Key Components of Air Distribution Systems (Fans)

September 18, 2023

Fans move air by pushing it with impellers (blades) powered by a motor. Pushing the air increases pressure which is measured in inches of water.

As the air is pressurized, it is heated by friction as it passes through the fan. More heat is added if the fan motor is located in the air stream. The air temperature can rise from 1 to 5 or more degrees, depending on the system operating pressure.

The most common fan designs used in HVAC systems are centrifugal and axial.


With centrifugal fans, air enters the fan housing through the rotor, makes a 90° turn, is captured by the scroll-shaped housing and fan blades, and forced through the fan discharge. Three types of centrifugal fans are common in HVAC applications:

Airfoil: Fans with backward-curved fans and deep airfoil blades. These are the most efficient centrifugal fans and rotate at higher speeds.

Backward curved: Fans with flat wheel blades that lean away from the direction the fan is turning. This design is only slightly less efficient than the airfoil design.

Forward curved: Fans using small blades that are curved forward in the direction of the rotation. These are the least efficient of the three centrifugal designs and are used mainly in low-pressure HVAC applications


With axial fans, air moves in a straight path through the propeller-type rotors to produce airflow. Three types of axial fans are common in HVAC applications:

Vane axial: Fans with fixed, adjustable, or controllable-pitch blades. These are the most efficient axial fans. Guide vanes in front of or behind the fan blades increase pressure capability and efficiency. These are direct-drive units without fan belts. Vane axial fans are noisy and usually require sound attenuation.

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