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Thread: Main trunk sizing

  1. #1

    Default Main trunk sizing

    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. #2
    HVAC Contractor Marc46's Avatar
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    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. #3

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    Quote Originally Posted by Marc46 View Post
    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.
    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. #4
    HVAC Contractor Marc46's Avatar
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    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. #5

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    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. #6
    HVAC Contractor Marc46's Avatar
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    Quote Originally Posted by angus6 View Post
    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
    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!

  7. #7
    DIY Senior Member Hube's Avatar
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    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. #8

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    Had 22 on may mind but typed 20

  9. #9
    HVAC Contractor Marc46's Avatar
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    Quote Originally Posted by Hube View Post
    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.
    He means a 32 foot long trunk,...........not takeoffs!

  10. #10

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    Quote Originally Posted by Marc46 View Post
    He means a 32 foot long trunk,...........not takeoffs!
    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
    Thanks again

  11. #11
    HVAC Contractor Marc46's Avatar
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    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. #12

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    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. #13
    DIY Senior Member Hube's Avatar
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    I think you need more than a "duct wizard"..... you need a MANUAL D

  14. #14
    DIY Senior Member Hube's Avatar
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    Smile

    [QUOTE=angus6;147757]Wondering if I can get away with a main trunk size of 16x10 and a 32 run,
    ************************************************** ******
    It's amazing how just leaving out a simple measure symbol like a ' can make such a difference in the reading of a post.
    Now, if you had said..... "and a 32' run", it would have meant the distance of the run, not the amount of runs.
    Note; the correct symbol for a measurement pertaining to a distance in feet is '

  15. #15

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    [QUOTE=Hube;148607]
    Quote Originally Posted by angus6 View Post
    It's amazing how just leaving out a simple measure symbol like a ' can make such a difference in the reading of a post.
    Now, if you had said..... "and a 32' run", it would have meant the distance of the run, not the amount of runs.
    Note; the correct symbol for a measurement pertaining to a distance in feet is '
    Hube please don't tell me you thought there were 32 runs in this little 1160 sq.ft. house yeah I left the " and ' out sorry bro.

    Had 1 of the local guys over today to get a quote on building the duct work and I'll pick it up and do the install, as there is no place local to buy duct. Been told they come in with a price that makes spending half a day to go pick it up not worth it. plus I'll get what he sized all the runs for each room at.

    He came up using duct at 12"x8" as the unit sits right in the middle of the run and he figured the air flow needed was basically equal on each side of the unit. Said that if I was running the unit from the end of the run he would have started with a duct that gave about 180 sq.in. opening and stepped it down to-wards the end . Does this sound right ?
    Last edited by angus6; 07-30-2008 at 07:38 PM.

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