Ventilation

# 26 Fan Substitutions

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There are three important considerations to keep in mind when changing the motors of HVAC fans, if a change in speed or horsepower is also involved.

The first is that airflow varies directly with fan speed, so doubling the speed of the fan will double the airflow.

$\text{Fan Speed}=\text{Air Flow}$

The second is that static pressure varies with the square of the fan speed, so that doubling the fan speed will quadruple the static pressure in the ductwork.

$\text{Fan Speed}={\text{Static Pressure}}^{2}$

The third consideration is that electrical power consumed by the motor varies with the cube of the fan speed, so that it requires a motor with eight-times the horsepower rating of the previous motor to double the fan speed.

$\text{Fan Speed}={\text{Power Consumed}}^{3}$

This will result in a very significant change in the demand of the electrical draw of the motor. Be sure not to exceed the current, voltage, or horsepower ratings of any electrical circuits, or overcurrent devices.

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