Main trunk sizing

Discussion in 'HVAC Heating & Cooling' started by angus6, Jul 24, 2008.

  1. angus6

    angus6 New Member

    Messages:
    8
    Wondering if I can get away with a main trunk size of 16x10 and a 32 run, House is 1160sqf 3br 1 1/2 b on basement, furnace is 69k btu lists max CFM@0.5 ESP of 1,200
    Reason of size is there is a wall in the way:( , 14 x 10 would be a easer bet figure thats a no go
    There well also be 2ton of a/c
  2. Marc46

    Marc46 HVAC Contractor

    Messages:
    102
    Location:
    Central Florida
    You are really pushing it on the trunk size.
    I would say it will work, but you will be increasing the "static pressure", and you may end up with some noisy grille outlets.

    Your furnace is pushing 1200 cfm which is good for 3 tons of AC.
    Your 2 ton AC system would only require 800 cfm.
    The rule of thumb for AC is 400 cfm per ton.

    16X11 is right on the line for 1200,.............keep in mind this is "ID", not "OD" measurements.
    If you have any way to use round duct a 16" round will do you for around 1400 cfm. 14" round is the equivalent of your 16X10 rectangular.
  3. angus6

    angus6 New Member

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    8
    Marc Thanks for the help , So I'm figuring that if I bite the bullet and move the wall that 18x10 would be a good size to use ?
  4. Marc46

    Marc46 HVAC Contractor

    Messages:
    102
    Location:
    Central Florida
    Angus,
    Yes,......an 18X10 is the size I would use here, if trying to keep one measurement at 10 inches. It is right on the money!

    Your other option is to cut your furnace blower motor speed, since the max of 1200 CFM is rated on the high tap,........you most likely have a 3 or 4 speed blower.
    The less CFM moving through the heat exchanger, the higher the outlet air temp will be, due to "hang time" per se.

    A 13X14, or 15X12, will also give you the same as the 18X10 trunk.
    We use 1.5" fiberglass ductboard here in Florida for residential trunks, unless doing a "spider" system,.........which is round flex duct, and "splitter" boxes.
    Unfortunately I really can't see what you are trying to work around!:)
  5. angus6

    angus6 New Member

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    8
    Marc what I'm working around/between is the center beam in the basement and a wall and wanting to keep no deeper then 10", So I'm on the fence over 18x10 or 20x8. the 20x8 tucks up in behind the beam best as in stay s above the bottom. just need to make up my mind and start:)
    Thanks again for the help
  6. Marc46

    Marc46 HVAC Contractor

    Messages:
    102
    Location:
    Central Florida
    No problem Angus.
    Keep in mind that trunk sizes are not "relative" as you stated above.
    You don't add 2 inches to the 18 dimension, and then subtract 2 inches from the 10.
    The 18X10 is 180 square inches,.......the 20X8 is 160 square inches.
    A 20X8 gives you around 1000 cfm. The same as your original 16X10.

    Either way bud,.......it will work, you just may have a few noisy supplies, or may need to cut your blower speed down one tap.

    Not trying to confuse you BTW!:D
  7. Hube

    Hube New Member

    Messages:
    156
    Location:
    Ontario
    Are you actually saying there are 32 runs ?? if so, (wow) what size are these takeoffs (runs)?
    Note;The size of any runs/takeoffs will determine the main trunk size.
  8. angus6

    angus6 New Member

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    8
    Had 22 on may mind but typed 20 :D:D
  9. Marc46

    Marc46 HVAC Contractor

    Messages:
    102
    Location:
    Central Florida
    He means a 32 foot long trunk,...........not takeoffs!:D
  10. angus6

    angus6 New Member

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    8
    Yeah , what he said :)
    Being a millwright I'm hoping I can stumble through this with a bit of guidance and not screw the pooch to bad:D
    Thanks again
  11. Marc46

    Marc46 HVAC Contractor

    Messages:
    102
    Location:
    Central Florida
    Angus,
    You are going to have to figure the take-offs correctly also.
    The info I gave is using your trunk like a simple "pressure plenum", since it is not overly long.

    You need to figure the number of rooms, and the square footage of each to determine the size to go to each supply grille.

    Maybe you already have that figured out though!
    Just remember,........you have 1200 cfm to work with in distributing it to each room.

    I can't see a problem, as you are only running a 2 ton evaporator coil on top of the furnace, plus the overall size of the home.
    If you need anything else,.....speak up!:)
  12. angus6

    angus6 New Member

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    8
    Marc
    I have a duct wizard calculator coming that should show up today, Figured it would help get me in the ballpark
    Thanks again for the help
  13. Hube

    Hube New Member

    Messages:
    156
    Location:
    Ontario
    I think you need more than a "duct wizard"..... you need a MANUAL D
  14. Hube

    Hube New Member

    Messages:
    156
    Location:
    Ontario
  15. angus6

    angus6 New Member

    Messages:
    8
    Last edited: Jul 30, 2008
  16. Marc46

    Marc46 HVAC Contractor

    Messages:
    102
    Location:
    Central Florida
    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!:cool:
    Good luck!
  17. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,266
    Location:
    New England
    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.
  18. angus6

    angus6 New Member

    Messages:
    8
    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
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