ACH stands for Air Changes per Hour. What does it measure?

ACH stands for Air Changes per Hour. What does it measure?

• A. The Percentage of Air Exhausted and Replaced Each Hour
• B. The Rate of Outside Air Replacement Per Hour
• C. The Total House Volume Per Hour
• D. The Speed of Air in the Ducts

Answer: (B)

You’re being tested on how ventilation is quantified. Air changes per hour measures how often the entire indoor air volume is replaced with outdoor air in one hour. It’s a rate that tells you, for a given space, how quickly fresh air is brought in and stale air is pushed out. Think of it this way: ACH is calculated from the outdoor air flow rate (the amount of outside air entering the space per hour) divided by the room’s volume. If a room is 5,000 cubic feet and outside air is supplied at 2,500 cubic feet per hour, the ACH would be 0.5, meaning the room’s air is replaced half a time per hour (or once every two hours). This concept emphasizes turnover of air, not the percentage of air moved, the total volume moved per hour, or the speed of air in ducts.

You’re being tested on how ventilation is quantified. Air changes per hour measures how often the entire indoor air volume is replaced with outdoor air in one hour. It’s a rate that tells you, for a given space, how quickly fresh air is brought in and stale air is pushed out.

Think of it this way: ACH is calculated from the outdoor air flow rate (the amount of outside air entering the space per hour) divided by the room’s volume. If a room is 5,000 cubic feet and outside air is supplied at 2,500 cubic feet per hour, the ACH would be 0.5, meaning the room’s air is replaced half a time per hour (or once every two hours). This concept emphasizes turnover of air, not the percentage of air moved, the total volume moved per hour, or the speed of air in ducts.