# Panasonic lower cfm is stronger than higher cfm??

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I have a bathroom fan that is 70-90cfm Panasonic whisper boost which is on 90cfm, dimensions of the opening is around 9” across the cover

I have another bathroom fan panasonic whisper remodel 110cfm which is around 12” accross the cover

why does the lower cfm suck much more air? I put toilet paper 3 pieces and it sucks it up and holds it while the 110cfm holds 1 but does not try to grab the other 2 unless i put it against it??

the 110cfm bathroom duct is around 5 feet rigid pipe and the 90cfm is 2-3 feet flexible duct

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### Replies

1. | | #1

You're looking at linear air flow velocity, not volumetric. Try dividing the claimed air flow by the surface area of the grill (minus the toilet paper). Which one is higher?

If you want a rough relative comparison of volume flow, get a large trash bag and tape it to a big frame like a cardboard box without bottom and top. Fill the bag with air, press it against the ceiling over the fan, and time how long it takes to deflate.

2. | | #2

I am not sure that your test is a valid way to compare fan performance.

3.14 x (6x6) = 113 square inches 3.14 x (4.5x4.5) = 64 square inches.

The larger fan moves 20% more air but has 175% more grill to move the air thru so the velocity will be lower while moving more air.

Walt

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