Calculating Cubic Feet
The first step in determining what size exhaust fan is needed, is to calculate the volume of the room. To do this, you'll simply multiply the length of the room times the width of the room time the height of the room.
- Length = 10 ft.
- Width = 8 ft.
- Height = 8 ft.
10 ft. * 8 ft. * 8 ft. = 640 ft.3
Calculate Equivalent Duct Length (EDL)
The next step is to measure the length of the duct run, and then apply some additional factors to determine the equivalent duct length of the run. For example, if we had this situation.
- Duct type = Insulated Flex.
- Duct length = 15'.
- Duct diameter = 4".
- Number of elbows = 2.
- Number of wall caps = 1.
We'll use these numbers, and the chart below to determine the EDL.
So the example above would look like this.
15' of 4" insulated flex duct x 1.5 = 22.5'
4" elbow adds 15' x 2 = 52.5'
1 4" roof cap adds 30' = 82.5'
Which means in our example, the EDL is 82.5'.
Determine Required CFM
Exhaust fans are sized using Cubic Feet Per Minute (CFM), so you'll have to use the two values calculated above to determine the size of the required fan. To do this, you'll use the chart below (you'll always round up).
Using our example numbers, we'll place our left finger on 640 cu. ft.. Then place our right finger on 90 ft. (because we have to round up). When we slide our left finger to the right, and our right finger down. we'll see that we need a fan rated for at least 150 cubic feet per minute.
Running The Fan Long Enough
Selecting an appropriately sized fan, is not the only thing to worry about. You also want to make sure that the fan runs for long enough, so that the moisture can be completely exhausted. It's often recommended to run the fan for between 20 - 30 minutes, after a shower. This is often made easier by installing an in wall timer to control the fan.
When selecting a timer, make sure it's rated for motor (inductive) loads and does not say "Incandescent only".
Everybody Wants It Quiet
In my opinion, getting the hot moist air out of the building is the highest concern. Though it would seem, some tend to like it quiet while the air is cleared.
You'll find that exhaust fan loudness is measured in Sone, where the lower the value the quieter the fan. If quiet is important to you, you'll want a fan at or below 1-2 sone. Here is a chart from Panasonic, that might help put sones into perspective.