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Unread 03-10-2021, 07:28 AM   #35
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Join Date: Jul 2005
Location: Fairfax, Va
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The general rule of thumb for bathroom exhaust fans is 1 CFM for each square foot of floor.

Regardless of the fan's design the more CFM's it moves the louder it will be. So, though it won't hurt anything the 80 CFM for the toilet closet is too much, by a lot. 80 CFM's going out means that there also has to be 80 CFM coming in so you'd need to either leave the door ajar or the gap under the door needs to be large. If the available make up air isn't sufficient the fan will be even louder. Finally, though the fan may have a damper at it's outlet it won't likely seal completely when closed so it will leak some air. The one on the relatively expensive Panasonic fan I just installed sure doesn't.

The duct size for the existing fan is likely 4".

Static pressure of the duct work also comes into play; the length of the run, how many turns it takes, the type of duct (flexible, rigid smooth, etc) even the dampers all factor in to how much air can be shoved through it.

It sounds like the 2nd, new fan will will need to be at least 100 CFM. I forget what the the sizing charts say but I'm pretty sure you're not going to be jamming 180 CFM through a 4" duct, and even 150 CFM might be pushing it.

As far as the wiring goes I'd just make it simple; run the fan and light off the same GFCI drop.
If I recall correctly my memory is excellent, but my ability to access it is intermittent.
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