What might be the reason for high discharge pressure after a fan motor has been replaced?

High discharge pressure in a system can often be attributed to the fan motor being oversized. An oversized motor can lead to excessive airflow through the system, causing the pressures to rise beyond normal operating levels. This occurs because the fan may push more air than the system was originally designed to handle, leading to a disturbance in the overall airflow dynamics and creating higher pressure at the discharge.

When a system is designed, specific parameters, including the size and capacity of the fan motor, are calculated to ensure efficiency and proper balance of airflow. If the new motor does not match the specifications of the original one, such as if it delivers a greater volume of air than needed, the system may not function optimally, leading to problems like high discharge pressure.

In contrast, while other issues like the fan blade not being changed or a defective replacement motor could potentially lead to problems, they typically would not directly cause an increase in discharge pressure due to the reasons established. Running backward could affect airflow and pressure but would be less likely to lead to consistently high discharge pressure compared to the significant impact of an oversized motor.