Air Change

Air change is the rate at which the air within a space is replaced with fresh air.  It is often expressed as the number of air changes per hour (ACH), indicating how many times the entire volume of air in a room is replaced with outside air in one hour.

Air Changes are Important for Several Reasons

Indoor Air Quality (IAQ)  -  Adequate ventilation helps maintain good indoor air quality by removing pollutants, odors, and airborne particles from the indoor environment.  Introducing fresh outdoor air helps dilute and remove contaminants, promoting a healthier indoor atmosphere.
Temperature Control  -  HVAC systems are designed not only to maintain comfortable temperatures but also to control humidity levels.  Fresh air exchange can help regulate temperature and humidity by preventing the buildup of excess moisture or heat.
Odor Control  -  Proper ventilation helps prevent the buildup of odors within enclosed spaces.  Introducing fresh air helps to displace stale air and any associated odors.
Disease Control  -  In certain environments, such as hospitals or other healthcare facilities, controlling the air change rate is crucial for minimizing the spread of airborne contaminants and preventing the transmission of diseases.

The recommended number of air changes per hour can vary depending on factors such as the type of space, its purpose, and local building codes.  For example, residential buildings might have different requirements than commercial or industrial spaces.  Local building codes and ventilation standards typically provide guidelines for minimum air change rates.  To achieve the desired air changes, HVAC systems may incorporate various components, such as ventilation fans, air exchange systems, and air filters.  It's important to strike a balance between providing sufficient ventilation and avoiding excessive energy consumption, and HVAC systems are often designed to be energy efficient while meeting ventilation requirements.