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Air Change Rate

Calculate air change rates - equations in imperial and SI units.

air change rate

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)

Related Topics

  • Ventilation Systems

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

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