Air Change Rate (ACPH)
Air change rate (ACPH) is the number of times a space's entire air volume is replaced in one hour — the standard way ventilation requirements are specified for basements, kitchens, battery rooms, plant rooms and cleanrooms.
Why it matters on an MEPF project
ACPH converts a fuzzy requirement ("ventilate this room properly") into a computable duty: room volume × ACPH = airflow, and airflow sizes the fans, ducts and louvres. Different spaces carry very different targets — car parks and kitchens run high double digits of air changes; comfort spaces need far less; cleanrooms are specified in ACPH bands by ISO class.
The failure mode is designing ACPH once and losing it silently: filters load, ducts leak, and the measured airflow drifts below the number the safety case assumed — which matters most exactly where ACPH is a safety barrier (basement CO, battery-room hydrogen, kitchen grease vapour). Measured re-verification belongs on the AMC calendar.
How it's specified in practice
| Parameter | Typical / working position | Governing reference |
|---|---|---|
| Basement car parks | High air-change duty with CO-based control common | NBC 2016 Part 8 practice |
| Commercial kitchens | High extraction with capture hoods | kitchen ventilation practice |
| Battery charging rooms | Calculated from hydrogen evolution, not a generic ACPH | IEC 62485 method |
| Cleanrooms | ACPH band by ISO class | ISO 14644 framework |
Common mistakes
- Quoting an ACPH with no room volume — the fan cannot be sized from a slogan.
- Designing supply with no relief/extract path, so the room simply pressurises.
- Never re-measuring after commissioning; loaded filters eat the number.
- Using a comfort ACPH where a hazard calculation (H₂, CO, solvents) governs.
Related on this site
Frequently asked
How do I convert ACPH to fan CMH?
Room volume in m³ × ACPH = airflow in m³/h (CMH). Our ventilation calculator does this with the usual duty presets per space type.
Is more ACPH always safer?
It is always more fan energy; safety follows the governing calculation. Hazard rooms need the computed rate maintained and proven — beyond that, extra air is cost, noise and (in conditioned spaces) load.
Standards referenced: NBC 2016 Part 8 · ISO 14644 (cleanrooms) · IEC 62485 (battery rooms)