When is return air ducted to one or several ceiling grills?

The return air is often ducted to one or several ceiling grills primarily to ensure proper air circulation and ventilation in a space with potential combustible materials. Ceiling grills facilitate the efficient collection of return air, which is crucial in spaces where flammable items may be stored since it helps maintain an adequate airflow and reduces the risk of heat or flames reaching these materials. Proper airflow can help prevent the accumulation of hazardous conditions that could lead to fire risks.

In contrast, ducting return air under the conditions described by the other choices does not address airflow efficiency or safety. For example, ducting based solely on pressure, moisture, or temperature may not be the most effective method for managing air return. Each of those factors can influence HVAC performance, but they do not specifically relate to the safe handling of air in environments where combustibles are present. Thus, ducting to ceiling grills is particularly important in settings with potential fire hazards.