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rjordan392 05-26-2008 02:47 PM

Static Pressure: Rule of Thumb?

I want to boost the cfm output in a 4 inch duct by 50 to 100 cfm that comes from my bathroom blower fan with a booster fan. The existing run of the duct plus 4 elbows has cut back on the rated cfm of the blower which is rated at 270 cfm free air. Now depending on the static pressure value of 0.05 to 0.375, the cfm will be from 250 to 70 cfm.
So how does one calculate by rule of thumb, the static pressure near the end of the duct run which is probally where I will install the booster fan? If I can determine the approximate static pressure, then I will know the approximate cfm delivery at that point and be able to pick out the correct booster fan.

To complicate matters, the dischage flange of the blower housing is 3-1/4 by 10 inches and I used reducing adapters down to 4 inch round duct because the duct run has to go through a 4 inch opening in my concrete foundation. Now I can increase the size of the duct run for most of it but still need to reduce it to 4 inches to go through the foundation wall. Can this be solved?

ro9beam2002 06-05-2008 08:47 PM


comp 06-05-2008 08:52 PM


Originally Posted by ro9beam2002 (Post 128234)

i agree,,, haven't a clue

Chimichanga31 06-06-2008 04:54 AM

How big is your bathroom? 270 cfm is quite a bit of air.

try this site. they have some sizing program called compute-a-fan.

rjordan392 06-06-2008 12:55 PM

I solved my problem by installing a light/fan/timer switch. The timer can be set for up to 60 minutes and has a preset adjustment in the switch. The bathroom is 9 x 8 x 7 foot. It takes a while to vent it properly due to the resistance to air flow in the ducts. I set the timer to run an additional 10 minutes after each use of the bathroom.

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