In a BE workplace, what does air changes per hour (ACH) measure?

• A. The rate at which air is replaced in the room
• B. The total airflow in CFM
• C. The ceiling height influence on ventilation
• D. The ratio of outdoor to indoor air

Answer: (A)

Air changes per hour tells you how often the air inside a room is replaced with fresh or conditioned air in one hour. It’s a rate that normalizes for room size, so you can compare ventilation effectiveness across spaces of different volumes. It’s calculated by dividing the ventilation rate (outdoor air moved per hour) by the room’s volume. For example, a 60 m^3 room with 60 m^3/h of outdoor air has an ACH of 1 h^-1; the same room with 600 m^3/h would have 10 h^-1. This is different from total airflow, which is the amount of air moved per unit time without regard to room size. It’s also not the outdoor-to-indoor air ratio itself; ACH focuses on how quickly the space’s air is replaced. Higher ACH generally improves contaminant dilution and removal, but very high values can cause drafts or energy waste.