Bathroom exhaust fan sizing

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
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ultimately you will want a variable speed with the motor mounted in the attic for quiet. see fantech. otherwise look at noise specs, see panasonic.
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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 apartments.
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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).
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Joseph Meehan wrote:

Many bathroom doors are cut a little short on the bottom to let air into the room just because of exhaust fans.
Lou
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As far as I know, all interior doors should have a 1/2-3/4" gap on the bottom for air circulation if you have forced air HVAC.
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No need if you have an air return in that room.
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True, but the most places I've seen they don't exactly load all the BR's with returns.
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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
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wrote:

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.
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Phisherman wrote:

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.
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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.
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pipedown wrote:

Variable speed heat recovery central ventilator zoned with humidistats in each served room...
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MiamiCuse wrote:

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.
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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 same effect. Interestingly there are millions of homes suffering from mould due to this habit. Perry
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