When are static measurements taken on an air system?

Static measurements on an air system are taken while the fan is moving air. This is essential because static pressure is the pressure exerted by the air when it is in motion through the ductwork. Taking measurements when the fan is operating allows for an accurate representation of the airflow dynamics within the ventilation system. This includes understanding how much pressure loss occurs as air moves through ducts and across obstacles, which directly impacts the efficiency and performance of the system.

The other options do not provide the appropriate conditions under which static measurements would reflect the system's operational efficacy. For instance, measuring with the fan off would provide inaccurate readings as there would be no airflow, making it impossible to gauge the system's performance effectively. Similarly, conducting these measurements during maintenance checks or after cooling the air does not align with the need to assess the operational performance under actual working conditions. Hence, the fan must be in operation to ensure relevant and insightful data regarding the air system's static pressure.