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  1. #1

    Practical question regarding ductwork

    So, I decided to install powerful inline fan in my attic capable of moving close to 3000 CFM and do air exchange every 4-6 minutes. This will be my whole house fan solution for the 2-nd floor only.

    I will install main trunk and then branch off to each individual bedroom/bathroom on a second floor. I have to use silencers because fan is definitely loud (seems like close to 70 db) and in addition to fan noise I would have to address aerodynamic noise coming from moving air @ pretty decent velocity through the ductwork.

    The immediate problem is that the largest available silencer off the shelf is only 12" and my fan requires 16" so I can't install it without further restricting air flow through both inlet and exhaust side of it.
    Secondly, I am torn apart what material/pipe to use as 16" galvanized pipe in 26 gauge is definitely not cheap and in addition I would have to spend extra time and money to further insulate it to reduce aerodynamic noise from air movement.

    Therefore I was thinking about insulated flexible pipe (very cheap and easy to install) but this one has its own issues as there are a lot of spirals/ridges inside it and in order to avoid potential problems I would have to increase its diameter from 16" to 18" or even 20" to reduce air velocity.

    So, what do I do? Any suggestions from Pro's?

  2. #2
    Join Date
    Feb 2009
    Location
    Tallahassee, FL
    Posts
    6,048
    Where does the air replacing all this exhausted air come in at?

    Is this a belt drive centrifugal fan?

  3. #3
    The air comes from open windows and it is large inline fan, like Fantech FKD series which I believe is direct drive

  4. #4
    Join Date
    Feb 2011
    Location
    Georgia
    Posts
    192
    silencers only reduce sound of air velocity at terminals. not the sound of the fan if you were standing next to it or standing underneath it in your bedroom.

  5. #5
    Hmm..exactly opposite to what I was told by Fantech tech support

  6. #6
    Join Date
    Feb 2011
    Location
    Georgia
    Posts
    192
    doubt they have ever installed one themselves
    Quote Originally Posted by ProperVenting View Post
    Hmm..exactly opposite to what I was told by Fantech tech support

  7. #7
    So what do I do?

  8. #8
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    7,046
    Live with the noise, or enter the 21st century and install an hvac system.

  9. #9
    Join Date
    Nov 2006
    Location
    Southeastern Pa
    Posts
    18,558
    Large ducts, large diameter fan(s), slower blade speed.
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist

    AOP Forum Rules:







  10. #10
    Quote Originally Posted by jtrammel View Post
    Live with the noise, or enter the 21st century and install an hvac system.
    If u refer to A/C and Heating system, I am already in 21-st century and I am installing this one in addition. And my last observation, you should be less presumptuous in your opinions.

  11. #11
    Join Date
    Feb 2011
    Location
    Georgia
    Posts
    192
    well... 16" round flex will do what you want at a velocity of roughly 1100fpm for about 50-70 feet. they do make a blanket material you can use to reduce sound. or live with it. using flex alone will silence some of the sound of the fan at the terminals.

  12. #12
    Quote Originally Posted by timebuilder View Post
    Large ducts, large diameter fan(s), slower blade speed.
    Tim, thanks for the suggestions. So forget silencers? I did mention that I am planning to oversize my ducts, what about using flex vs regular pipe? I really need to find a solution for the fan noise. If you guys are telling me that silencers will only help with aerodynamic noise and not fan noise, I am start questioning the whole installation idea.

  13. #13
    Join Date
    Feb 2011
    Location
    Georgia
    Posts
    192
    silencers reduce the sound at the registers only.

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