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
Be careful if there is also a connection to a chimney in the rooms
from which it draws air! It works against the pressure pushing smoke
up and sucks CO2, CO, dust, and whatever is produced by your heating
into your living room.
I recommend to ask a chimney sweep. In my apartment there is a small
fan blowing air into the chimney. Our chimney sweep told us not to
enlarge it since it might blow smoke and poisonous gas into the other
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No additional factoring needed. They are measuring room air changes per
time. The make up air will come from somewhere like the heat vent, open
door or window etc.
Basically physics says you can't draw air out of the room with out air
coming into the room at the same time, even with the door closed air has to
come in from somewhere (like around the door or cold air return).
Most manufacturers assume that you'll be drawing air from room or a
window anyaway, so its built in to the calculations.
It also depends how you duct it, and how far away the fan has to push
the air to the outside. Rigid duct is a little better than flex duct,
but don't go out of your way if the run is short.
For a 10X10 room, 100CFM is fine, but like others said noise is a
factor, so look into quiet fans like Panasonic
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.
Good thinking. If I were you, I'd also consider the humidity of the existing
air modified by the presumably less-humid replacement air. Then, too,
there's the temperature of the air to consider. Of course this gets
complicated inasmuch as the relative humidity of the bathroom's air goes
down quite dramatically (usually). That's for a bath.
On the other hand, if trying to remove offensive odors, there's a table
somewhere of bean consumption vs time vs CFM necessary to purge the space
down to the undetectable levels.
Plus, individual levels of acceptable humidity, odors, noise, and cost have
to be factored in.
The idea of having a bathroom fan is to exhaust the water vapour
outside into the garden where it will do no harm. While the fan is
running it pulls air into the room through cracks round the door and
holes in the walls and ceiling. Helping to ventilate your home.
When you leave the bathroom door open you negate the whole idea.
Most of that water vapour promptly makes its way into your home,
soaking into your bed and other soft furnishings, thats why your bed
feels cold. Where it can, it goes inside your walls and roof, leading
to wood rot, and starts green and black mould that will lead to
health problems. It makes a visible presence during the winter
condensing on your windows and any other cold surface.
Best to keep that door closed for at least 20 minutes, better still
all the time, as damp towels etc drying on the radiators produce the
Interestingly there are millions of homes suffering from mould due to
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