It doesn’t make much sense to use an air conditioner to cool a home when the outside air is cooler than the inside. So in areas with hot days and cool nights, people often use whole-house fans to exhaust the hot indoor air once the outside temperature drops below 78°F.
Whole house fans are installed in the ceiling, in an opening that is cut into the attic. They flush indoor air out through the attic, replacing it with outside air drawn in through the open windows. Residents turn on the fan and open windows when the outside temperature drops below the inside temperature, and for best results, they leave the fan on for several hours – preferably overnight. This cools the house down and also flushes built-up heat (much of which would otherwise find its way back into the home) from the attic.
In some climates – those with wide swings between day and nighttime temperatures, such as drier, inland areas – whole-house fans can sometimes replace air conditioning altogether. In others they can reduce the run time of the air conditioner and precool the home so the air conditioner doesn’t have to work as hard. Residents with air conditioning are often able to use a whole-house fan exclusively in spring and fall, and during much of the summer, turning on the air conditioner only on extremely hot days.
Many people prefer to use outdoor air for cooling, because they find the air conditioner dries the air out too much, or because they enjoy the gentle breeze a fan can create. However, whole-house fans should not be used during very humid days because they bring in moisture with the air.
But how big should a whole-house fan be? Traditional whole-house fan sizing methods are based upon getting enough air flowing through the home to create a cooling breeze while providing 15-20 air changes per hour (ACH) and flushing heat out of the attic. A gentle breeze causes evaporation off the human body and therefore can make the temperature feel several degrees cooler than it actually is. However, whole-house fans that are big enough to create a breeze sometimes produce unintended effects, such as heat loss, noise, and house depressurization.
The Fan and the House System
Traditional whole-house fans use anywhere from 700 to 1100 watts and are typically sized to move 4,000 to 7000 cubic feet per minute (CFM) of air, depending on the size of the home. The fan blades are usually 24 to 36 inches long; fans with larger blades have higher CFM ratings than those with smaller blades. Common sizing methods include dividing the volume of the house by three (for 20 ACH) or four (for 15 ACH), or–assuming 9-foot ceilings–multiplying the floor area of the house by three to get the fan’s CFM for 15 ACH. Although this presents quite a range of possible fan sizes for a given house, a fan sized by any of these methods would provide a complete house air change every few minutes.
When this much air is being sucked out of the house, it is vital that the house system be set up to handle it without causing harmful side effects from depressurization. This includes having enough attic vents for the air to escape through and having enough open window area to replace the air being exhausted.
To make sure the air drawn up into the attic can escape, there should be about 1 sq. ft. of net free area for every 750 CFM of fan airflow. Not all houses have enough existing attic ventilation to accommodate a traditional whole-house fan. For instance, to install a 4,800 CFM fan, there should be about 6.4 sq. ft. of net free vent area in the attic. Standard building code requires a minimum of 1 sq. ft. of net free vent area in the attic for every 300 sq. ft. of attic floor area. For example, a 1,500 sq. ft. single story house would probably have about 5 sq. ft. of net free vent area already installed to meet the minimum code. A 4,800 CFM fan would require an additional 1.4 sq. ft. of net free vent area to this attic.
What happens if there is not enough vent area in the attic? First, the fan cannot move as much air, because the lack of an escape route causes back pressure on it from the attic air. The fan cannot work well, even though it may look as if it does. Second, the pressurized and hot attic air will find its way back into the house through leaks in ceiling and walls. And finally, the back pressure on the fan will cause it to be noisy.
Residents need to remember to open windows while the whole-house fan is running. Without sufficient open-window area, the fan motor has to work too hard to pull air through the smaller openings within the structure, and thus may burn out early.
For air to flow freely through the house without causing depressurization, the total open-window area should be approximately the same as the attic’s net free vent area through which the air is escaping. So for the 4,800 CFM fan discussed above, there should be about 6.4 sq. ft. of total open window area.
Insulation and Air Sealing
Whole house fans are installed in a hole in the ceiling leading into an attic. This creates an uninsulated area of the attic, up to 9 sq. ft. for a 36-inch 6,000 CFM fan. The fans themselves generally have louvers that close when they’re turned off, but these louvers are not airtight.
Energy specialists sometimes recommend that homeowners build insulated boxes to place over the fans in the winter. However, a homeowner has to be pretty dedicated to build and use a box cover, and none of the traditional fan manufacturers sell such covers. Homeowners who do build an insulated cover must remember to take it off before turning on the fan, to avoid burning out the fan or starting a fire (this is likely the reason why no company sells the covers).
Traditional whole-house fans tend to be very noisy. This can cause residents to use them sporadically, turning them off whenever the noise bothers them.
Some fans are noisier than others. Belt drive fans are a little quieter than direct-drive fans because they can move more air at a slower fan speed. However, belt drive fans require maintenance every two years. Other factors can cause fan noise as well. Loose installation will cause the unit to vibrate and make excessive noise. And a fan trying to force air out through vents that are too small will emit an annoying whistle.
Keeping a Whole House Approach
Given their drawbacks, can whole-house fans be included on a modern energy efficiency retrofit list? Yes, they can. In many cases, they provide efficient and effective cooling without causing problems. And they are very cost-effective – most fans cost only about one-tenth as much to operate as a central air conditioner. However, it is essential that the fan and vents be properly installed and that homeowners understand the need to open enough windows when the fan is on.
A better solution to the traditional whole house fan
An alternative to traditional whole-house fans is a modern and ultra quiet whole house fan from Centric Air. Although Centric Air fans move less air than the traditional style whole house fans, they require no framing to install; no extra attic vents; and keep the home’s insulation and air sealing barriers intact. They also offer a range of whole house fans that will accommodate the existing attic ventilation for most homes ranging from 750 sq. ft. to 4400 sq. ft.
The key to using a smaller fan effectively is to run it for a longer time so it can remove built-up heat from the home and the attic. Even with a small fan, however, homeowners need to understand the importance of opening windows to prevent backdrafting.
Whole-house fans have been around for decades and for many homeowners they are a great alternative to expensive air conditioning for cooling their home.
To learn more about the benefits of Centric Air whole house fans please visit www.CentricAir.com