Air Changes Per Hour


The process by which fresh air is introduced and contaminated air is removed from an occupied space is termed ventilation. Ventilation is necessary to provide a continuous supply of oxygen. Air changes per hour (ACH) is a measure of the air volume added to or removed from a space in one hour, divided by the volume of the space.

Cross Ventilation

Air Changes per Hour (ACH) is calculated through:

ACH = (cfm × 60)/Vol
cfm = (ACH × Vol)/60
Vol = (cfm × 60)/ACH

  • ACH = air changes per hour
  • cfm = quantity of airflow
  • 60 = constant, minutes per hour
  • Vol = conditioned space volume in cubic feet (length × width × height)

Example: The conditioned space (Building A) is 109’ long and 48’ wide with an 8’ high ceiling. Determine the cubic volume of the room which is length × width × height. I am going to do it in two calculations instead of one to talk about room area in square feet.

109’ × 48’ = 5232 sf
5232 sf × 8’ = 41,856 cf
ACH = (cfm × 60) ÷ Vol
ACH = (5232 × 60) ÷ 41,856
ACH = 7.5

Acceptable room air changes per hour for an office space are 5−10 per hour.

Less than 5 ACH: possible IAQ concern. More than 10 ACH: possible draft problems.

Example: Air changes per hour if the airflow design is 1 cfm per square foot with 8’ ceiling. A quick way to estimate ACH is to count ceiling tiles or measure area of the space. Most retail spaces are designed for 8’ ceilings and an average of 1 cfm per sf. Building A is 5232 sf. The design cfm is 5250. With 1 cfm/sf and an 8’ ceiling, the ACHs are 7.5. With 1 cfm/sf and a 9’ ceiling, the ACHs are 6.6, and with a 10’ ceiling, the ACHs are 6.

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Variable Air Volume System

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