Whole house fans sizing and installing

Sizing the whole-house fan and the attic ventilation demanded by the fan, is as important as a proper installation.

Traditional guidelines for whole-house fan sizing

Whole house fans are sized in CFM: Cubic Feet of air per Minute of ventilated power.

To determine the minimum size of a whole-house fan to a specific home, practicioners often use the formula: S x H x 0,5 (S = House square feet; H = ceiling height, typically 8 feet).

An example: a 900 square feet house with a typical ceiling height of 8 feet, will need at least a 900 x 8 x 0,5 = 3,600 CFM whole house fan.

Alternative sizing method

Many contractors use another rule of thumb to determine the size of the whole-house fan. They just multiply the square footage of the living space by 3. For example: a 900 square-foot house requires a fan around 2,700 CFM (900 x 3).

Compared with the first method, this one produces a lower estimation, which responds better to modern well-insulated homes.

Taking into account qualitative elements

Another approach when sizing a whole-house fan, is to consider the insulation, shading and other elements that determines the cooling needs of the building.

If your attic has sufficiently high amounts of insulation, if your roof uses reflective materials and has a light color, and if your windows are shaded by trees or other effective mean, than your home will demand a relatively small-whole house fan. In this case a whole-house fan with a capacity of 1000-3000 CFM will be sufficient.

Fans with capacities above 5,000 or 6,000 CFM should only be installed in traditional homes, with low attic insulation, traditional roofs and inadequately shaded walls and windows.

Vent space

Whole house fans demand high attic ventilation for good performance. Usually you'll need 2–4 times the normal area of roof vents.

To calculate the attic ventilation needed for your home, you may divide your whole-house fan CFM by 750 (this amount should be doubled, if you use insect screens and louvers). If you have, say, a 3,000 CFM fan, you will need approximately 3,000/750 = 4 square feet of attic ventilation area (soffit, gable, roof, ridge vents) .

Location and installing

Whole house fans are usually located in the hallway ceiling.

Installing a whole-house fan demands skills and experience in wiring, and a proper assessment of the attic area. As a rule, it’s better to call an experienced professional to perform these tasks.

Seal penetrations between the attic and rooms

When installing the fan, pay attention to cracks and gaps between the attic and the rooms. The fan will create positive pressure in your attic, and the air will be driven back into your rooms through cracks and gaps, if they exist. To avoid that you should caulk properly any penetration between the attic and the living space.