Why fan selection isn't just about the ductwork.
A supply fan doesn't just fight the ductwork. It fights everything the air passes through from the moment it enters the return grille to the moment it leaves the supply diffuser — and roughly half of that resistance, in a typical packaged air handling unit, never leaves the equipment's own cabinet. Confusing the ductwork-only number with the number the fan actually has to overcome is one of the most common, and most avoidable, reasons an installed HVAC system quietly delivers less airflow than it was designed for.