What is the recommended external static pressure of a supply air plenum on a forced air furnace?

The recommended external static pressure range for a supply air plenum on a forced air furnace is typically established to ensure efficient airflow and proper system operation. A range of 0.2" to 0.5" W.C. is considered optimal because it provides a balance between adequate airflow delivery throughout the duct system and the prevention of excessive energy consumption and noise. This pressure level is conducive to maintaining optimal furnace performance while minimizing the strain on the blower motor and avoiding airflow issues that could lead to inconsistent heating or cooling in the space being served.

When the static pressure falls below this range, there may not be enough airflow to effectively distribute conditioned air, potentially resulting in cold spots and increased energy costs. Conversely, if the static pressure exceeds this range, it can cause excessive wear on equipment, increase noise levels, and may lead to system failures.

The other options represent ranges that are either too low or too high, which would not support the effective operation of the furnace as intended. Thus, the choice of 0.2" to 0.5" W.C. is aligned with industry standards for forced air furnaces.