(206) 949-5683, Top Rated Plumber, Seattle
Page 2 of 2 FirstFirst 12
Results 16 to 18 of 18

Thread: Main trunk sizing

  1. #16
    HVAC Contractor Marc46's Avatar
    Join Date
    Jul 2008
    Location
    Central Florida
    Posts
    102

    Default

    Angus,
    Yes, that should work fine.
    I didn't know it was in the middle of the trunkline. I have a better picture in my head of what your install looks like now.

    Basically what you do, is size your trunk for your max CFM..........after you take off X number of drops, you minus the proper amount of CFM used, and step your trunk size down to the new CFM rating remaining.
    This is done to maintain your air velocity,..........especially on long trunklines.

    Only one suggestion. Since your furnace is sitting in the middle, and you are going to "split" the CFM per se,.............I would not just go straight up into a 12X8 trunk with your supply plenum. It really should be brought in with 2 nice "radius" bends if possible. That is a mighty small trunk, for 1200 CFM to just "slam" head on into.

    As the other poster suggested,...........nowadays all of this is usually done on the computer, and several "manuals" are generated for the sake of permits.
    I couldn't tell you how many BIG jobs that I designed back in the "old" days, using nothing but a pencil, and a handheld ductulator.
    Funny thing is most of them worked as good, or better, than some of the computer generated layouts I have seen. Oh well,........times change!
    Good luck!

  2. #17
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    Location
    New England
    Posts
    21,428

    Default

    The backpressure and turbulance of a right-angle bend is huge in comparison to a radius bend (dont' quote me, but I think it is something like 5x). Part way in between is to put an angled deflector vane in. Another thing often called for but often omitted, is to use a decoupling band to separate the ductwork from the vibrations of the air handler/furnace from the ducts. It is often just a 2" or so piece of vinal coated canvas connecting the two halves of rigid metal ductwork. This will go a long ways to isolate noise propagation into the ducts.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #18

    Default

    Quote Originally Posted by jadnashua View Post
    The backpressure and turbulance of a right-angle bend is huge in comparison to a radius bend (dont' quote me, but I think it is something like 5x). Part way in between is to put an angled deflector vane in. Another thing often called for but often omitted, is to use a decoupling band to separate the ductwork from the vibrations of the air handler/furnace from the ducts. It is often just a 2" or so piece of vinal coated canvas connecting the two halves of rigid metal ductwork. This will go a long ways to isolate noise propagation into the ducts.
    I do know he spec'd in the decoupling band as to the transition from the plenum to the duct I'll have to check , I'm guessing the it's a angled deflector as that's what they used in some stuff the did for us at work

    Once again thanks guys for the sharing of your knowledge

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •