At what CFM is powered make-up air required?

Powered make-up air is required when the exhaust systems in a building remove larger volumes of air, creating a negative pressure that can lead to insufficient air replacement. The specific threshold at which powered make-up air becomes necessary is typically determined by building codes and standards pertaining to ventilation and indoor air quality.

The correct answer of 400 CFM is based on guidelines that indicate when exhaust systems exceed this airflow, it becomes essential to introduce powered make-up air to ensure adequate ventilation and a balanced air pressure within the building. This measure helps prevent issues such as backdrafting of combustion appliances, reduced indoor air quality, and inadequate ventilation in occupied spaces.

The other options, while they may represent air movement levels relevant to certain scenarios, do not meet the criteria established by the relevant codes for requiring powered make-up air. Therefore, 400 CFM is recognized as the pivotal airflow volume triggering the need for this system to maintain a safe and comfortable environment.