If you ever went through the specifications of a fan, you would have seen a CFM rating. It is actually an important parameter indicating the performance of a fan, and it is very important that you use a fan with the right CFM rating.
So what is the CFM of a fan? CFM is a measurement of airflow. It is the volume of air moved by a fan per minute measured in cubic feet. So if a fan has a CFM rating of 1000 it means that this fan can move 1000 cubic feet of air in a minute.
As you know, a fan, whether it is a ceiling fan or an attic fan, has variable speeds. So for what speed is the CFM rated for? The CFM rating shown in the specifications sheet of any fan is of its highest speed, and definitely, this is the highest CFM that this particular fan can produce. However, for some types of fans, like ceiling fans, the CFM ratings at different speeds are also listed.
What does the CFM rating depends on?
The CFM rating of a fan depends on three things;
- The power of the motor
- The pitch of the blades
- The length and shape of the blades
A powerful motor can create more CFM than a weaker motor. This is because a powerful motor can give the fan more RPMs. However, this increase in CFM with motor power is not linear if all the other measurements of the fan remain the same.
The pitch of a fan is the angle between the blade of a fan and the horizontal plane it runs. This pitch need not remain a constant for the whole length of the fan blade though. There are many models with varying blade pitch, for eg: turbines, to facilitate more airflow.
The length and the shape of the blades also determines its CFM. The longer the blade is, the more air it can move. Couples with an adequately powered motor a long blade can move more air than a shorter one.
Why is the CFM rating important?
If you need a fan you can’t simply go and buy any you like. First, the CFM requirement for space should be calculated, and the fan should be selected based on this value. CFM is a strong indicator of the amount of air a fan can move. The more the CFM the more air it can move. However, if you install the wrong size fan it may not work as intended, and moreover, it can cause serious damages.
For example, if you buy an over-powered attic fan, it can actually depressurize your attic causing back drafts and even fire hazards. So it is always important to choose a fan with the right CFM.
There is a wide range of fan types for a wide range of applications so it is absolutely impossible to know what CFM fan you need without knowing what it is for along with a myriad of other parameters.
If you are looking for an attic fan here is a size guide.
If you are looking for a whole house fan, here is its size guide.
How is the CFM of a fan calculated?
The CFM ratings of consumer fans are pre-calculated before production since these are mass-produced. However, if you want to know the CFM of your fan you can simply buy a device called Anemometer aka Wind Meter. By holding this device infront of the fan you can measure the air velocity. Multiplying this air velocity (which is in meter per second) with the diameter of the fan will give you the amount of air it moves in a minute. However, you will need to convert this to cubic feets since the air velocity is measured in meters per second.