Airflow Factors

Ceiling fans come in a huge variety of sizes, styles, colors, features, and even levels of efficiency. If you are thinking about installing a fan in your home, you need to know the various factors involved, specific to efficiency. With this, you will feel more confident with your purchase, knowing you made the right choice for your particular needs.


A fan's efficacy is defined as the ability to create airflow. For ceiling fans, efficiency is measured using Cubic Feet of air moved per minute, or CFM. To determine the efficacy of fans being compared, consider the following factors listed below.

Blade Length

A good rule of thumb is that the longer the fan blade is, the greater the percentage of air in a room that will be affected. If you are trying to cool a large room, you need to know this information to understand overall efficiency. Today, most ceiling fan blades measure 36, 42, or 53 inches. Although you can also find smaller and larger sizes, these are the most common.

Blade Pitch

The pitch is also an important consideration, which is the angle at which the blades tilt in relation to the X-axis. For instance, if the blade has a significant pitch, it would slow down the airflow generated. Now, as the pitch increases, the fan experiences what is known as drag, which means the motor is having a difficult time rotating the blades. Typically, you want a blade pitch around 15.

Blade Surface

Another important factor of efficacy is the surface of the blade. In this case, the more surface a blade has the more it can produce airflow. Keep in mind that too much surface can have a negative impact on efficiency.

Blade Surface Area to Air Feed Ratio

This ratio indicates that the more surface a blade has the more airflow it will produce. Now, it is important to understand that if the blade has too large of a surface area, the spacing between the blades is likely inadequate, causing air to draw upwards. As a result, airflow is affected adversly.

Ceiling Height vs Fan Height

You also need to know the height of your ceiling so you can choose the right fan. A ceiling of eight foot or less would need a low profile or hugger type of fan, which reduces risk of injury. If the fan to ceiling height measures greater, then a down rod would be required. In this case, the length of the down rod depends on the height. Ceilings of nine to ten feet would need a down rod of about six to eight inches whereas a ceiling of 20 feet would need a 120-inch down rod.

Rotation Speed

Blades will rotate at different speeds, which are measured in Revolutions per Minutes or RPM. The speed of the blades will determine the amount of airflow with faster rotation creating more and slower rotation creating less.

How it Feels

In addition to the factors listed above, the effectiveness of the fan or how it feels to the user can vary based on several factors that are independent of the fan. First, the height of the person experiencing the fans airflow affects the perceived effectiveness or coolness. Second, the humidity level or dew point of the room can have an impact on how cool it feels. In addition, the the angle of the fan blades can also matter since this will direct the full power of the fan to a specific location.

Efficiency Measurement

When you are looking at a fan to determine if it is energy efficient, there is one main measure of efficiency. This measure is defined as the airflow generated by the fan divided by the energy required to generate that airflow. The way to look at whether a fan is energy efficient is to take the number Cubic Feet Per Minute of the fan on its best setting and then divide it by the energy required in Watts. Since most fans publish their cubic feet per minute on the box and the watts required to run, this measure of efficiency can be quite handy for comparison purposes.