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.


What is the purpose of ventilation in an occupied space?
Ventilation is necessary to provide a continuous supply of oxygen to occupants and remove contaminated air from the space. This process helps maintain indoor air quality, reduces the risk of airborne diseases, and improves overall comfort and well-being.
How does air changes per hour (ACH) relate to ventilation?

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. It’s a key parameter in ventilation design, as it determines the frequency of air replacement and the effectiveness of removing contaminants and introducing fresh air.

What are the factors that influence the required ACH in a building?

The required ACH in a building depends on various factors, including occupancy density, activity level, and type of occupancy (e.g., residential, commercial, industrial). Other factors, such as outdoor air quality, indoor pollutant sources, and desired indoor air quality, also impact the required ACH. ASHRAE standards and local building codes provide guidelines for determining the minimum ACH requirements for different types of buildings.

How is ACH calculated, and what are the units of measurement?

Air changes per hour (ACH) is calculated using the formula: ACH = (cfm / 60) / Vol, where cfm is the quantity of airflow in cubic feet per minute, and Vol is the volume of the conditioned space in cubic feet. The unit of measurement for ACH is air changes per hour, which represents the number of times the air in the space is replaced in one hour.

What are the implications of inadequate ACH on indoor air quality and occupant health?

Inadequate ACH can lead to poor indoor air quality, which can cause discomfort, fatigue, and even health problems for occupants. In extreme cases, inadequate ventilation can result in the buildup of airborne pollutants, such as carbon dioxide, volatile organic compounds (VOCs), and particulate matter, which can exacerbate respiratory issues and other health conditions.

How can ACH be measured and monitored in a building?

Air changes per hour (ACH) can be measured using various methods, including flow hood measurements, capture hood measurements, and tracer gas testing. Building management systems (BMS) and building automation systems (BAS) can also be used to monitor ACH and adjust ventilation rates in real-time to optimize indoor air quality and energy efficiency.