View Full Version : Inline bathroom exhaust fan sizing

09-17-2010, 06:00 PM
I bought an inline batrhoom exhaust fan to install in the attic to replace the builder fan in my bathroom. The new inline fan is sized for 6 inch duct pipes, but the existing exhaust pipes are only 4 inches.

Will this be a problem, or can I just get a pipe attachment to fit the 4 inch pipes to the 6 inch input on my new fan?

Replacing the duct pipes shouldn't be too hard if I need to. If I do, should I replace them with flexible metal tubes or rigid ones?

Here's the inline fan I bought. Thanks

09-17-2010, 06:19 PM
You might find that the intake whistles because of the increased airflow and the restrictions. My thought is you'd be better off keeping the diameter the same all the way to the exit outside. Best to ask the manufactuer. What does the installation manual say about it?

09-17-2010, 07:02 PM
Thanks for the response. The manual didn't say anything that I could find on this. So I'll probably just look at changing out the duct tubing. It's all exposed in the attic so I don't think it should be too tough.

09-18-2010, 07:02 AM
The new inline fan is sized for 6 inch duct pipes, but the existing exhaust pipes are only 4 inches.

You need the fan curve for your make and model.
Then you calculate the duct resistance from the calculators that are on the Web. The intersection of these two lines will give you your operating point, i.e., the pressure and the CFM.
Depending on the shape of the fan curve you can get some pretty weird results if you mix and match fans and duct sizes.

I think for bathrooms they recommend 8 ACH so for a 5'x7'x8' high bathroom this is 280 cu ft times 8 = 2240 CFH = 37 CFM.
Smaller pipes will appear to be longer lengths of larger pipes.
Using their friction loss calculator on this page I see that at 37 CFM the loss for 4" pipe is 10x the loss for 6" pipe, other things being equal.

The fan will be noisier if it pushes against more backpressure. Your max 1.4 sone is at 0.2" WC and 240 CFM.

The calcs are not that hard, they are just tedious and you have to use consistent units, either English or metric.

We're definitely having fun now; what size is your bathroom?