Smoke detectors shall be installed in return air systems with a design capacity greater than ______, in the return air duct or plenum upstream of any filters.

The correct answer is based on the codes and regulations that govern the installation of smoke detectors in HVAC systems. The requirement for smoke detectors to be installed in return air systems with a design capacity greater than 2,000 cubic feet per minute (cfm) is established to ensure that large air handling systems can effectively detect the presence of smoke. This is crucial because larger systems have a greater volume of air cycling through them, and the faster air movement can influence the spread of smoke.

Installing smoke detectors in these systems helps to provide an early warning in the event of a fire, allowing for prompt action to be taken. Placing the detectors upstream of any filters is also important, as it ensures that the sensors are exposed to the unfiltered air, thus maintaining their sensitivity and effectiveness in smoke detection.

By adhering to the 2,000 cfm threshold, the code aims to create a balance between safety and practical installation requirements, as very small systems may not require such stringent measures where the risk of significant fires is lower.