What does Air Exchange per Hour (ACH) measure in a defined space?

Air Exchange per Hour (ACH) measures the frequency of air replacement in a defined space, specifically how many times the total volume of air within that space is replaced with outdoor air over the course of an hour. This metric is essential in understanding ventilation effectiveness and indoor air quality, as it provides insights into how often fresh air is brought in and stale air is expelled.

A higher ACH value indicates that a space is receiving more fresh air, which can contribute to improved air quality, while a lower ACH may suggest insufficient ventilation, potentially leading to issues with indoor pollutants, humidity, and overall comfort levels. This measurement is particularly important in environments where air quality is critical, such as in residential buildings, offices, or healthcare facilities.

While speed of air movement through ducts is relevant for HVAC design and efficiency, it does not directly relate to the concept of air exchange within a room. Additionally, total energy consumption of HVAC systems pertains to the efficiency and performance of the heating and cooling systems themselves, rather than the direct measurement of air replacement. Temperature change of incoming air is important for HVAC performance but does not define how often air is exchanged within a space. Understanding ACH is crucial for effective design and operation of ventilation systems, ensuring spaces are adequately ventilated for occupant health