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New return duct sizing

I have a 1970's 1100 sq ft ranch on a slab. The return duct is in the attic and is in need of replacement. Instead of putting a new return duct back into the attic, I would like to move it into the conditioned space.

There is only one large return vent, located in the hallway between the bedrooms. It would be about a 30ft. run of duct from the furnace, with three 90 deg elbows. An online ductalator calculated the diameter of a round duct at around 14.5" for my 2 ton system.

The best size for where I want to put the duct would be a 14" max diameter. Would a 14" duct work or would I need to move up to the 16"? Thank you for any help.


    HVACNUT Member Posts: 5,829
    Mine must be getting old.
    At 800 CFM, round duct equivalent is under 13" at .10 sp.
    14" can move up to 1,000 CFM so you're good.
    What's going on? The new return will be in the same location?
    But instead of running through the attic, and the house is on a slab...
  • hvacfreak2
    hvacfreak2 Member Posts: 500
    edited November 2018
    For residential return duct you should really consider velocity versus friction. The 16 inch will keep you under 600 fpm and may be a better choice.

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  • Jamie Hall
    Jamie Hall Member Posts: 23,276
    I'd go with the 16 inch if I could -- remember that that poor fan is sucking on that puppy and you want it to breathe as well as it can.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    EBEBRATT-Ed Member Posts: 15,517
    A rectangular duct would be a better choice and take less room and easier to box in. 800 cfm is right for 2 tons.
  • jr365
    jr365 Member Posts: 2
    Thank you all for your replies.

    The plan was to run it in the house along the corner of the wall and ceiling. Basically following the same path as the existing return, but inside the home.

    I wanted to go with spiral duct, to avoid having to frame out and drywall another type. I don't mind the look of an exposed duct, and it would avoid a lot more work and expense. Biggest reason to keep it no bigger than 14" is so it fits in the space above the doors. Any larger and it would overlap the top of the doors.

    Would making the vent opening larger, help make up the difference in the duct size difference, or maybe a larger air filter? Or both?
    HVACNUT Member Posts: 5,829
    Can I ask why you want to remove the return from the attic?
    If you run exposed spiral duct, how does the intake grill configure? I assume its going round to square/rectangle at some point.