What air changes per hour mean
Air changes per hour (ACH) is the number of times the air in a space would be fully replaced in one hour at a given ventilation airflow. It turns an airflow in cfm into a figure you can compare between rooms and buildings of different sizes.
The formulas
- Volume = floor area × ceiling height
- ACH = airflow (cfm) × 60 ÷ volume (ft³)
- ASHRAE 62.2 minimum for whole-house mechanical ventilation = 0.03 × floor area (ft²) + 7.5 × (bedrooms + 1) cfm
Worked example
A 2,000 ft² three-bedroom house with 8 ft ceilings and a 100 cfm continuous exhaust fan:
- Volume = 2,000 × 8 = 16,000 ft³.
- ACH = 100 × 60 ÷ 16,000 = 0.375: the air is replaced about every 2 hours 40 minutes.
- ASHRAE 62.2 minimum = 0.03 × 2,000 + 7.5 × 4 = 90 cfm, about 0.34 ACH.
- The fan meets the minimum.
Typical values
| Situation | ACH |
|---|---|
| Very tight new home without ventilation | below 0.1 |
| Whole-house ventilation to ASHRAE 62.2 | 0.3–0.4 |
| Older, leaky home (natural infiltration) | 0.5–1.5 |
| Classroom or office (ASHRAE 62.1) | 2–4 |
| Hospital patient room | 6 |
| Operating room | 15–20 |
Ventilation and airtightness
Building codes now require new homes to be tested for airtightness, often to 3 to 5 ACH at 50 pascals (ACH50) under the IECC. Natural infiltration is roughly ACH50 ÷ 20, so a tight home leaks only about 0.15 to 0.25 ACH and needs mechanical ventilation to keep indoor air healthy. The ASHRAE 62.2 formula sets that airflow.
Indoor air and infection control
For rooms where people gather, the CDC suggests at least 5 equivalent air changes per hour of clean air. That can combine outdoor air with filtration: a portable HEPA purifier with a clean air delivery rate (CADR) of 200 cfm adds 6 ACH in a 2,000 ft³ room, such as a 250 ft² bedroom with 8 ft ceilings. A CO₂ monitor helps check ventilation; readings below about 800 ppm indicate good fresh air supply.
Energy cost
Every cubic foot of outdoor air must be heated or cooled. A balanced system with an energy recovery ventilator (ERV) or heat recovery ventilator (HRV) recovers 60% to 80% of that energy, which makes generous ventilation affordable in cold and hot climates.
Measuring airflow
Fan ratings assume ideal ducts; real airflow is often 20% to 50% lower. A flow hood, an anemometer at the grille or a simple garbage-bag test gives a better number to enter.