Bathroom Exhaust Fan Duct

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infinity

infinity

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I have a bathroom exhaust fan which will require about 25' of duct to reach the outside of the house. The fan has a 4" outlet and is 110 cfm, also the run will contain one 90° elbow. How would you size the duct for this to have maximum output from the fan?
 
kwired

kwired

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Ask a HVAC guy that knows his air flow stuff well?

Even without regards to air flow, I would likely increase to 6 inch and avoid flexible duct as much as possible. If a typical in ceiling fan, you get more fan noise if there is too much restriction. If an inline fan and the blower is further away you usually won't hear increased noises because of duct size, plus flex duct becomes somewhat a muffler in those kinds of applications, though it does lessen air flow.

I believe my HVAC buddy once told me 6 inch was what is recommended for ~110 CFM as the base rule when they are sizing their duct runs, sort of like we automatically select 12 AWG for a 20 amp circuit, there can be factors that cause adjustments though.
 
synchro

synchro

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Chicago, IL
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EE
kwired is correct that 6 inch will be much better than 4 inch duct in this situation.

Using the pressure vs. airflow performance curve of the 110 CFM Broan unit below and the online calculator for flexible duct linked below I get the following results:

4" 25ft duct with one 90 deg elbow:
Friction loss 0.213 inH2O, velocity 894 ft.min (10 MPH)
The 110 CFM rating is at 0.1 inH2O pressure head.
From the performance curve, at 0.213+0.1 = 0.313 inH2O we get 78 CFM

6" 25ft duct with one 90 deg elbow:
Friction loss 0.038 inH2O, velocity 104 CFM at 530 ft.min (6 MPH)
At 0.038+0.1 = 0.138 inH2O we get 104 CFM which is very little degradation from the 110 CFM rating.



Your fan will be a little different but the large improvement with a larger duct will be the same.
 
infinity

infinity

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Thanks, I posted this question in the engineering forum because I knew someone would know how to do the calculation. My calculation would be just to run a larger duct (6"). :rolleyes:

Now there is the 4" flexible plastic junk run through the kitchen ceiling and out of the side of the house. The kitchen will be demo'd in the near future so when the ceiling is replaced the duct will be replaced too.
 
kwired

kwired

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Location
NE Nebraska
Dennis Alwon said:
If the duct run thru the attic or if you want to reduce noise then use insulated duct
Click to expand...
All depends on what noise you want to reduce.

With typical heating cooling where the blower is further away, you usually get more noise at the outlet when it is smooth wall duct as you usually get more velocity of flow through the smooth wall duct. Even placing a short length of flex near the outlet end can muffle the sound enough to make a difference in those situations.

For typical bath exhaust fan where the blower is right at the "inlet" you have opposite thing going on. You want as little restriction in flow as possible because restriction usually makes more noise in the blower itself and it is right there at the inlet, you typically won't care so much if there is noise at the outlet end.
 
kwired

kwired

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Location
NE Nebraska
Dennis Alwon said:
So are you denying that insulated duct doesn't dampen the sound if the fan is at the inlet?
Click to expand...
It dampens air flow sounds in the duct, but more restriction causes more noise in the blower itself. A compromise to get the best of both is increased duct size to allow more air flow and insulated duct to dampen sounds of air flowing in the duct.
 
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