It seems to me that this question refers to IMC 601.2 (2009) Air movement in egress elements. This is what I read from this post; I have a fully ducted HVAC system (supply and return). I have supply diffusers and return grills in the exit access corridor, the open lobby that the corridor terminates into, and the private office spaces that the corridor serves. Basically I am ventilating all these spaces with my single HVAC system.
This is permitted. The corridor is not serving as a supply, return, exhaust, relief or ventilation air duct. The corridor is simply being ventilated. This would be different if the private office spaces were not provided with a return grill, and the only return grill location was in the corridor. With no return grill in the private office space, the supply air to the office would have to return through an undercut in the door leading to the corridor. The corridor would then be used as a return duct for the office. This is not permitted.
The purpose of this section is to prevent smoke from filling the corridor. If there was a smoldering fire in one of the offices, a return grill in the office would pull the smoke back to the HVAC unit, and the smoke detector mounted in the HVAC unit (if required) would shut down the air movement. If there were no return grill in the office, the smoke would be drawn into the corridor, filling it with smoke before being pulled back to the HVAC unit and shutting down the air movement. We don't want to design a system that will fill our egress elements with smoke. The example above does not do this, therefore it is permitted.
