What does ACH stand for in the context of air handling?

• A. Air Changes per Hour
• B. Actual Circulation per Hour
• C. Absolute Contamination Heuristic
• D. Air Consumption per Hour

Answer: (A)

In air handling, ACH is a standard way to express how often the air in a space is replaced each hour. It stands for Air Changes per Hour. You calculate it by comparing how much air is moved into or out of the space (the ventilation flow rate) to the room’s volume. If you have a ventilation rate of Q cubic meters per hour and the room has a volume V cubic meters, ACH = Q / V. For example, 300 m3/h of supply air in a 50 m3 room gives 6 air changes per hour, meaning the room’s entire air content is replaced six times each hour. This metric helps gauge how quickly contaminants and CO2 will be diluted and how the space will respond to ventilation needs.

The other phrases don’t represent the standard ventilation concept used in air handling. They don’t describe the rate at which the space’s air is replaced, so they aren’t the term practitioners rely on to quantify ventilation.