LouB wrote in :

Joseph Meehan wrote:
"MiamiCuse" wrote in message
...
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

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.