Air Changes Per Hour

Air Changes Per Hour, often abbreviated as ACH or ac/hr, refers to the number of times the total volume of air in a specific space (like a room or building) is replaced with fresh air in one hour. This metric takes into account both mechanical ventilation (like HVAC systems) and natural ventilation (like windows or vents). It is calculated by dividing the total ventilation rate (in cubic feet per minute or cubic meters per hour) by the volume of the space being ventilated. Essentially, it tells us how often the air in a space is completely “refreshed,” which is an important factor for maintaining good indoor air quality and managing temperature and humidity levels.