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Air Change Rate

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Air Change Rate - Imperial Units

Air change rate - air changes per hour - expressed in imperial Units

n = 60 q / V                                  (1)


n = air changes per hour (1/h)

q = fresh air (make up air) flow through the room (Cubic Feet per Minute,  cfm)

V = volume of the room (Cubic Feet)

Air Change Rate - SI Units

Air change rate expressed in SI-units

n = 3600 q / V                             (2)


n = air changes per hour

q = fresh air (make up air) flow through the room (m3/s)

V = volume of the room (m3)

Example - Air Change Rate SI Units

With an air flow of 3 m3/s in a 20000 m3 room the air change rate can be calculated as

n = 3600 (3 m3/s) / (20000 m3)

= 0.54 (h-1)

Typical Air Changes per Hour

Typical air changes per hour for well-insulated spaces:

  • no windows or exterior doors - 0.33 (1/h)
  • windows or exterior doors on one side - 0.67 (1/h)
  • windows or exterior doors on two sides - 1 (1/h)
  • window or exterior doors on three sides - 1.33 (1/h)
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Related Topics

Ventilation Systems

Design of systems for ventilation and air handling - air change rates, ducts and pressure drops, charts and diagrams and more.

Related Documents

Air Change Rates in typical Rooms and Buildings

Fresh air (make up air) rquirements - or recommended air change rates (ACH) for typical rooms and buildings like auditoriums, kitchens, churches and more.

Garage Ventilation

Exhaust ventilation from garages and workshops.

Outdoor Supply Air Rates

Recommended supply air rates of outdoor air - with or without smoking - in rooms like banks, assembly halls, hotels and many more.

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