While shopping for ceiling fans, it is likely that you will encounter websites and sales people touting how important it is to consider the blade pitch of a ceiling fan before you buy it. Blade pitch is the angle at which the blades are set relative to parallel to the ground. It is commonly stated that "the steeper the blade pitch, the more air a fan will move" or "a good fan will have a blade pitch of 14 degrees or more". These common misnomers are often used to entice people to purchase a ceiling fan regardless of how well it actually performs. Blade pitch alone does not determine the airflow of a ceiling fan, although it is part of the equation. The power of the motor, size and shape (aerodynamics) of the blades, the material the blades are made of, the RPM at which the blades spin...and the blade pitch are all factors that contribute to airflow. For example, ceiling fans with real palm leaf blades won't move hardly any air no matter what angle the blades are set at. Buy one of these fans and you may be very disappointed with the airflow even if it boasts a 20 degree blade pitch.
What Really Matters is CFM and Wind Speed!
The EPA now requires all fan manufacturers to publish the airflow rating of their ceiling fans in CFM (Cubic Feet per Minute). They are also required to publish the energy usage in Watts as well as the efficiency, which is measured as CFM/Watt. Before this information was available, blade pitch was an important consideration when comparing fans to try and determine if one fan "might" move more air than another, but without considering the other factors I mentioned, it was not very accurate.
Because the CFM data for ceiling fans is now readily available, blade pitch should not even be part of your consideration, because you can easily compare the performance between fans with actual airflow ratings. But don't be fooled by the CFM numbers either, because they can be misleading if you do not also account for the size of a fan. Yes, CFM is a great direct comparison when considering fans of the exact same size, but when you are trying to decide what size fan is best for your application, CFM alone does not tell the whole story and can be misleading. What you really need to do is compare Wind Speed
What is the difference between Wind Speed and CFM?
CFM is the measurement of the volume of air being moved in cubic feet per minute, whereas wind speed is the measurement of how fast that air is actually moving in MPH or Linear Feet per Minute. Wind speed is what actually creates a the wind-chill effect of a ceiling fan, not the volume of air being moved. When operated in the forward direction (for cooling) a ceiling fan will produce a column of air directly beneath the fan that is about the same as the diameter of the fan. It is within this column of air directly beneath a ceiling fan where you will actually feel the most intense airflow and most benefit from the cooling effect. If you take the exact same fan and put different sized blades on it, larger blades will put more drag on the motor and cause it to spin slower at high speed, which lowers the intensity of the airflow, but spreads it over a larger area. Conversely, smaller blades will allow the fan to spin faster, which will generate more intense airflow (higher wind speed), but over a smaller area. This is why manufacturers will use larger motors in fans with larger blade spans, so that the larger fan will spin just as fast as the smaller fan, and hence move more air. But there are limitations to how fast a fan can spin based on size (read more about this further down).
I like to equate CFM - vs - Wind Speed to a garden hose with or without a pressure nozzle. With no nozzle, the water flows unrestricted in a wider column with little pressure. If you reduce the area where the water comes out of the hose with a pressure nozzel, the water comes out in a highly pressurized stream. The actual volume of water (flow rate) coming out of the hose without a nozzle may be about 12 gallons per minute, compared to about 6 gallons per minute with the nozzle on. So if you were shopping for a hose or nozzle because you wanted to hose down your driveway, the 12 gallons per minute number seems larger, but you know for a fact that having a nozzle will do a better job. So it is not the amount of water used that is important to clean your driveway, it is the amount of water pressure. With ceiling fans, wind-speed in MPH is like water pressure and CFM airflow is like water volume or flow rate. So when shopping for ceiling fans, the size of the fan (area it will cover) and actual wind-speed (MPH) is what you need to know.
If you just want to move a lot of air around in the house without feeling it, then you want CFM, which is wind volume. But who wants that? You want to feel cool, right? So the bottom line is, you want to be able to compare the wind speed between ceiling fans. Now if you have 2 fans that are exactly the same diameter and one produces more airflow (CFM), then that fan will produce a higher wind speed. But how about 2 fans that are different sizes, yet produce the same airflow (CFM)? The smaller fan will actually generate a higher wind speed because it must move the air faster in order to generate the same CFM as the larger fan. Take a look at the comparison between the two fans below. The large 84" diameter fan produces a very high airflow over 10,000 CFM, whereas the smaller 54" fan produces about 20% less CFM airflow. If you consider just the difference in CFM between the fans you might conclude that the larger fan will keep you 20% cooler. But the actual wind speed of the larger fan is only 2.98 MPH compared to 5.94 MPH for the 54" fan. Although the smaller fan moves 20% less air, it actually provides twice as much cooling effect than the larger fan where the wind speed is diluted over larger area.
Why not just make the larger fan even more powerful?
So why don't ceiling fan manufacturers just put a more powerful motor in the larger fans to make them spin faster and increase the wind-speed? Unfortunately, there are regulations that limit how fast ceiling fans are allowed to spin in residential applications. These limitations are a safety measure to minimize bodily harm in cases where someone sticks their hand in the air or does something really stupid that causes them to get hit by the fan (you can search YouTube for examples). The maximum allowed RPM is based on how fast the tip of the blade travels as it goes around. The larger a fan is, the faster the blade tip travels through the air during each revolution. So larger fans like the 84" fan above simply cannot be allowed to move faster in order to increase the wind speed. The only way to increase wind speed once the maximum allowed RPM has been reached is to improve the efficiency of the blades with aerodynamics. If you look closely at the larger fan, you will see the blades already have a more aerodynamic design than the smaller fan, which has flat traditional style wood blades.
So what is the takeaway from this is?
For a large area, two fans with high wind speed will keep you much cooler than a single larger fan with a lower wind speed. If you can only install a single fan, look for a model with the highest CFM, but do not expect to feel as much cooling effect as you would from a smaller fan.
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.
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