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Bathroom exhaust fan sizing MiamiCuse 05-06-2008
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Posted by MiamiCuse on May 6, 2008, 1:26 am
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



Posted by buffalobill on May 6, 2008, 3:33 am
> 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

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.

Posted by virtuPIC on May 6, 2008, 5:32 am
> However when you use finish using your bath typically you have the fan on
> and the bathroom door open right? =A0So it's drawing the air in the bath P=
LUS
> the air from outside the bath. =A0Would it be a good rule of thumb to

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.

virtuPIC
--
Airspace V - international hangar flying!
http://www.airspace-v.com/ggadgets for tools & toys

Posted by Joseph Meehan on May 6, 2008, 6:32 am
> 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

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).


--
Joseph Meehan

Dia 's Muire duit




Posted by LouB on May 6, 2008, 8:01 am
Joseph Meehan 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
>
> 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).
>
>

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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