It seems to me that when a bathroom fan is running with the door
closed, the makeup air comes primarily from the gap between the door
and the floor. Is there a formula or rule of thumb for the needed
area of this gap depending on the CFM of the fan?
ASHRAE recommends 200-300 FPM through the undercut area of doors to minimize
drafts and not cause a noticeable resistance when opening the closed door.
The pressure loss at these velocities are negligible.
I'm not sure what you are talking about.
I assume FPM is "feet per minute", and you are calculating this by
dividing the CFM of the fan by the square footage of the opening?
Right now I have a half bath with a 50 CFM fan, so I should have at
least 50/250 = 0.2 ft^2 of gap. With a 28" door, that's 1" of gap, so
my current 7/8" is marginal. In the full bath with a 80 CFM fan and a
28" door, I need to have 1.6" of gap at the bottom. Do these numbers
An FPM of 80 sounds crazy, since, for example, a 50 CFM fan would need
5/8 ft^2 of area, or a 3.2" gap beneath a 28" wide door.
You can stretch the velocity limit above 300 FPM.
For example at 400 FPM the velocity pressure is (400/4005)^2 =~ .01" water
The total loss (assuming a 0.5 entry coeff.) is .015 " water which will have
negligible affect on your fan capacity.
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