EKS-HUM-019 / Moisture load and airflow
Air Change Rate
换气次数
Air change rate is a defined volumetric airflow divided by the volume of a space, commonly reported as air changes per hour.
- Abbreviations
- ACH
- Aliases
- air changes per hour
QUANTITY, SYMBOL AND UNIT
Quantity / objectAir change rate
SymbolN, ACH
SIh⁻¹reciprocal hour
CommonACHair changes per hour
Definition
Air change rate is a defined volumetric airflow divided by the volume of a space, commonly reported as air changes per hour.
The airflow may refer to outdoor air, supply air, infiltration or another stated stream. Because those meanings produce different numbers, every ACH value must identify both the airflow basis and the volume boundary.
Formula and variables
ACH = 3600 Q / V Conditions: Q is in m³/s, V is in m³ and the same air-stream definition applies throughout. Use ACH = Q/V when Q is already in m³/h.
| Symbol | Meaning | Unit |
|---|---|---|
ACH | air changes per hour | h⁻¹ |
Q | defined volumetric airflow rate | m³/s |
V | defined zone or enclosure volume | m³ |
Relations and distinctions
Outdoor-air ACH and supply-air ACH
Outdoor-air ACH excludes recirculated air; supply-air ACH can include it. They answer different questions.
ACH and air distribution
ACH is a nominal turnover rate. It does not show short-circuiting, dead zones or local air age.
Engineering meaning
- Label the air stream and space volume whenever ACH is used in a moisture or ventilation calculation.
- For moisture load, convert the relevant outdoor or infiltration airflow to mass flow and apply the humidity-ratio difference.
Common misconceptions
MisconceptionA higher ACH always means faster moisture removal.
Accurate explanationThe result depends on which air stream is counted and on the moisture state of that air; humid outdoor exchange can add load.
Application boundaries
- ACH alone does not describe contaminant removal effectiveness or local air distribution.
- ACH alone cannot determine moisture load, dehumidifier capacity or room humidity response.
Supporting sources
-
ASHRAE / SRC-ASHRAE-F17-CH16
ASHRAE Handbook - Fundamentals, Chapter 16: Ventilation and Infiltration ↗
2017 SI edition / Chapter 16, Air Change Rate section, nominal and space air change discussion
Supports: Air change rate has units of reciprocal time and is commonly reported as ACH; Air change rate is airflow divided by a defined volume; Outdoor, supply and recirculated airflow definitions must not be mixed
Maintenance record
Air Change Rate
- Maintained by
- Yakeclimate Engineering Team
- Authoritative sources verified
- August 3, 2026
- Terminology checked
- August 3, 2026
- Last updated
- August 3, 2026