What would disallow duct from penetrating a third floor without a fire damper?

The requirement for a fire damper at each floor penetration is in place to prevent the movement of smoke and flames through ductwork during a fire. Building codes are established to enhance safety and protect occupants by minimizing the spread of fire and smoke between floors. When a duct penetrates a floor without an appropriate fire damper, it compromises the fire-resistance-rated assembly of that floor, allowing smoke and heat to travel from one floor to another unchecked. Therefore, if the building code mandates the installation of a fire damper at every floor penetration, failing to install one would indeed disallow the duct from passing through that third floor.

The other options touch on considerations that might affect the installation of ductwork in different ways but do not directly address the specific requirement imposed by fire safety codes regarding floor penetrations. The absence of fire-rated walls might indicate a different set of risks, while the size of the duct or the design regarding the vertical run might affect overall duct performance but not the necessity for fire dampers established by building code requirements.