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  1. #1

    Sizing my supply duct

    Hey,

    I live in a older home that is around 900-950 sqft. on the main level. The ac unit is 3ton. It currently has 7, 6" supplies coming out of the unit. The supplies then connect into bigger ducts that range from 8-10''. I think is used to have a gravity system and when they replaced the furnace they just hooked back up to the old lines. They house heats and cools with no problem now, but I want to finish off the basement and replace the old ducts with a square supply. How do I know how big it has to be? The run is about 20ft long. One straight shot with 7 takeoff's would be no problem, but how big does it need to be? I would like to keep it as thin as possible. I have had 3 people look at it and one said 8x16 the other 8x24. Could I go with something 6" deep?

    Thanks in advance.


    Vorp

  2. #2
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,967
    Quote Originally Posted by Vorp View Post
    Hey,

    I live in a older home that is around 900-950 sqft. on the main level. The ac unit is 3ton. It currently has 7, 6" supplies coming out of the unit. The supplies then connect into bigger ducts that range from 8-10''. I think is used to have a gravity system and when they replaced the furnace they just hooked back up to the old lines. They house heats and cools with no problem now, but I want to finish off the basement and replace the old ducts with a square supply. How do I know how big it has to be? The run is about 20ft long.

    One straight shot with 7 takeoff's would be no problem, but how big does it need to be? I would like to keep it as thin as possible. I have had 3 people look at it and one said 8x16 the other 8x24. Could I go with something 6" deep? Thanks in advance. Vorp
    First, I'm betting you have more cooling tonnage than you need.
    I'd see what I could do to reduce the heat-gain, then do a load-calc.

    Well, at 1200-CFM, a 16" metal rd gets 893-FPM Velocity., with 2-90's; 20'; 0.03" friction rate over the 20-feet.

    A 16" metal rd is 201-sq.ins.
    You'll need it deeper than 6".

    An 8X26 is 208-sq.ins., & might work.

    Also, 7, " take-offs will have a problem delivering the 1200-cfm at reasonable residential velocities.

    That's 171-cfm through each 6" duct run, that's 871-fpm velocity through branch runs.

    I'd drop in at least one more branch run where it's needed most. That still averages too high at 150-cfm @ 764-fpm each run. Nine runs would drop it to 133-cfm @ 678-fpm each run.

    It's usually best not to go much over 600-FPM on branch runs, using diffusers with a little more sq.ft of free-air-area would help reduce air noise.

    Oversize the Return-Air! - Darrell

  3. #3
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,879
    Contact a contractor that we do a Manual D, then he can size it right.
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    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  4. #4
    Join Date
    Jun 2005
    Location
    SW Wisconsin
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    Quote Originally Posted by beenthere View Post
    Contact a contractor that we do a Manual D, then he can size it right.
    Beenthere, gave you the correct advice; contact a contractor that knows how to do a manual D, as that is the way to get it done right!

    Too many unknowns from here. - Darrell

  5. #5
    Join Date
    Nov 2008
    Location
    Chapel Hill, NC
    Posts
    1,070
    Quote Originally Posted by udarrell View Post
    Oversize the Return-Air! - Darrell
    Just out of morbid curiosity (as I'm not planning any more HVAC work for the next 15-20 years!) what's the consensus on oversizing return air ducts? Ive seen numerous recommendations to oversize the return and some opposing views saying the return and supply must be the same to maintain static within spec.

    ???????????

  6. #6
    Join Date
    Jul 2008
    Location
    Metro Atlanta
    Posts
    70
    you must have enough return to equal your supply and they must equate to the tonnage and cfm desired for the application. After that im pretty sure you can have as much return as you want, the blower will not return more than it can supply.

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