The International Mechanical Code requires a duct smoke detector in the return for units over how many cfm?

The correct choice reflects the requirement set by the International Mechanical Code, which mandates the installation of duct smoke detectors in the return air duct systems for air handling units that move significant airflow, quantified as over 2000 cfm (cubic feet per minute). The rationale behind this requirement is predominantly safety-oriented; duct smoke detectors play a crucial role in ensuring that smoke does not circulate through the HVAC system, potentially affecting larger areas of a building before a fire alarm can be triggered. When the airflow exceeds this threshold, the risk of smoke travel through the duct system increases, making the presence of a smoke detector essential for early detection and response to potential fire incidents. This is why understanding the specific airflow criteria outlined in mechanical codes is vital for the design and implementation of effective fire alarm systems in commercial and industrial settings.