Hi Roc, The motion sensor might not be adequate. It is suggested that the fan be run an additional 30 minutes following a shower. So a timer is great for this. I hooked my 2 ceiling vents to one motor in the attic. The sizing was provided by Fantech, the brand I used. There is some loss in efficiency in length of duct. My ducts are rigid then insulated to try and cut down on noise. By connecting my 2 ducts, I could use the existing roof vent, instead of 2 roof openings. Of course I think one larger one might have worked, I wanted one over the toilet and laundry area, then one in the shower.

White or background noise I am sure you mean enduring, lol............

Thanks Millwork for pointing this out! I just added the comma to my post.

LMAO........................

Millwork, we aren't much help, are we :)

too much air would be removed

Is that a thing?

My existing is Panasonic. It's very quiet and effective. Timer is a good idea.

I'm sorry for going off-topic, but in response to enduring's punctuation comments, I have to post this:

"Let's eat Grandma".

"Let's eat, Grandma".

I laugh every time I see it.

Rococogurl, figure out the cubic feet in your planned space then you get a rough estimate. But length of ducting will impact fan size too. Also there needs to be enough incoming air from outside the room to actually let the fan do its job. I have 3/4" gap under my 2 entry doors to the bathroom and that takes care of my space just fine. I had considered a louvered panel in one of my pocket doors to the room but went with regular door.

SJhockey, lol, very cute. Good to hear from you :)

By my calculations I have 576 cubic feet (6x12x8). Again, question is whether a second fan is needed or a single 110 cfm fan, not near the shower entry, is sufficient.

Duct run would be 4 feet max, straight up. It doesn't seem practical to tie it into the other fan which would be 5 feet away.

There would be plenty of incoming air as the doorway to the room is expanding to 30". No issues now.

I wouldn't pick on enduring's punctuation considering the general state of grammar around here. Bigger fish to fry.

Its ok, its wonderful that these folks pick on me, it was pretty funny. But back to the question. I went to Fantech and found this bit of information, and posted at the end. So fantech looks at sq foot, not cubic feet. I didn't remember. My bathroom is 80sf, (8x10'). I have a vent near the toilet/washer stack, and one in the shower. I have a 170cmf fan that is sized to accommodate the 2 (not long) runs of ducting, Y connection, and short run out the roof. I chose to have a vent in my shower because in my old smaller bathroom, there was mold always growing on the ceiling in the shower. The room was sized for the fan, but for what ever reason it did not vent properly. I don't think there was enough makeup air with the doors shut. I wasn't going to have mold happen in my new beautiful bathroom. Now, still 2 doors, but with ample clearance at the bottom, my bathroom never steams up. I don't really feel any draft from the ceiling vent in the shower either. I have french shower doors on the front that go the span of the 5' alcove shower pan (kohler cast iron). There is a 1' gap between the top of the glass and the ceiling, which is 8'.

Anyway, here is the Fantech website comment I found.

"A small tip to help you size a fan for your bath:

IF your bathroom is 100 sq.ft. or smaller. According
to the guidelines of HVI (Home Ventilating Institute) baths that are
100 sq.ft. or smaller require 1 CFM per square foot of bathroom or a
minimum 50 CFM of fresh airflow. To find the square footage simply
multiply the length of the bath by the width. For proper airflow in
typical installations (when duct losses aren’t calculated), use the 0.4”
Ps rating point for correct fan selection.

IF your bathroom is over 100 sq.ft. Ventilation for
baths over 100 sq.ft. is calculated based on the fixtures in the bath.
Allow 50 CFM for each standard toilet, shower or tub. Whirlpool and
jetted tubs are calculated at 100 CFM. Add up the CFMs for all fixtures
and you will have calculated the total CFMs required. We recommend that
steam showers use a separate fan on a timer so that the fan may be shut
off during shower use and then run afterward to dry the shower stall."

It sounds like your existing fan would be OK. And you could always go larger if you find it isn't, as long as the ducting didn't have to change diameter it would be easy to do.

I'm reading this while in the gym on the stationary bike and everyone keeps looking at me wondering why I'm laughing.

Do. Not. Get. Me. Started. Consider yourself warned @jackson2348 LOL!

rococogurl-

We. Will. Not. Be. Intimidated...

A panda walks into a cafe. He orders a sandwich, eats it, then draws a gun and fires two shots in the air. "Why," asks the confused waiter, as the panda walks toward the exit. The panda produces a badly punctuated wildlife manual and tosses it to the waiter. "I'm a panda...look it up." The waiter turns to the relevant entry:

Panda: Large black-and-white bear-like animal, native to China. Eats, shoots and leaves.

LOL kudzu. However, do you not get bored of achieving punctuation fail notifications? OMG this-is-the-internet and we don't want to be corrected even when we're wrong? Are you old? If so, that's the reason. And, like, I went ok, that's the game but really he went yeah, and like I'm there.

Warned you. It's waaay worse than punctuation.

Isn't there a comma after "shoots"?

According to the guidelines @enduring posted, at 72 square feet I can get by with the single fan even though it isn't near the shower, correct?

Still would like to see what Sophie Wheeler thinks.

Eats, Shoots & Leaves: The Zero Tolerance Approach to Punctuation

One of my colleagues gave me a copy of that when I left IBM Research. Not sure what that says about me ...