Air changes per hour

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Air changes per hour, or air change rate, abbreviated ACH or ac/h, is a measure of the air volume added to or removed from a space (normally a room or house) divided by the volume of the space.[1] If the air in the space is either uniform or perfectly mixed, air changes per hour is a measure of how many times the air within a defined space is replaced.

In many air distribution arrangements, air is neither uniform or perfectly mixed. The actual percentage of an enclosure's air which is exchanged in a period depends on the airflow efficiency of the enclosure and the methods used to ventilate it. The actual amount of air changed in a well mixed ventilation scenario will be 63.2% after 1 hour and 1 ACH.[2] In order to achieve equilibrium pressure, the amount of air leaving the space and entering the space must be the same.

ACH equation in Imperial units

 \quad N = \frac{60Q}{Vol}


  • N = number of air changes per hour
  • Q = Volumetric flow rate of air in cubic feet per minute (cfm)
  • Vol = Space volume L × W × H, in cubic feet

Ventilation rates are often expressed as a volume rate per person (CFM per person, L/s per person). The conversion between air changes per hour and ventilation rate per person is as follows:

 \quad Rp = \frac{ACPH*D*h}{60}


  • Rp = ventilation rate per person (CFM per person, L/s per person)
  • ACPH = Air changes per hour
  • D = Occupant density (occupants per square foot, occupants per square meter)
  • h = Ceiling height (ft, meters)

Air change rate[edit]

Air change rates are often used as rules of thumb in ventilation design. However, they are seldom used as a the actual basis of design or calculation. For example, laboratory ventilation standards indicate recommended ranges for air change rates,[3] as a guideline for the actual design. Residential ventilation rates are calculated based on area of the residence and number of occupants.[1] Non-residential ventilation rates are based on floor area and number of occupants, or a calculated dilution of known contaminants.[4] Hospital design standards use air changes per hour,[5] although this has been criticized.[6]

Basement Parking 15–30
Commercial kitchens & Toilets 15–30
Smoking rooms 10–15
Laboratories 6–12[3]
Classrooms 3–4
Warehousing 1–2

The Passive House standard requires a 0.6 ACH.[7] Many if not most uses of ACH are actually referring to results of a standard blower door test in which 50 pascals of pressure are applied (ACH50), rather than the volume of air changed under normal conditions.


  1. ^ a b "ANSI/ASHRAE Standard 62.2-2013: Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings". Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers. 2013. 
  2. ^ Bearg, David W. (1993). Indoor Air Quality and HVAC Systems. CRC Press. p. 64. ISBN 0-87371-574-8. 
  3. ^ a b "Lab Ventilation ACH Rates Standards and Guidelines" (PDF). Retrieved 9 June 2014. 
  4. ^ "ANSI/ASHRAE Standard 62.1-2013: Ventilation for Acceptable Indoor Air Quality". Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers. 2013. 
  5. ^ "ANSI/ASHE/ASHRAE Standard 170: Ventilation for Healthcare Facilities". Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers. 2013. 
  6. ^ "Engineers' Perspectives on Hospital Ventilation". Retrieved 9 June 2014. 
  7. ^ "International Passive House Association - Guidelines". Retrieved 23 March 2013.