The Most Important Consideration When Shopping For Ceiling Fans
Preface: When shopping for ceiling fans the most overlooked consideration is wind speed, or how much breeze you can expect from a fan. That's why you buy a fan in the first place, right, to cool you off? Those who do consider performance usually compare CFM ratings, which is a flawed comparison as you will learn from this article.
"Choose the fan with the highest wind speed and largest blades that will fit your space!"
Which fan creates the strongest breeze:
a 96" fan at 11,000 CFM ~ OR ~ a 50" fan at 7,000 CFM?"
Most people would assume the 11,000 CFM fan will create the strongest breeze, but that is incorrect. Plug the numbers into our calculator below and you will find the larger fan only has a 2.12 MPH wind speed factor, while the smaller fan is 4.34 mph. The fact is, smaller blades put less drag on the motor, so it spins at a higher RPM moving the air much faster, creating a stronger breeze in the area directly beneath the fan. So wind speed tells you how fast the air is moving (MPH), which is directly related to how much breeze a fan creates. CFM only tells you how much air a fan moves by volume, not how fast it moves the air. The key is to find the balance between blade span and wind speed.
Instructions: Enter the blade span and CFM of any ceiling fan to calculate it's Wind Speed Factor. By default, the calculator has the numbers in it for the 60" Emerson Carrera Grand ceiling fan
Rating System: (1) < 1.5 MPH, (2) 1.5 to 2.49 MPH, (3) 2.5 to 2.99 MPH, (4) 3 to 3.75 MPH, (5) 3.75 to 3.99 MPH, (5.5) 4 or More
Our exclusive Wind Speed Calculator converts CFM to MPH by factoring in the blade span. The result is what we call "Wind Speed Factor" represented as MPH (Miles Per Hour). This number gives you the ability to compare the breeze you might expect between various size fans. Most of the fans on our site already have the Wind Speed Factor calculated, so you can easily compare them. You can use our calculator to compare our fans with other websites.
Ceiling fans cool you off by creating a wind-chill effect, they do not lower the temperature of the room. So how cool you feel is directly related to how fast the air is moving (MPH), not how much air is moved (CFM).
Ceiling fans create an air column that is roughly the same diameter as the ceiling fan (Fig. 2), so the main cooling effect is in the area directly beneath the fan. The surrounding area receives significantly less breeze. So it is important that you try to install a fan directly over the area you want to cool or you choose a fan large enough to cover as much of the area as possible. As long as a portion of the blade passes over the area you want to cool, you will feel the strongest breeze. In many cases, this means using a much larger fan that you might have considered.
If you cannot handle a single fan that is as large as the area it needs to cool, consider two or more smaller fans. If you can only use one fan, use the advice we suggested at the beginning of this article: Choose the fan with the highest wind speed and largest blades that will fit your space. We also highly recommend DC motors that have at least 6 speeds. If you buy a high wind speed DC motor fan, most of the time running it on medium or low will be sufficient. You will always have those higher speeds for extra hot days when you really want that strong breeze.
Fig.2 above shows the intensity and variation of wind speed from a ceiling fan measured with anemometers. Notice the large amount of blue area labeled as "Still Air Zone" where the air is hardly moving at all. This test result shows that substantially less, and in some places, almost no airflow is felt outside of the direct column of air. Considering the silhouettes we added to the diagram, the woman would feel the air at it's maximum flow (3.13 MPH) as she is standing directly beneath the fan in the main column of air. The man talking to her would feel less than 1/3 of the breeze (less than 1 MPH). The man sitting in the office chair may not feel anything, except at his ankles (less than 1/2" MPH).
This image shows the airflow pattern using a smoke test with and without a desk in the room. Notice how similar the airflow pattern of the smoke is to the above picture of air velocity. Also, notice how the wind flows around the desk on the picture in the right showing that some obstructions will divert the wind. In this case, if you are sitting and your chair is not directly beneath the fan yet the desk is, you will benefit from the diversion of air and be able to feel the breeze from the fan. Consider the same thing in a bedroom. If part of the fan is over your bed, the air will flow to the end of the bed, thus covering you with a breeze. If the fan blades are not at all over the bed, the air will flow under the bed and you will not feel much of it at all. This would be a case where running the fan in reverse may actually allow you to feel the breeze since it will be pushed out to to the walls and down. If your bed is against the wall, the air will be forced to flow over you.
The above images were taken from a study titled "Ceiling fan air speeds around desks and office partitions" published online at: eScholorship.org
There is no scientific testing done to prove or disprove our calculations, they are an interpolation based on the data supplied by the manufacturer and we openly admit that the our wind speed factor imperfect. Yet It is based on the scientifically proven fact that the column of air generated by a ceiling fan is roughly the same diameter as the fan itself (see images below). This is why we use the term "Wind Speed Factor" rather than actual "Wind Speed". If you were to hold an anemometer beneath a fan and measure the actual wind speed, it most likely would not be the same as our wind speed factor. With that said, our wind speed factor does provide a valuable tool that can help you compare ceiling fans and decide which size and model will be best for your application.
"The largest fan with the highest wind speed that will look good in the area is the best choice!"