# Bathroom Exhaust Fan Duct Sizing



## Lyndon (Jun 2, 2010)

I am replacing an old noisy bathroom exhaust fan that currently has a 4" roof jack/cap and 4" ductwork. According to the proper calculations, I would need a new exhaust fan rated at 190 CFM. However, I can't find any 4" exhaust fans with CFM ratings this high. The roof is 7/12 pitch and I have no intention of climbing on it to change out the 4" roof jack with a 6" version - or hiring it done.

The duct work requires 3 turns and is 17' long. My goal is to replace the old fan with one that will do the best job possible using the 4" roof jack and that is less than 2 sones.

I just read on Nutone's website that you can use larger duct work than the duct outlet of an exhaust fan - it will actually improve performance by reducing static pressure.

My question is: (1) Should I buy the largest 4" CFM exhaust fan I can find and convert it at the fan outlet to 6" and then back to 4" at the roof jack. Or, (2) Should I buy a 6" exhaust fan rated at 190 CFM and convert it to 4" at the roof jack? Would this meet code? Which would work better - move more air and remain quiter?


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## Plumber101 (Feb 25, 2009)

Internation residential code states for bathrooms that the exhaust fan needs to be 50 cfm for intermittent and 20 cfm for a continuous fan.

I'm courious how you came up with the cfm you need?

4 " duct is only rated about 80 cfm 
6 " duct is rated about 110 cfm

Requardless of what you do if you restrict it to the 4" roof cap (Approx 80 cfm) then you are only wasting money unless you run it all in 6" to include the termination cap.


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## joecaption (Nov 30, 2011)

Your also loosing a lot of CFM's with the long run and all those turns. Not a great lay out.


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## joecaption (Nov 30, 2011)

http://www.panasonic.com/business/b...?cm_mmc=MC-_-Panasonic Vent Fans-_-msn-_-1348
May find what you looking for here.


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## Lyndon (Jun 2, 2010)

Plumber101 said:


> Internation residential code states for bathrooms that the exhaust fan needs to be 50 cfm for intermittent and 20 cfm for a continuous fan.
> 
> I'm courious how you came up with the cfm you need?
> 
> ...


Per Panasonic's website:

17' solid ductwork
45' = 3 elbows/turns
30' = 4" Roof cap

For a total of 92' Equivalent Duct Length (EDL)

Room is 11' 4" x 11' 7" x 9' ceilings = 1182 cubic feet

Using Panasonic's scale I would need an exhaust fan rated at 190 CFM.

Calculations using other means: 1190 cubic feet / 7.5 = 160 CFM

or 1190 x .13 = 160 CFM

or 1190 / 60 minutes per hour x 8 ACH = 160 CFM

However, these other calculations does not take into effect the bends in the duct work or the length.


So if 6" duct is rated at 110 CFM, how would you ever install an exhaust fan that would exhaust more than 110 CFMs? I've seen 6" exhaust fans rated MUCH higher than 110 CFMs.


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## Lyndon (Jun 2, 2010)

joecaption said:


> Your also loosing a lot of CFM's with the long run and all those turns. Not a great lay out.


I am well aware of this fact but the facts ARE the facts!


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## titanoman (Nov 27, 2011)

I had no idea a fart fan was such a technical apparatus.


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