Makeup Air for Bath Fan and Door Gap

• posted on June 7, 2004, 3:55 am
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?
Thanks, Wayne

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• posted on June 7, 2004, 2:50 pm

Around 250 FPM max

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• posted on June 7, 2004, 3:10 pm

250???? Christ mine are 70-80. An inch would probably be enough.. WAG

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• posted on June 7, 2004, 5:44 pm

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.

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• posted on June 7, 2004, 8:44 pm

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 seem right?
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.
Cheers, Wayne

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• posted on June 8, 2004, 12:35 am

Yes, FPM=ft/min
You can stretch the velocity limit above 300 FPM. For example at 400 FPM the velocity pressure is (400/4005)^2 =~ .01" water gauge. The total loss (assuming a 0.5 entry coeff.) is .015 " water which will have negligible affect on your fan capacity.