Which statement describes the effect of increasing external static pressure on a ducted system?

• A. Increasing external static pressure improves airflow.
• B. Increases in external static pressure reduce airflow and can degrade performance and energy use.
• C. External static pressure only affects indoor humidity.
• D. External static pressure has no effect on modern variable-speed fans.

Answer: (B)

Increasing external static pressure means the fan has to push air through more resistance in the ductwork. As this resistance grows, the flow rate the fan can deliver at a given speed drops, and meeting the desired airflow often requires spinning the fan faster. That higher speed consumes more power and can raise energy use and noise, while also potentially stressing the system and reducing overall performance. Maintenance and aging (like dirty filters or long, complex ducts) can raise external static pressure over time, making these effects more pronounced.

So the statement is the best describes that effect: higher external static pressure reduces airflow and can degrade performance and energy use. The other ideas don’t fit because ESP doesn’t improve airflow, it can affect more than humidity, and modern variable-speed fans still face limits where increased ESP leads to reduced airflow or higher energy draw.