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Thread: Reducing 6" to 4" on exhaust duct

  1. #1

    Default Reducing 6" to 4" on exhaust duct

    I just got a NuTone QTXEN110 and it is much larger than the old fan and also uses a 6" exhaust. The old fan is connected to a 4" exhaust duct. Is it OK to use a reducer and go from 6" to 4" or would that be bad for the fan? The bathroom is 45sqft with 8' ceilings so I believe the 110cfm exhaust is more than large enough, therefore, if it did not function to capacity, I dont see it as a total issue, although I dont want this to stress the fan causing issues. Although, I will leave it to the folks with experience to get the right solution.

    Also, the exhaust duct appears to just run to the eave/ridge/(I dont know the exact term) There is no actual cap at the end as far as I can tell, so I believe it is just venting out the ridge vents. Is this OK? It is in my attic and just want to confirm that the configuration and setup is OK and done in such a manor that will lead to moisture being pumped into the attic as opposed to outside.

    Thanks guys.

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    It probably will end up noisier and won't push as much air. Fan life probably won't change much. Venting into the soffit isn't a good idea. Much better to run it either through the roof, or a gable end wall. If you are going to run it to a new position, why not just run new duct?
    Jim DeBruycker
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    DIY Senior Member BimmerRacer's Avatar
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    Venting into the soffit is a surefire recipe for mold. Our bath used to be "ventilated" that way.

  4. #4

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    Quote Originally Posted by jadnashua View Post
    It probably will end up noisier and won't push as much air. Fan life probably won't change much. Venting into the soffit isn't a good idea. Much better to run it either through the roof, or a gable end wall. If you are going to run it to a new position, why not just run new duct?
    Yeah, I will most likely want to exhaust it outside, so I will then replace it with 6". The only issue now is figuring out where to exhaust to...UGH! (ceiling or wall and the big question, how)

    May have to pay a guy to just do it.

  5. #5
    Radon Contractor and Water Treatment 99k's Avatar
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    A 4" duct is more than enough to handle 110CFM

  6. #6

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    Quote Originally Posted by 99k View Post
    A 4" duct is more than enough to handle 110CFM
    OK, but if I use the 4" duct, what are your thoughts on the venting only to the soft vent?

    I misspoke before, it goes to the sofit vent instead of the ridge vent as originally stated...

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    Plumbing Designer FloridaOrange's Avatar
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    Quote Originally Posted by 99k View Post
    A 4" duct is more than enough to handle 110CFM
    Too much static pressure for the fan. As stated above it will be noisier and will likely not pull 110 cfm.

    OP, why not get the 50cfm model instead. As for termination point why not inspect to see if YOU actually have a problem before you go changing things.
    Matt
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    Radon Contractor and Water Treatment 99k's Avatar
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    Quote Originally Posted by FloridaOrange View Post
    Too much static pressure for the fan. As stated above it will be noisier and will likely not pull 110 cfm.

    OP, why not get the 50cfm model instead. As for termination point why not inspect to see if YOU actually have a problem before you go changing things.
    There are two factors to consider regarding the flow curve, CFM and water column (strength of vacuum). If the 50 CFM fan has twice the lift, then I would recommend that choice since the space is small.

  9. #9

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    Quote Originally Posted by FloridaOrange View Post
    Too much static pressure for the fan. As stated above it will be noisier and will likely not pull 110 cfm.

    OP, why not get the 50cfm model instead. As for termination point why not inspect to see if YOU actually have a problem before you go changing things.

    What do you mean to see if I have a problem? Do you mean to crawl out to th corner to see how the exhaust runs to sofit vent and to see if it is blowing out properly?

  10. #10
    Retired Defense Industry Engineer jadnashua's Avatar
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    There could be mold, mildew, rot where the old fan was exhausting...you won't know if you don't look. Keep in mind that the soffit vents are designed to be inputs to the attic...and exhaust out the ridge vent. So, what goes out from your bath exhaust fan will want to rise right back up through adjacent inlets back into the attic. In the winter, you could end up with frost on the inside of the roof deck, or wet insulation. None of these things are good.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  11. #11
    Radon Contractor and Water Treatment 99k's Avatar
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    Quote Originally Posted by jadnashua View Post
    There could be mold, mildew, rot where the old fan was exhausting...you won't know if you don't look. Keep in mind that the soffit vents are designed to be inputs to the attic...and exhaust out the ridge vent. So, what goes out from your bath exhaust fan will want to rise right back up through adjacent inlets back into the attic. In the winter, you could end up with frost on the inside of the roof deck, or wet insulation. None of these things are good.
    All very good points! My choice to vent is the gable side of the home with a standard dryer vent. It is much easier to install too.

  12. #12
    DIY Senior Member seaofnames's Avatar
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    Another idea is to use ABS or PVC as venting, instead of tin to not worry about rust/corrosion.

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