# Air Change Rate

## How to calculate air change rates - air change rate equations in imperial and SI units

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### Air Change Rate - Imperial Units

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

n = 60 q / V (1)

where

n = air changes per hour (1/h)

q = fresh 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 can be expressed in SI-units as:

n = 3600 q / V (2)

where

n = air changes per hour

q = fresh air flow through the room (m^{3}/s)

V = volume of the room (m^{3})

### Example - Air Change Rate SI Units

With an air flow of *3 m ^{3}/s* in a

*20000 m*room the air change rate can be calculated as

^{3}

n = 3600 (3 m^{3}/s) / (20000 m^{3})

= 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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