How to calculate ventilation rates

Written by mercedes martinez
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How to calculate ventilation rates
Blower doors have a similar pressure gauge: the tighter the building, the less pressure is needed by the blower door fan. (pressure meter image by Thor Jorgen Udvang from

Proper ventilation is required to eliminate toxins in the indoor air from off-gassing furniture and gas appliances. Air movement is also important to decrease the feeling of a stuffy room. On the other hand, if your home has too much ventilation, it will be draughty and energy inefficient. According to the Environmental Protection Agency (EPA), most ventilation ducting supplies homes with a combination of outside air and recirculated air. The rate of air circulation is expressed as cfm, or cubic feet per minute. Use calculations to find the minimum amount of ventilation to keep your home healthy, comfortable and energy efficient.

Skill level:

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Things you need

  • Blower door test
  • Paper
  • Pencil
  • Calculator (optional)

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  1. 1

    Identify parts of the calculation. r= 60a/V

    "r" equals the air change rate per hour. "a" equals the supplied air flow into the room (cfm). "V" equals the rooms volume (cubic feet or cf).

  2. 2

    Find blower door testing equipment to get accurate numbers (in cfm). Seal an exterior door using the blower door and turn the fan on. The meter will give you the air flow ("a").

  3. 3

    Replace your numbers for the air flow "a". Enter in the volume of the room for "V".

  4. 4

    Multiply your air flow by 60, then divide by the room's volume to find the air change rate per hour ("r").

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