This article was originally published in the May/June 1999 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
Home Energy Magazine Online May/June 1999
Finding the Whole-House Fan That Fits
by Jeanne Byrne Jeanne Byrne is a former managing editor of Home Energy.
Whole-house fans cool homes very efficiently in many parts of the country. But they have come under fire for creating leaks into the attic, causing potential depressurization problems, and being noisy. What is the best way to fit the fan to the house?
|Many whole-house fans are too large to fit between ceiling joists. Instead of cutting joists, contractors can use wooden H brackets installed between them. The brackets create a framed box that raises the fan above the joists. Note that the joist below the fan will create extra resistance, slightly decreasing the fan's effectiveness.|
It doesn't make much sense to use an air conditioner to cool the air in a home when the outside air is cooler than the inside. So in areas with hot summer days and cool nights, people often use whole-house fans to clear out hot indoor air once the outside temperature drops below about 80°F. High-velocity whole-house cooling fans operate at night during summer months to cool the house. These whole-house cooling fans should not be confused with low-velocity whole-house ventilation fans, which operate continuously to provide fresh air and to control levels of indoor polllutants.
The dramatic cooling potential of whole-house fans has been well established (see Florida Cooling, the Natural Way, HE Nov/Dec '91, p. 32). 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 almost to the outside temperature and also flushes built-up heat (much of which would otherwise find its way back into the home) out of the attic.
In some climates--those with wide swings between day and nighttime temperatures, such as drier, inland sites--whole-house fans can 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.
Whole-house fans are not well suited to very humid climates because they bring in moisture with the air. In some cases, dust and pollen sucked in by a fan can also cause problems. On the other hand, many people prefer to use outdoor air for cooling, find that the air conditioner dries the air out too much, or enjoy the gentle breeze a fan can create.
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
Whole-house fans use anywhere from 265 to 700 watts and are typically sized to move 2,500-6,800 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 (see Oversized Kitchen Fans--An Exhausting Problem, HE Jan/Feb, '99, p. 37). This includes having enough attic vents for the air to escape through and having enough open window area to replace the air being exhausted. A contractor or energy specialist taking a whole-house approach should also consider the loss of heat through an uninsulated, leaky fan in the winter; security issues from leaving windows open at night; and the noise created by a large fan.Attic Vents
To make sure the air drawn up into the attic can escape, there must be about 1 ft2 of net free area for every 750 CFM of fan air flow. Not all houses have enough existing attic ventilation to accommodate a whole-house fan. For instance, to install a 4,800 CFM fan, a contractor should make sure there are at least 6.4 ft2 of net free vent area in the attic. Standard building code requires a minimum of 1 ft2 of net free vent area in the attic for every 300 ft2 of attic floor area. For example, a 1,500 ft2 one-story house would probably have about 5 ft2 of net free vent area already installed to meet the minimum code. A contractor installing a 4,800 CFM fan would have to add 1.4 ft2 of net free vent area to this attic. (Note that the contractor should physically check the existing attic vent area, rather than assuming that the house meets code.)
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 possibly quite hot) attic air will find its way back into the house through leaks in the attic floor. And finally, the back pressure on the fan will cause a whistling noise at the fan.
Even if the vent area is adequate, a large whole-house fan can cause turbulence in the attic, possibly stirring up silica, dust mites, or insulation. These particles may even be blown out of eave vents and sucked back into the home through the open windows.Open Windows
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. Even worse, a fan desperate for air may backdraft a gas water heater or clothes dryer. In winter, when the fan is not meant to be run, an accidental flip of the switch can backdraft a furnace or fill the house with smoke from a fireplace.
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 ft2 of total open window area. However, if there are screens on the windows, this number should be multiplied by three to account for the extra resistance. That would be 19.2 ft2 of open, screened windows. For vertical sliding windows 2.5 ft wide, for example, this could be accomplished by opening four windows to a height of 2 ft. However, a little depressurization of the home helps to create the breeze effect, and many people use somewhat less open window area despite the problems it may cause.
Leaving windows open all night can be a security issue. Where this is a concern, people will close the windows and turn the fan off when they go to sleep. By cutting short the fan's running time, one of the main benefits of whole-house fans--removing built-up heat from the home (particularly a home with masonry construction) and from the attic--is lost. Heat stored in the walls, floors, ceilings, and furnishings is then slowly released into the home throughout the night and is not flushed out.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 ft2 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 usually 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 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 probably why no company sells the covers). Most fans are controlled simply by switches on the wall--similar to light switches--so the risk of accidentally turning the fan on is considerable. Installing warning labels at the switch and an override on/off button at the fan can help to prevent such accidents.Noise
Typical whole-house fans use industrial motors and tend to be 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 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, while direct-drive fans are usually maintenance-free. Other factors can cause fan noise as well. Loose installation will cause the unit to vibrate and make excessive noise. (Sponge-edged mounts on the frame help to reduce noise from vibration.) And a fan trying to force air out through vents that are too small will emit an annoying whistle. In general, the cheapest fans (which typically have only a one-year warranty, versus five years for higher-quality fans) are the noisiest.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 contractors install the fan and vents properly and educate homeowners well about the need to open enough windows when the fan is on. Contractors should also install safeguards such as warnings at the switch and an override on/off switch at the fan for the off-season.
Another solution to whole-house fan problems may be to use a smaller fan. One company now offers an efficient, quiet, 1,000 CFM fan with built-in insulation (see Tamarack's Small-Fan Alternative). Used for homes up to 2,500 ft2, this fan requires no extra attic vents and keeps the home's insulation and air sealing barriers intact. 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 are an old technology, and they have been all but replaced by air conditioning in many areas. But opening windows and using a whole-house fan is still an effective, energy-efficient way to cool a home. Even occupants who already have air conditioning can usually benefit from using a whole-house fan in the transition season or to precool the house before switching on the air conditioner.
Tamarack's Small-Fan Alternative
Tamarack's HV1000 actually consists of two 500 CFM fans, which together use about 120 watts. The fans fit in a rough opening 14.5 inches x 22.5 inches, so the unit can be installed in between attic floor joists either 16 or 24 inches on center. The 1,000 CFM of air flow requires only 1.33 ft2 of net free vent area. So a contractor does not need to install any more attic vents than are required by minimum housing codes. (A house with 1,000 ft2 of attic floor area should already have more than 3 ft2 of net free vent area to meet code.)
The HV1000 has insulated damper doors that automatically open and shut when the fan is turned on and off. The damper doors are airtight and are insulated to R-22. (Tamarack also sells a model with R-38 insulation.) The insulated dampers are better than a box cover, because the fan is kept insulated and airtight all year round, whenever it is not in use. The fan is rated at 3 sones, quieter than standard whole-house fans, and costs about $400.
A 1,000 CFM fan does not require as much open window area as a larger fan would. To get about 1.33 ft2 of open window area (or 4 ft2 of screened window area), a resident would need to raise only a couple of 2.5-ft windows to a height of 1 ft. If they are concerned about nighttime security, residents could open four windows about 6 inches each and use locking security stops.
Another advantage of the HV1000 is that unlike other whole-house fans, it can be installed vertically. For instance, it can be located in a kneewall to an attic from a room with a vaulted ceiling. Other fans cannot handle the right-angle stress that a vertical installation would place on the motor.
A larger fan--if used properly--will certainly cool down a house faster than Tamarack's HV1000. And the breeze from a larger fan will make the occupants feel more comfortable while the house is cooling down.
However, Tamarack's engineers believe that users get most of the gains from a whole-house fan with the first 1,000 CFM, and that there is a decreasing rate of return from adding power above that. Circulating fans (such as ceiling or oscillating fans) can always be used in conjunction with a small whole-house fan to create a cooling breeze.
The human factor is also important. People are more likely to leave a 1,000 CFM fan on for long periods because it is quieter and they are more likely to maintain window security at night than with larger fans. Left on all night, the fan can continually remove stored heat from the home and attic until the outside temperature rises again in the morning and residents turn it off.
The HV1000 is clearly a good opiton when a homeowner does not want to add attic venting; cannot leave windows wide open at night; is very concerned about fan noise; or lives in an area with cold winters, where there is a large energy penalty from an uninsulated fan.
The advantages of an efficient small fan may be especially enticing for energy savings programs. Whole-house fans are often left out of utility and government programs due to real or perceived problems with homeowner acceptance, winter air leakage and heat loss, or potential backdrafting. Certainly 1,000 CFM is enough to cause backdrafting and residents still need to be educated to open windows, but the risks are much lower than with a 2,500-6,000 CFM fan.
While no independent technical studies of the HV1000 have been done, many cooling energy experts believe that the concept of a small, efficient whole-house fan is sound. It may be just what we need to bring this old-fashioned cooling method into the era of house-as-a-system thinking.
For information on the HV1000, contact:
Tamarack Technologies Inc.
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