Basics of Air Change Rates
ASHRAE Journal, Vol. 67, no. 12, December 2025
Author:
Kishor Khankari
Abstract:
Air changes per hour (ach) are commonly used in the HVAC community as a metric for specifying ventilation
airflow rates. However, a lack of proper understanding regarding the use of ach can lead to unnecessary
penalties in the initial and operating costs of building ventilation systems. Moreover, specifying the same ach
irrespective of the generation rates can make smaller spaces more susceptible to higher levels of contaminant
exposure.
There are two related aspects of ach: a) the level of dilution and b) the rate of decay or purge. In many
ventilation applications, when the primary goal is to dilute contaminants, the volume of the space does not
affect the desired level of dilution. Instead, the extent of dilution is determined solely by the rate of
contaminant generation and the rate of ventilation airflow.
Conversely, under certain special circumstances where there are incidental or accidental surges in contaminant
generation, the rate of recovery becomes the primary goal of ventilation. In these cases, the space volume and
therefore ach plays a crucial role in recovery.
Indoor airflow patterns have a more significant impact on ventilation effectiveness than ach or supply airflow
rates. Higher ach does not guarantee perfectly mixed spaces or uniform levels of contaminant concentration.
An optimized layout, along with the appropriate number of supply and return locations, can achieve better
ventilation effectiveness even at lower airflow rates, potentially saving on both initial and operating costs. As
recommended in various standards, merely adjusting the airflow rate to improve ventilation effectiveness
does not address the root causes of ineffective ventilation. Similarly, the reciprocal of ach serves only as a
theoretical indicator of the average age of air and does not accurately reflect the actual air exchange rate.
When ach is used simply for convenience – perhaps to facilitate a better perception of ventilation airflow rates
– it should be distinguished from when ach is used as a design metric. This distinction will help clarify various
misconceptions around ach and ensures that space volume is not mistakenly used in estimating ventilation
airflow rates.
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About the author
Dr. Kishor Khankari
ASHRAE Fellow, ASHRAE Distinguished Lecturer
Dr Kishor Khankari is the founder of AnSight LLC. As a specialist in Computational Fluid Dynamics (CFD), his passion for solving engineering problems and providing sound scientific solutions has led to innovations and optimized designs in the industry.
A noted expert in his field, he has a Ph.D. from the University of Minnesota and has published in several technical journals and trade magazines. As a well sought-after speaker Dr. Khankari makes regular presentations in various technical conferences and professional meetings worldwide.
