Sizing Ventilating Fans for Optimal Results
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Sizing Ventilating Fans for Optimal Results


Lack of ventilation in your home leads to poor indoor air quality, increased levels of moisture, and elevated amounts of particulates and contaminants in the air. Installing and properly sizing ventilating fans helps alleviate these problems throughout your home.

Ventilation fans must be large enough to completely replace the volume of air in the area they’re ventilating within a specific time period, usually an hour. This is called ACH, or air changes per hour. Kitchens, for example, usually require 15 air changes per hour. Bathrooms need 8 ACH, while other rooms usually need only 6 ACH.

Ventilation fans are rated based on the number of cubic feet per minute (CFM) of air they can ventilate, so the size of the fan you need will be based on that number.

  1. Calculate the cubic feet of the room where the fan will be installed: Cubic feet is calculated by multiplying the room’s length, width, and height. For example, a room that is 8 feet long by 10 feet wide by 8 feet tall will have an air volume of 640 cubic feet. To achieve a rate of 1 ACH, the ventilation fan must be able to ventilate this 640 feet of air each hour.
  2. Determine the necessary ACH of the room: A standard room will need 6 ACH, so multiply 640 cubic feet by 6. To be effective, the ventilation fan will need to move 3,840 cubic feet of air per hour.
  3. Convert to CFM to determine fan size: Divide 3,840 by 60 to determine how many cubic feet per minute the fan must be able to ventilate, which in this example is 64. The ventilation fan you install in this room must be rated at 64 CFM to be effective.

Certain characteristics of rooms may require adjustments to these sizing calculations. Ask your HVAC contractor for advice.

Roth Heating & Cooling has provided reliable professional heating, cooling, plumbing, and electrical services in Portland and the surrounding Oregon communities for more than 36 years. Contact us today for expert help sizing ventilating fans to give you the best level of indoor ventilation and comfort.

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