Airflow guide
Air changes per hour explained
ACH expresses an identified airflow as a multiple of an enclosed room’s volume each hour. It is useful for a room-volume comparison, but it does not identify the air source or prove performance.
What ACH means
ACH is hourly volume flow divided by an enclosed room volume. It offers a compact way to compare nominal air movement between rooms of different sizes. Label the flow before calculating: supply, return, outdoor air, exhaust, and recirculated air have different roles.
Formula
For metric inputs, m³/h divided by room volume in m³ gives ACH directly. Use one consistent unit system rather than mixing CFM with cubic metres.
Worked example
A 2,400 ft³ room supplied with 240 CFM has 240 × 60 ÷ 2,400 = 6 ACH. With the same 240 CFM in a 4,800 ft³ room, ACH is 3. The fan flow did not change; only the reference volume did.
What ACH can and cannot tell you
| ACH can indicate | ACH cannot establish by itself |
|---|---|
| Nominal airflow relative to a stated room volume. | Whether the air is outdoor, recirculated, filtered, supply, return, or exhaust. |
| A way to compare an identified flow across room sizes. | Uniform mixing, contaminant removal, or the effectiveness of air treatment. |
| A conversion input for early airflow planning. | Automatic compliance with ventilation, health, or code requirements. |
Use ACH in the right workflow
- Identify the room boundary and calculate or measure its volume.
- Identify the air stream and why it is being evaluated.
- Use the formula to convert a known CFM to ACH, or a stated ACH basis to CFM.
- For ventilation requirements, use the applicable standard, code, and project design basis.
- For heating and cooling, verify the load, supply-air conditions, distribution, ducts, and fan capability separately.
FAQ
Is one ACH a complete air replacement?
No. ACH is a nominal flow-to-volume relationship. Mixing, short-circuiting, distribution, leakage, and outdoor-air fraction can be different.
Can ACH be used for duct sizing?
It can yield a preliminary CFM after a room volume and explicit ACH basis are known. Duct design still requires actual area, velocity, fittings, pressure loss, and fan checks.
Can ACH size HVAC equipment?
No. Capacity should follow a documented heating or cooling load and design conditions. ACH does not capture those inputs.
Related tools and resources
- ACH Calculator and CFM Calculator
- CFM vs ACH and HVAC Airflow Reference Table
- Duct Velocity Calculator — review geometry only after the airflow is defined.