Final answer:
Duct static air pressure affects the supply fan load by creating either more or less resistance to the airflow. Higher duct static air pressure increases the load on the supply fan, while lower duct static air pressure decreases the load.
Step-by-step explanation:
In forced air systems, the supply fan is responsible for pushing air through the ductwork and into the building. The duct static air pressure is the pressure exerted by the air inside the duct. This pressure affects the load on the supply fan.
When the duct static air pressure is high, it creates a greater resistance to the flow of air. The supply fan has to work harder to overcome this resistance and maintain the desired airflow. This increases the load on the fan, making it consume more energy.
On the other hand, when the duct static air pressure is low, there is less resistance to the flow of air. The supply fan doesn't have to work as hard to maintain the desired airflow, resulting in a lower fan load and energy consumption.