properly vented so that the air is able to escape back outdoors as quickly as it is being blown into the attic. To calculate the required vent space, divide your CFM by 750 to 1000. This gives you the number of square feet of attic vent space needed. A good rule of thumb is to provide the same amount of venting as the size of the shutter hole in the ceiling. Attic venting is measured by "net free area". Net free area is the actual opening of a vent after deducting blockage or restrictions, such as screening or vent slats. (Minimum venting for fans: 24"= 4 sq ft, 30" = 6 sq ft, 36" = 9 sq ft)
When you're using a whole-house fan, it's important to allow for good airflow—make sure you leave a window or door open to keep the air moving and avoid damaging your system. Again, for high speed operation, minimally allow as much window opening as the shutter hole in the ceiling; for low speed about two-thirds as much.
Insulate the fan opening during the winter—the louvers don't seal tightly, making it a prime place for heat loss. A piece of foam board, available at most art supply or hobby stores, secured with Velcro™ to the shutter will prevent most air loss, and provide some insulating value, while still allowing easy access to the fan operation.
New generation whole house fans that are insulated, quiet and easy to install can now be found online at [1] or here [2]. These fans move less air than the traditional whole house fans, are quieter and have automatic insulated doors. They are meant to be run continuously through the night. One argument for their use is that time is required to cool the great mass of the house and its contents. Pre-cooling this mass is viewed as an opportunity to save on the next day's cooling costs and/or extend the comfort zone for those without air-conditioning.
