During a hot day, your home can build up heat, much like a car sitting in the sun. Installing a whole-house fan can quickly wisk away the unwanted heat, giving your air conditioner a helping hand and an easier start to cooling your home. Amazingly enough, a whole-house fan can change the temperature of your home in less than three minutes. Just try doing that without one!
When the fan is turned on, it moves the air quickly, measured in cubic feet per minute (CFM), out of the living quarters of your home, through the ceiling opening, into your attic space where the air is dispersed through eave or roof vents.
To determine the appropriate sized fan to match capacity of air flow to size of the home, we’ll need to find the cubic volume of your home. Along with that, we’ll need to consider the CFM of the fan in determining the appropriate amount of venting to handle the air being moved to prevent air pressure from the room below to the attic space.
Whole-house fans require the airflow draft through the louvered doors and screen must be at very least, equal to the airflow through the venting of the roof and eave vents. Too little air ventilation in the attic will cause a push on the roof structure due to air pressure, which we do not want.
To determine fan size, you’ll need to calculate the total volume of all the rooms. To do this, simply multiply with this equation: length x width x height = volume. Do this for each room and then add the totals together to reach the cubic footage of the home. OK, so let’s say the room is 20′ long, 10′ wide, and 8′ tall. So 20 x 10 x 8 = 1600 cubic feet. Now, let’s say the overall square footage of a home is 2,400 square feet, that being the length x the width, with 8′ tall walls. So we’d multiply the 2,400 x 8 = 19,200 cubic feet. Confused yet?
Now we need to determine the appropriate sized fan. To do that, simply divide the cubic feet by 2 like this: 19,200 ÷ 2 = 9,600 CFM. The vent rate would be 1 square foot of unobstructed space per 750 CFM of airflow. In this example, you’ll need around 13 square feet of vent for air removal. * Keep in mind that screen size determines the multiplication factor of screen restriction. For example, a 1/16″ screen requires a 2.00 multiplication, requiring 26 square feet of vent, more vent than a 1/4″ screen with a 1.00 multiplication, thus needing only the 13 square feet of vent.
Remember to open your windows and doors at the end of the day, when the sun is setting, before turning on the fan to allow maximum air flow and room heat dissipation. By maximizing air flow and adding the right sized whole-house fan, cooling your home will be easier and save you money on air conditioning. In the chart below, you can compare fan sizes and exhaust ratings. Use this to determine the appropriate sized fan for your home.