On May 6, 6:26 am, "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

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