I need to provide an exhaust fan equivalent to ventilation by a 4 square feet of window. What size of exhaust fan do I need, how many air changes, CFM? My client wants to replace a bedroom window with a second door but per code a 4 sq feet of ventilation area is needed. Alternatively I can provide an exhaust fan. How do I pick the exhaust fan size?

2 Answers 2


First you need to define the volume of the room, that will give you how much air will need to be moved per unit time based on how many air changes per hour are required.

Then you can work out the amount of air to move per hour or minute etc based on how long the fan will be running. Will it run when the room is empty or only when occupied.

Once you have defined those conditions then you will be able to select a suitable fan.


The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) provides guidelines for air changes per hour, and they vary depending on the room. Bedrooms are 5-6 air changes per hour.

So you want to take the volume of the room, and that turns into either a fan size or a window size. It's also possible to get a door with a window built in.

Your climate zone matters also.

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