What does ACH stand for and why is it important in ventilation design?

What does ACH stand for and why is it important in ventilation design?

• A. Air Circulation Hydraulics; measures air exchange rate
• B. Air Changes per Hour; measures how often the air in a space is replaced per hour.
• C. Automatic Cleaning for Health; for air quality
• D. Air Conversion Hours; unit of ventilation

Answer: (B)

Air Changes per Hour is a ventilation design metric that tells you how many times the air in a space is replaced in one hour. It is calculated by taking the volumetric airflow rate (how much air moves in or out per hour) and dividing it by the room’s volume, giving units of h^-1. This rate matters because it directly relates to dilution of contaminants, CO2, and humidity; higher ACH means more fresh air is available to dilute pollutants and improve comfort. In design practice, engineers use ACH to ensure IAQ targets are met while balancing energy use, since increasing air changes typically raises ventilation and conditioning energy demands. For example, a 100 m³ room with 300 m³/h of ventilation air has an ACH of 3, meaning the room’s air is replaced three times each hour. The other options don’t reflect the standard term or its meaning as a rate of air replacement per hour.

Air Changes per Hour is a ventilation design metric that tells you how many times the air in a space is replaced in one hour. It is calculated by taking the volumetric airflow rate (how much air moves in or out per hour) and dividing it by the room’s volume, giving units of h^-1. This rate matters because it directly relates to dilution of contaminants, CO2, and humidity; higher ACH means more fresh air is available to dilute pollutants and improve comfort. In design practice, engineers use ACH to ensure IAQ targets are met while balancing energy use, since increasing air changes typically raises ventilation and conditioning energy demands. For example, a 100 m³ room with 300 m³/h of ventilation air has an ACH of 3, meaning the room’s air is replaced three times each hour. The other options don’t reflect the standard term or its meaning as a rate of air replacement per hour.