HVAC Star Practice Exam 2026 - Free HVAC Certification Practice Questions and Study Guide

Air changes per hour shows how many times the air in a room is replaced in one hour. To get this, use the ventilation rate in CFM (cubic feet per minute) and convert it to cubic feet per hour by multiplying by 60. Then divide by the room’s volume in cubic feet. This gives ACH in per hour: ACH = (CFM × 60) / room volume (ft^3). The logic is that you’re measuring how many roomfuls of air pass through the ventilation system every hour. If the room is larger, the same CFM replaces the air more slowly, resulting in a smaller ACH; if the room is smaller, ACH is higher. The other forms would mix units or misplace multiplication and division, which wouldn’t yield a per-hour replacement rate.