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 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

Measure of Airtightness[edit]

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. The Passive House standard requires airtightness so that there will be less than 0.6 ACH with a pressure difference between inside and outside of 50 PA.[7]

Effects of ACH due to forced ventilation in a dwelling[edit]

Forced ventilation to increase ACH becomes a necessity to maintain acceptable air quality as occupants become reluctant to open windows due to behavioural changes such as keeping windows closed for security.[8]

Air changes are often cited as a means of preventing condensation in houses with forced ventilation systems often rated from 3 - 5 ACH though without referencing the size of the house. However, where ACH is already greater than 0.75 a forced ventilation system is unlikely to be of use at controlling condensation and instead insulation or heating are better remedies.[8] Seven out of eight houses studied in NZ in 2010 had an ACH (corrected for ventilation factors) of 0.75 or greater.[8] The presence of forced ventilation systems has been shown in some cases to actually increase the humidity rather than lower it.[8] By displacing air inside a dwelling with infiltrated air (air brought in from outside the dwelling), positive pressure ventilation systems can increase heating (in winter) or cooling (in summer) requirements in a house.[8][9] For example, to maintain a 15 °C temperature in a certain dwelling about 3.0 kW of heating are required at 0 ACH (no heat loss due to warmed air leaving the dwelling, instead heat is lost due to conduction or radiation), 3.8 kW at 1 ACH and 4.5 kW are required at 2 ACH.[8] The use of roof space for heating or cooling was seen as ineffectual with the maximum heating benefits occurring in winter in more southerly regions (being close to the South Pole in these southern hemisphere reports) but being equivalent only to about 0.5 kW or the heating provided by about five 100 W incandescent light bulbs; cooling effects in summer were similarly small and were more pronounced for more northerly homes (being closer to the equator); in all cases the values assumed that the ventilation system automatically disengaged when the infiltrating air was warmer or cooler (as appropriate) than the air already in the dwelling as it would otherwise exacerbate the undesirable conditions in the house.[9] Ultimately forced


  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. 
  8. ^ a b c d e f Pollard, AR and McNeil, S, Forced Air Ventilation Systems, June 2010, Report IEQ7570/3 for Beacon Pathway Limited
  9. ^ a b Warren Fitzgerald, Dr Inga Smith and Muthasim Fahmy, Heating and cooling potential of roof space air: implications for ventilation systems, May 2011, Prepared for the Energy Efficiency and Conservation Authority (EECA)