Mechanical Ventilation: Whole-House Fans

A whole-house fan can substitute for an conditioner most of the year in most climates. Whole-house fans combined with fans and portable fans pro- vide acceptable summer comfort for many families, even in hot .

How Whole-House Fans Work

The whole-house fan pulls air in from open windows and exhausts it through the attic and roof. It provides good attic ventilation in addition to whole-house ventilation. You can regulate cooling by simply closing windows in the unoccupied areas and opening windows wide in occupied areas. Many people cool the bedrooms at night and the living areas during the daytime.

Whole-house fans should provide houses with 30 to 60 air changes per hour (varies with climate, floor plan, etc.—check with a professional to determine what is appropriate for your home). The air-change rate you choose depends on your climate and how much you depend on the whole-house fan for cooling. Cooler, shadier areas don't require as much ventilation as warmer, sunnier ones. Houses entirely dependent on whole-house fans require a bigger fan because there is no air conditioning to fall back on.

Sizing a Whole-House Fan

Whole-house fans are sized in cubic feet per minute (cfm) of ventilating power. To determine the size you'll need, first calculate the volume of your house in cubic feet. To do that, multiply the square footage of the floor area you want to cool by the height from floor to ceiling. Take that volume and multiply by 30 to 60 air changes per hour (depending on the power you need). Then, divide by 60 minutes to get the cubic feet per minute of capacity your house requires. [(Square feet______ x room height______) x 30 or 60 / 60 = cfm required _________. ]

Fan Noise Levels

  • When shopping for circulating fans, be sure to test the fan for noise. Listen to the different qualities of each fan's sound. Fans also will be marked with their rated noise level, measured in “sones.” Very quiet fans are rated at 1.5 sones. Some are so quiet, they're rated as low as 0.5 to 1 sones.
  • Poor installation can affect a fan's noise level. If a fan isn't installed securely, it can vibrate and rattle, making as much noise as a poorer- fan.
  • Your fan will probably run for a good part of the day – be sure it's quiet as well as cooling.

Installing and Using a Whole-House Fan

Installing a whole-house fan is tricky and should be done by a professional. Attic measurements, dedicated circuit wiring, and possibly new attic vent installation should be performed by an experienced person.

Attic ventilation must be increased to exhaust the fan's air outdoors. You'll need 2 to 4 times the normal area of attic vents, or about one square foot of net free area for every 750 cfm of fan capacity. The net free area of a vent takes into account the resistance offered by its louvers and insect screens. More vent area is better for optimal whole-house fan performance.

Some fans come with a tight-sealing winter cover (or you can build one). If you switch between air conditioning and cooling with a whole-house fan as the summer weather changes, build a tightly sealed, hinged door for the fan opening that is easy to open and close when switching cooling methods.

Drawbacks of Whole-House Fans

Whole-house fans can be noisy, especially if improperly installed. In general, a large- capacity fan running at low speed makes less noise than a small fan operating at high speed. All whole-house fans should be installed with rubber or felt gaskets to dampen noise. You can set a multispeed fan to a lower speed when noise is a problem.