How to Calculate Air Turnover Rate (ACH)
Calculating the Air Turnover Rate, commonly referred to as Air Changes per Hour (ACH), is critical for maintaining indoor air quality, ensuring proper ventilation in HVAC systems, and meeting safety standards in industrial or medical environments. This metric indicates how many times the entire volume of air in a room is replaced or filtered within one hour.
Air Turnover Rate Calculator
Understanding the Air Turnover Rate Formula
The calculation for Air Changes per Hour is straightforward but requires precise measurements. The core concept connects the volume of the space with the rate at which your ventilation equipment moves air.
ACH = (CFM × 60) / Room Volume
Where:
- CFM (Cubic Feet per Minute): This is the flow rate of air provided by your fan, HVAC system, or air purifier.
- 60: The number of minutes in an hour (converts minutes to hours).
- Room Volume: Calculated by multiplying Length × Width × Height (in feet).
Step-by-Step Calculation Example
Let's say you want to calculate the air turnover rate for a standard bedroom with a portable air purifier.
- Measure the Room: Suppose the room is 12 feet long, 10 feet wide, and has an 8-foot ceiling.
Volume = 12 × 10 × 8 = 960 cubic feet. - Determine Airflow: Your air purifier specification sheet says it delivers 200 CFM.
- Apply the Formula:
Multiplier = 200 CFM × 60 minutes = 12,000 cubic feet per hour.
ACH = 12,000 / 960 = 12.5 ACH.
In this example, the air in the room is completely replaced (or filtered) 12.5 times every hour, which is excellent ventilation.
Recommended ACH Values by Room Type
Different environments require different turnover rates to ensure safety and comfort. Below are general guidelines accepted in the HVAC industry (ASHRAE standards vary based on specific occupancy).
| Space Type | Target ACH Range | Ventilation Need |
|---|---|---|
| Residential Bedroom | 2 – 4 ACH | Low/Moderate |
| Kitchen / Bathroom | 5 – 15 ACH | Moderate/High (Odor control) |
| Office Space | 4 – 8 ACH | Moderate (CO2 reduction) |
| Classroom / Daycare | 3 – 8 ACH | Moderate |
| Laboratory / Hospital | 6 – 20+ ACH | Critical (Contaminant control) |
| Server Room | 20 – 50+ ACH | High (Heat removal) |
Why is Air Turnover Important?
Understanding how to calculate air turnover rate is vital for several reasons beyond simple comfort:
1. Health and Safety
Adequate air turnover dilutes airborne pathogens, viruses (such as COVID-19 or Influenza), and bacteria. In medical settings, high ACH is non-negotiable to prevent cross-contamination between patients.
2. Pollutant Removal
Indoor air accumulates volatile organic compounds (VOCs) from paint, furniture, and cleaning supplies. It also traps dust, allergens, and pet dander. A higher turnover rate ensures these contaminants are filtered out or exhausted outside more frequently.
3. Moisture Control
In humid environments or spaces like bathrooms and basements, stagnant air leads to mold growth. Ensuring a sufficient turnover rate helps regulate humidity levels and keeps building materials dry.
How to Improve Your Air Turnover Rate
If your calculation shows that your current ACH is too low for your needs, consider the following steps:
- Upgrade Fans: Install fans with higher CFM ratings. Remember that ductwork resistance (static pressure) can lower the actual CFM delivered compared to the rating on the box.
- Add Air Purifiers: Portable HEPA air purifiers add to the total CFM of the room. If your HVAC provides 100 CFM and you add a purifier providing 200 CFM, your total effective airflow is 300 CFM.
- Open Windows: Natural ventilation can significantly increase ACH, though it is dependent on weather conditions and wind speed.
- Clean Filters: Clogged HVAC filters restrict airflow, drastically reducing CFM and consequently your ACH. Regular maintenance ensures the system runs at design specifications.
Frequently Asked Questions
Does a higher ACH mean better air quality?
Generally, yes. A higher ACH means pollutants are removed faster. However, there is a point of diminishing returns where increasing airflow increases noise and energy costs without significantly increasing health benefits for a standard residential room.
How do I calculate CFM if I know my target ACH?
If you know you need 6 ACH for a 2,000 cubic foot room, you can reverse the formula: CFM = (ACH × Volume) / 60. In this case: (6 × 2,000) / 60 = 200 CFM required.
Can I use this for metric measurements?
Yes, the logic remains the same. If you use meters for dimensions, you calculate Volume in cubic meters. You must then ensure your airflow is in Cubic Meters per Hour (CMH) or convert it appropriately. Note that 1 CFM ≈ 1.7 CMH.