What's more important, CFM or Wind Speed? Read this article to find out.
Ceiling Fan Wind Speed Factor is a rough calculation of the potential Wind Speed any given ceiling fan produces in the column of air directly beneath it. The calculation is based on the width of the fan and the CFM (Cubic Feet Per Minute of Airflow), which the EPA currently requires ceiling fan manufacturers to publish. Wind Speed Factor allows you to compare the actual cooling capability between ceiling fans that are of different sizes and CFMs.
WIND SPEED FACTOR
Rating System: (1) < 1.5, (2) 1.5 to 2.49, (3) 2.5 to 2.99, (4) 3 to 3.75, (5) 3.75 to 3.99, (5.5) 4 or More
Theory Behind Our Wind Speed Factor Calculations:
Hypothesis: Ceiling fans push air down in a column of air that is roughly the same width (or diameter) as the ceiling fan. If you have 2 fans that move the same Cubic Feet per Minute of air (CFM) and one is smaller than the other, the smaller fan will have more concentrated airflow, which will make you feel cooler if you stand or sit directly beneath it. The larger moves air over a larger area, so you will NOT feel as cool when you are directly beneath it because the main column of airflow is less intense.
Ceiling fans cool you off by creating a wind-chill effect, which is directly related to wind speed (MPH), not the amount of air that is moved (which is CFM). All ceiling fan manufactureres are now required to test and publish the amount of airflow their fans produce in CFM. In my opinion, this is a flawed comparison because CFM does not in and of itself tell you how much cooling efffect a fan produces. Yes, it is a valid comparison if you have two fans that are exactly the same size, then the one with a higher CFM will produce a higher wind speed. But what about comparing fans of different sizes. Let's say you are not sure if you would be better off with a 52" fan or a 60" fan. If you just compare the CFM, it is likely that the 60" fan will be higher. But that does not mean it produces a higher wind speed because the amount of air being moved is measured in a larger area and may be less concentrated. This is why I divised the Wind Speed Factor Calculator, which provides a means of calculating the airflow intensity, which I have termed "Wind Speed Factor", giving you the ability to compare different size fans with various CFM. The Wind Speed Factor is expressed in MPH and is calculated based on the CFM reported by the manufacturer and the diameter of the fan plus 8", which is the diameter of the cylinder used for testing ceiling fan CFM. These are the basic factors necessary to calculate wind speed in an air column.
There is no scientific testing done to prove or disprove my calculations and I will say openly that the calculation is somewhat flawed as it is based on the assumption that the column of air generated by a ceiling fan is cylindrical in shape, where in reality, the pattern varies between models and actually spreads out wider the closer you get to the floor. However, the CFM testing procedure required by the federal government is the foundation for this flaw because it is performed in a cylinder as opposed to an open area. This is why I use the term "Wind Speed Factor" rather than "Wind Speed", because it is a factor based on the CFM as reported by the testing procedure rather than an actual scientific measurement of the wind speed. So although the actual wind speed may be different, the wind speed factor we are calulating does provides a useful way to compare the actual cooling effect between ceiling fans of differing diameters and CFM.
Fan Size -vs- Wind Speed -vs- CFM
Note: The formula for calculating wind speed was revised on 1/31/2015. The previous formula did not take into account that the testing chamber (cylinder) used to determine the CFM of a fan is 8" in diameter larger than the blade span. The revised formula ads 8" to the fan diameter to account for this. The wind speed results are now slightly lower than previosly stated
"When it comes to ceiling fan size...the largest fan with the highest wind speed that will look fine in the area is the best choice!"
Ceiling fans cool you off by creating a wind-chill effect, the degree of which is determined by the wind speed produced by the fan; the higher the wind speed, the cooler you feel. It is critical to know the following; CFM is not the same as wind speed; the wind chill effect you feel from a ceiling fan is concentrated in the column of air directly beneath the fan blades; the wind chill drops off dramatically just a foot or two outside that area.
Ceiling fan manufacturers are required by the DOE to publish the airflow of each fan in CFM (Cubic Feet per Minute) which is not the same as wind speed. CFM is the volume of air being moved every minute, whereas wind speed how fast the air is moving in MPH (Miles per Hour) or LFM (Linear Feet per Minute), both of which are directly related to the amount of wind chill effect. When you compare two fans that are different sizes yet have the same CFM you will find they produce quite different wind speeds. The smaller fan must move air faster in order to move the same amount of air per minute (CFM) as the larger fan. The airflow generated by the smaller creates more wind chill because it is more concentrated and has a higher wind speed. The larger fan of equal CFM moves air over a larger area so it is less concentrated at a lower wind speed. So when determining what size fan to buy you must consider the amount of space a fan will cover (the diameter of the fan plus a foot or two), the intensity of the airflow directly beneath the fan (the wind speed) balanced with how any particular size fan will look or fit in the space. Ultimately the largest fan with the highest wind speed that will look fine in the area is the best choice for performance.
The required testing method for ceiling fans is not done in a way that translates well to real world applications, so the CFM data can be somewhat deceiving if you do not know how to interpret it.
The current required test procedure (shown left) shows a ceiling fan hanging above a 3 foot tall metal cylinder that is 8 inches wider than the ceiling fan (blade span) and stands 4 feet above the floor. Sensors are located at the bottom of the cylinder to measure the wind speed. The wind speed is then converted into CFM (Cubic Feet per Minute). Although this testing procedure can be valuable for comparing the performance of fans equal in size, it does not emulate a real application inside a home and does not make it easy to compare fans that are different sizes in order to determine the best size fan(s) for any particular application.
Therefore, the best way to compare the actual performance between fans of various sizes is to compare the wind speed. The wind speed of a ceiling fan can be mathematically calculated based on the CFM and Blade Span. Here is a comparison where we performed the calculations between 3 fans of different blade spans and CFMs. The largest fan has the highest CFM yet the smallest fan produces the highest wind speed.
84" Fan with CFM of 10000 produces wind speed of 2.46 MPH (216.62 LFM)
52" Fan with CFM of 6500 produces wind speed of 3.76 MPH (331.04 LFM )
The above example shows how deceiving CFM data can be. Even though the 52" fan moves 35% less air than the 84" fan, the intensity of the breeze that produces a wind-chill effect is over 50% more than that of the larger fan and will make you feel much cooler if you are directly beneath it. So, to cover a larger area, two or more smaller fans with good performance may be a better choice than a single large higher CFM fan.