Need advice for bathroom exhaust fan upgrade.


Old 08-18-06, 01:22 AM
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Question Need advice for bathroom exhaust fan upgrade.

We currently have a Broan 50 CFM fan in our 100 sq ft. ensuite bathroom.
The 50 CFM is not enough for a bathroom of our size and we want to upgrade to a 110 CFM unit.

The current 50 CFM uses a 3" duct which runs through the attic to a 3" roof cap.
The new 110 CFM exhaust fan requires a 4" duct.

My question is, can I run a new 4" duct up to the existing 3" roof cap and connect the two using a 4" to 3" adapter?

The Nutone instructions say "no" as it will impede the flow and cause excess noise.

Normally I would follow the instructions but I have no way of getting on the roof (30+ feet and steep) and a contractor quoted me $500 to switch out the roof cap.

Old 08-18-06, 05:32 PM
Join Date: Mar 2006
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Yes, you may use the 4-to-3 reducer at the roof jack. It is not the prefered method, it will add some back pressure which will cause increased noise and decreased performance but it will still be far, far better than simply exhausting into the attic.

Try to find a tapered reducer as opposed to a concentric reducer. The concentric reducer is more likely to creat eddy currents that will further impede the air flow.

I don't know what kind of a "contractor" you asked but I would ask a roofer. If there are any re-roof jobs being done in your area you might ask the job foreman if he or any of his crew is interested in a small "side" job. I'm pretty darn sure that you could get the proper roof jack installed for far less than $500.
Old 10-17-06, 12:04 PM
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I have been researching this because I must go from 4" to 6". The prior answer is right. Trouble is, you will not get the full output because of static pressure, and will get some noise. You can reduce the noise by using an insulated duct. But most noise is a function of the fan itself.

But it's also right that there is no way replacing one roof vent with another, without other work, costs $500. It takes a few minutes for a skilled person.
Old 10-17-06, 09:39 PM
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It also help some to make the tapered reducer
longer. Any sheet metal shop or hvac can make you one up.


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