Which statement best describes the proper use of ACH calculations to improve IAQ?

Which statement best describes the proper use of ACH calculations to improve IAQ?

• A. Determine desired ACH, compute required airflow, and adjust system to meet target
• B. Increase fan speed everywhere without calculation
• C. Only increase outdoor air without considering target ACH
• D. Use a fixed ACH value from a different building without calculation

Answer: (A)

The key idea is using a quantitative link between space size and ventilation to achieve the desired IAQ. You start by setting a target air changes per hour (ACH) based on IAQ goals, then translate that into a concrete airflow requirement using the room’s volume. The needed supply airflow is simply Q = ACH × V, so you multiply the target ACH by the space’s volume to get how much air must be moved per hour (and you can translate to cubic feet per minute or cubic meters per hour as needed). With that number, you adjust the system—fans, dampers, and controls—so the space actually receives that airflow and maintains the target ACH under real operating conditions. This approach is effective because it is explicit, space-specific, and energy-conscious, ensuring the ventilation meets IAQ goals rather than relying on guesswork. Raising fan speed without calculation can overshoot or undershoot the target and wastes energy; increasing outdoor air without considering the target ACH may lead to over-ventilation or under-ventilating indoor contaminants; and using a fixed ACH from another building ignores differences in room size, occupancy, and contaminant sources, making the result inappropriate for the current space.

The key idea is using a quantitative link between space size and ventilation to achieve the desired IAQ. You start by setting a target air changes per hour (ACH) based on IAQ goals, then translate that into a concrete airflow requirement using the room’s volume. The needed supply airflow is simply Q = ACH × V, so you multiply the target ACH by the space’s volume to get how much air must be moved per hour (and you can translate to cubic feet per minute or cubic meters per hour as needed). With that number, you adjust the system—fans, dampers, and controls—so the space actually receives that airflow and maintains the target ACH under real operating conditions. This approach is effective because it is explicit, space-specific, and energy-conscious, ensuring the ventilation meets IAQ goals rather than relying on guesswork.

Raising fan speed without calculation can overshoot or undershoot the target and wastes energy; increasing outdoor air without considering the target ACH may lead to over-ventilation or under-ventilating indoor contaminants; and using a fixed ACH from another building ignores differences in room size, occupancy, and contaminant sources, making the result inappropriate for the current space.