Finding the Whole-House Fan That Fits

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by Jeanne Byrne

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.

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

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

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

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

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

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

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 fan consists of two small fans set side by side. It is designed to fit between joists either 16 or 24 inches on center. Tamarack Technologies has a different approach to whole-house cooling fans. Instead of trying to create a breeze, Tamarack engineers set out to make an efficient, quiet fan that would remove stored heat, cool the attic, and replace the air in the house at a somewhat slower rate than typical fans. Tamarack recommends its 1,000 CFM fan for homes up to 2,500 ft2. For homes bigger than 2,500 ft2, a larger fan would still be recommended, or the homeowners could install two 1,000 CFM fans. 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. PO Box 490 11 Patterson's Brook Road West Wareham, MA 02576 Tel:(800)222-5932 Fax:(508)295-8105 Web site: www.tamtech.com

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