What does ACH stand for?

• A. Air Change Rate Per Hour
• B. Air Changes Per Hour
• C. Air Conditioning Humidity
• D. Air Current History

Answer: (B)

ACH stands for Air Changes Per Hour. This term is the standard way to describe how many times the air inside a space is replaced with outdoor air in one hour, which is a key measure of ventilation performance. The plural “Changes” emphasizes multiple air replacements over the hour, and you’ll see this phrasing in HVAC guidelines and building codes when planning or evaluating ventilation.

To ground it with a quick example: if a room has a volume of 100 cubic meters and the ventilation system brings in 300 cubic meters of outdoor air per hour, the air inside is replaced three times per hour, so the ACH is 3.

The other options don’t describe ventilation change rate: one refers to humidity, another to an unclear concept, and the phrase with “Rate” is not the standard term used in practice.