Sizing a whole-house fan doesn't require any special methods or equipment, or sophisticated calculations. Traditional sizing charts are still used, but be cautious when using them.
Oversized and undersized units
A larger and ‘oversized’ whole-fan may cost you more and consume a little more electricity, but it will not interfere with the normal performance of the equipment or cause temperature swings, contrary to what happens with oversized furnaces or air conditioners. Whole-House Fans PM
The size of whole house fans is measured in CFM (Cubing Feet per Minute).
On the other hand, an undersized unit will require some minutes more to cool the house and it can be noisier than a larger unit operating at a lower speed, but just that.
Sizing the Whose-House Fan
Whole-house fans are powerful devices. Some models are able to move 10.000 cubic feet of air per minute (CFM) or more. These models have the capacity of changing all the air in the home every two minutes or less, which is often useless.
Fans with capacities of 1.000-4.000 CFM, able to change all the air in the home several times per hour, are a better choice.
Average sized homes (1.500 square foot) with good levels of insulation and shade will not need a big whole-house fan: a fan with a capacity of 1.000-4.000 CFM is powerful enough and easier to install. Larger homes with low levels of insulation may require larger units: say, fans of 5,000 or 6,000 CFM.
See box below, for an overview of traditional sizing methods.
Traditional Guidelines For Whole-House Fan Sizing
Some practitioners use the formula: S x H x 0,5 (S = House square feet; H = ceiling height, typically 8 feet). An example: a 1000 square feet house with a typical ceiling height of 8 feet, will need at least a 1000 x 8 x 0,5 = 4,000 CFM fan. Many contractors use another rule of thumb: they just multiply the square footage of the living space by 3. For example: a 1000 square-foot house will require a fan around 3,000 CFM (1.000 x 3). Compared with the first method, this second method produces a lower estimate, responding better to the needs of modern homes with reasonable levels of insulation.
DIY or Professional Installation
Whole-house fans are usually installed in the ceiling of a central common area of the house, and may require reinforcement of its framing and also improved attic ventilation (the fans move air into the attic, which may require additional attic venting).
To calculate the new attic ventilation requirements, some contractors use to divide the CFM of the fan by 750 (or by 1.500 if you are using insect screens and louvers). In other words: a 3,000 CFM fan requires, according to this rule of thumb, 3,000/750 = 4 square feet of attic ventilation area.
When installing the whole-house fan, pay close attention to possible cracks and gaps between the attic space and the rooms. Seal them, carefully. Do not forget that the fan will move air into the attic, forcing it out through the roof vents – or back into the living space through possible cracks and gaps...
Professional installation is highly recommended.