HVAC systems over _____ CFM must shut down on alarm to prevent the spread of smoke?

When dealing with HVAC systems in the context of fire safety, a crucial aspect is the prevention of smoke and fire spread throughout a building. The requirement for HVAC systems to shut down on alarm at a specified airflow rate, which in this case is 2000 CFM, is based on the consideration of how efficiently smoke can travel through ductwork and other pathways provided by the HVAC system.

At airflow rates exceeding 2000 CFM, the ability for smoke to be carried and distributed throughout a building significantly increases, posing a serious risk to life safety and property. Hence, by shutting down the HVAC system at this threshold, it helps to contain and control the spread of smoke during an incident, allowing for safer evacuation of occupants and improved conditions for firefighting efforts.

The other options represent lower thresholds that do not align with established codes and practices aimed at effectively managing smoke spread in larger structures. By adhering to the specification of 2000 CFM, fire safety protocols ensure that a sufficient level of control is maintained over potential smoke movement during an emergency situation.