Sizing and Installing - Whole House Fans


The size/capacity of the whole-house fan

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

To determine the right size of the fan just calculate the volume of your house (V) including the upstairs area (you may calculate V multiplying the number of your house square feet (S) by its ceiling height (H): typically 8 feet).

The size of the whole house fan should correspond to a number between V (maximum) and half of V (minimum).

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

In that case a two-speed whole house fan that delivers 4,000 cfm at the high setting and 3,200 cfm at low (120 watts) will be adequate.

Note:
Many contractors use another rule of thumb to determine the size of the whole house fan in CFM. They just take the total of square feet of living space and multiply it by three. For example, a 900 square foot house would require a fan of approximately 2,700 CFM (900 x 3). Compared with the first method, this one produces a lower estimation.

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 the your fan CFM by 750 (this amount should be doubled, in case of 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 study of the space in your attic. It may also demands insulation work.

As a rule, it’s better to call an experienced professional to perform these tasks.

Tight sealing

Buy a whole house fan with a tight-sealing cover (or build one…).

If you intend to use air conditioning and cooling with a whole house fan to respond to weather changes, buy (or build) a proper tightly sealed system and also an easy to open and close door for the fan opening (to facilitate switching cooling methods).

Seal penetrations between the attic and living room

Your 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 all penetrations between the attic and living space. Apply insulation materials directly against the fan box frame.





See also:
Basics and Usage Whole House Fans
Pros and Cons Whole House Fans

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