Bathroom fan duct

[BenRan207]

I have an inline bathroom fan that has a 4inch inlet. It pulls a lot of air but it's loud. Can I swap it to a 6inch inlet and reduce back to a 4 at the fan in my attic for noise reduction?

 

[Jadnashua]

You can, but it might decrease the air throughput. I've had really good results from Panasonic fans. Some of them will accept both 4" and 6" ducts. If I needed to do it again, I'd buy one with a built-in moisture sensor. Rather than a timer, one of these would turn itself on once the moisture got within certain limits, and shut itself off later. The one I put into my mother's house, unless you carefully listen in a quiet time of the day, you may not hear it running.

In my place, I have an in-line fan as part of a tubular skylight, and it, too, partly because the motor is remote, is very quiet. You do not have to live with a noisy bathroom fan...best to start off with a quiet, quality design than trying to patch what you have.

 

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[BenRan207]

Thanks for your help. The fan motor itself is virtually silent. It's the air whooshing through the 4" inlet grille that is making the noise. I figured by expanding the inlet it would decrease the speed of the air, not the overall volume input and therefore decrease the noise without a drop in performance. That's what I'm figuring anyways...not a pro so hoping that's correct. (Moisture sensors are a great technology but here in Jersey, the humidity is so oppressive the fan would constantly run so I went with a timer instead.)

 

[Jadnashua]

FWIW, the sensor used on the Panasonic units actually works with condensation rather than relative humidity. They're inter-related, but not the same thing. Plain high humidity wouldn't activate it. Once it's activated, it will run first until that condensation is removed, then a preset time afterwards. The unit I put in my mother's house had it in the wall switch. Were I to do it again, I'd get a fan with it built in rather than having the condensation reach the wall switch first. Still works, especially in that bathroom that tends to get guests more than 'regulars', so they might not think about turning the fan on or off.

The hassle is with using a timer is you have to remember to turn it on, and then, you might not select enough time to actually get rid of the moisture you need to if it's not run long enough, or waste conditioned air if it runs too long. Remember, the air you exhaust must be replaced, and it's pulling that in from outside through cracks unless you have an engineered heat recovery system.