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.

Read More
Variable Air Volume System

How useful was this post?

Click on a star to rate it!

Average rating / 5. Vote count:

No votes so far! Be the first to rate this post.

We are sorry that this post was not useful for you!

Let us improve this post!

Tell us how we can improve this post?