Understanding Air Changes per Hour (ACH)
**ACH** is a measure of how many times the entire volume of air in a space is replaced by new (or filtered) air every hour. It is a critical metric for indoor air quality and ventilation design.
ACH Requirements and Guidelines
Formula Breakdown
$$\text{ACH} = \frac{\text{CFM} \times 60}{\text{Volume (CF)}}$$
- **CFM:** Cubic Feet per Minute (Rate of airflow).
- **$60$:** Converts minutes to hours.
- **Volume (CF):** Room Length $\times$ Width $\times$ Height.
Recommended ACH Values (Residential)
| Area | Recommended ACH |
|---|---|
| Living Spaces (General) | 0.35 to 1.0 |
| Bedrooms | 0.5 to 1.0 |
| Kitchen (Intermittent Exhaust) | 8 to 15 |
| Bathroom (Intermittent Exhaust) | 8 to 12 |
| Basement/Garages | 1.0 to 2.0 |
ACH and Energy
While higher ACH is better for air quality (e.g., controlling humidity, odors, or viral load), it is also more energy intensive. In cold climates, a high ACH means continuously heating large volumes of cold incoming air.