I have a Broan model 676, 110 CFM Ceiling Mount Fan in my 8'x8' (500 cubic feet) bathroom with ~15' of duct and a couple elbows - and have no mirror fogging. Builder told me I was over sizing it and I probably did. No idea if it is the CFM or brand/model, just happy it works so well.

Interesting non-profit organization (http://www.efi.org/about) that shows how to calculate bathroom fan size: http://www.efi.org/factoids/bathroom_sizing.html

What kind of duct did you use? Are there any elbows? How short is "very short"? Does it actually exhaust to the outside?

And where does it exhaust? Roof, gable wall, in the soffit ?

Good morning aramaram! I specify a 100+ CFM Panasonic Whisper Quiet fan for practically every bathroom project that I undertake, unless the room is especially large (in which case I increase the CFM).I have found that this ventilates the bathroom faster & more effectively. So my answer to your question would be yes. Go for it!

The best of luck to you!

~LM Masiello LM Designs

You can try leaving the door open just a crack, maybe it's having trouble keeping up with the steam over such a short time period because it isn't able to draw in enough air? (Not too few CFM, but no air source?)

When do you start the fan? Is the unit exhausted to the outside?

Thank you very much for the replies. The unit does exhaust to the outside with a straight horizontal run of about 2' with a 4" duct. I usually start the fan a minute or so before the shower. Leaving the door open helps but not entirely.

You need more CFMs !

Got it! Should 110 CFM be sufficient? I need a low profile fan because the ceiling is made of 2 x 6 joists and 110 is the highest that I've found (from Panasonic). Does anyone know of other brands that I should consider with high outputs? Thanks again.

Broan, Fantech, Panasonic were the only brands I considered.

LOL, I also considered 150 cfm. Apparently, too much cfm can suck heat or AC out of the room.

If you have attic space above the bathroom, look at Fantech. I've never seen steam in my bathroom since installing one. and it's SO quiet because the motor is in the attic, not the room.

No real downside to increasing the size of the fan other than perhaps making it a bit chilly when you get out.

That said, and I'm no expert, I suspect a fan's performance is highly dependant on the supply air.

The most ready and easy supply of incoming air is probably a bathroom window.. Have you tried cracking the window?

-john

Unfortunately, there is no attic (it's a flat roof) and opening the window has limited benefit because, given the layout, the fan also ends up sucking the air from the window instead of the shower. The house is in Northern California with mild climates so I would prefer to put in a fan that is overkill (not worried about the chill factor) but the most powerful one I can find to fit 2 x 6 joists is 110 CFM. Thanks again for all your help.

fantech also makes a unit that places the motor under the eaves I believe. The interior fan is very unobtrusive but very powerful and quiet.