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How to Size a Whole House Fan with the Correct CFM

Utilizing a whole house fan is a great way to reduce the amount of energy and money needed for cooling without sacrificing any level of comfort at home. These types of fans are capable of cooling the entire home during the hours when the temperatures indoors are greater than outdoors, such as evening and morning hours, by bringing in cooler air from the outside. It does this using only a fraction of the energy required by the air conditioning unit. Whole house fans come in a variety of sizes, determined by the amount of air they circulate, or whole house fan cfm. Having an understanding of this makes choosing the correct fan size for your home an easier process.

Understanding Whole House Fan CFM

Cubic Feet per Minute or CFM is the amount of cubic feet of air per minute the fan moves. The amount of power a fan has depends on this measurement. The higher the CFM is, the more powerful the fan is. The amount of power necessary depends largely on the amount of space being cooled. Larger spaces, of course, would require larger fans.

Determine how much air is being moved
Figuring out the amount of air exchanges that will occur within a certain time frame with open windows and a whole house attic fan is done by using the measurement of your home in square feet along with the CFM rating of the fan. As an example, let’s use a home that measures 2200 square feet, has an average floor-to-ceiling height measurement of 8 feet, and a whole house fan with CFM rating of 1600:
  • Determine the home’s cubic measurement by multiplying the square footage by ceiling height:
    2200 square foot home x 8 inch ceiling height = 17600 cubic feet
  • Divide the cubic feet measurement by the CFM rating assigned to the fan:
    17600 cubic feet ÷ 1600 = 11

What this means is that every eleven minutes, the air in the home is being exchanged by a fan of this rating along with having windows open in each room. To facilitate faster air exchange would necessitate choosing a fan with a higher CFM. So as another example, moving to a 2500 CFM fan, would cut the air exchange time to roughly 7 minutes (17,600 / 2500). Another way to look at this is to calculate how many air exchanges are performed in an hour for a fan with a certain CFM. To accomplish this divide 60 (number of minutes in an hour) by the number of minutes it takes to do an air exchange for the whole house. So in our previous example with the 2500 CFM fan, that would be 60/7 – so roughly 8 air exchanges per hour. Most manufacturers recommend a minimum of 4 air exchanges per hour.

Some additional points for consideration in addition to CFM when choosing the correct venting fan include:
  • Power Requirements. The higher the CFM, the more wattage it required to operate the fan. However, there are numerous energy efficient models available in the marketplace that significantly cut down on power usage. Some fans even provide the option to operate on either a low or high speed each with varying CFM and hence power consumption levels. Having this option allows you to use even less energy on days that are cooler, but a fan is still needed.
  • Noise Level. Generally speaking, the higher the CFM the higher the noise level from the operation of the engine. There are however, many engineered products with advanced technology that dampen the noise level.
Once you have the correct measurements for calculating the whole house fan cfm, you will be able to choose the right fan to fit the needs of your home and start reducing the cost of cooling at home.
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