Exhaust fan 6" to 4" reducer

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  #1  
Old 12-18-15, 04:04 PM
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Exhaust fan 6" to 4" reducer

I purchased a Model # FV-15VQ5 WhisperCeiling 150 CFM Ceiling Exhaust Bath Fan, ENERGY STAR. It was an online only purchase and the description said it came with a 6” to 4” vent reducer. It did not. I was wondering if the unit was designed to only run using a 6” vent. I have 2 questions. 1. If I add a reducer will it reduce the effectiveness of the unit? 2. Will it cause the unit to be louder/nosier?

Bathroom specs. 10’x11’x8’=880 cubic feet. The ducting will be about 10’ long with 2 elbows.

My other alternative would be Model # FV-11VQ5
WhisperCeiling 110 CFM Ceiling Exhaust Bath Fan, ENERGY STAR

Thanking you in advance…
 
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Old 12-18-15, 04:20 PM
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1. If I add a reducer will it reduce the effectiveness of the unit?
Yes!

2. Will it cause the unit to be louder/nosier?
Probably.

Much of the noise from a ventilation fan comes from the velocity of the air movement. Decreasing the cross-sectional area of the exhaust duct WILL increase the velocity of the air AND increase the "back pressure" on the fan. Both of these will cause the amount of airflow to decrease substantially. Bottom line is that you NEVER want to decrease the duct size on the outlet of a fan to a smaller size than the fan outlet.
 
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Old 12-19-15, 07:39 AM
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Furd, what are your thoughts on a reducer used in the opposite fashion..e.g., a 4" fan to a 6" duct.
 
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Old 12-19-15, 08:54 AM
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A 4" duct can carry less than ˝ the volume of a 6" duct, and with more friction.
 
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Old 12-19-15, 01:28 PM
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thanks for the input. What are your thoughts about using a 6" soffit vent? i have about a 5' straight run to my soffit. Otherwise I'll have to buy a 110cfm fan and run it to my existing wall vent.
 
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Old 12-19-15, 04:22 PM
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I don't like soffit exhaust vents but if it is the most expedient then go for it. I would block any soffit INLET vents for several feet on either side of the exhaust.
 
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