"Pete C." wrote in message
...

Phisherman wrote:

On Tue, 6 May 2008 01:26:14 -0400, "MiamiCuse"
wrote:

I am trying to size my bathroom exhaust fan. Most sizing chart says to
use
the SQFT size and multiply by 1.1 to get the CFM if ceiling is standard
8'.
So for a 10x10 bath = 100 SF it would be 110 CFM.

However when you use finish using your bath typically you have the fan
on
and the bathroom door open right? So it's drawing the air in the bath
PLUS
the air from outside the bath. Would it be a good rule of thumb to
overshoot the 1.1 multiplier? or that already have a safety factor
built-in?

Thanks,

MC


There's nothing wrong with a more powerful fan, other than the noise.
The faster you can dry your shower, the less chance of mold/mildew.

Along the same lines, and avoiding the noise issue, you can install a
larger central ventilator fan that installs in the attic and has flex
ducts running from it to multiple bathrooms, kitchen, laundry, etc.
Since the fan is remote from the room there is less noise.


I'm wanting to do this too but I am concerned about ejecting too much of the
air I payed to heat or cool. A simple central fan will draw lots of air
from several ducts (depending on how many baths you have) and that could
eject several hundred CFM of heated air which you will just need to heat
again. Might as well leave a window open as far as the heater is concerned.

Sizing a fan close to what you need and having each fan controlled for a
single bath is more energy efficient depending on your climate and time of
year. Use a timer switch to operate the fan for convenience. Variable
speed would be nice as would automatic baffels to select which rooms are
ventilated. A well designed central ventilator can be efficient, you just
need to buy all the accessories.