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Thread: Undercutting Doors

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  1. #1
    DIY Senior Member chefwong's Avatar
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    Default Undercutting Doors

    OK. Which camp are you in.

    1/2 undercut for funtion (air return, etc)
    or
    1/4 -- nice and tight to flooring.

    Gonna do some door hanging and can't decide....

    Or maybe meet somewhere in the middle and do 3/8".

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    Don't do it for air, if you needed it, you'd need more than the 1/2" anyway - there should be an air return doing the job in the room. A bit more is handy to help the door go over whatever might be sitting there.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    Moderator & Master Plumber hj's Avatar
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    quote; there should be an air return doing the job in the room

    What universe are you living in? The majority of modern homes have only one or two central returns and depend on the gap under the door for return air when the door is closed. Make a "tight gap" and then develop muscles pulling the door open when the furnace is operating.
    Licensed residential and commercial plumber

  4. #4
    In the trades Dana's Avatar
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    In many locations where there isn't a dedicated return in the doored-off room, jump ducts or return grilles in partition walls are required by code. It would take about a 2" door cut to meet the pressure differential requirements for a duct return path. Making the door tight WITHOUT providing a low impedance return path means you're using incidental air leakage out to the Great Outdoors to provide the return path, which takes a toll on system efficiency (and capacity).

    If you want to use the door as the return path without creating a hamster & squirrel escape route, these suckers work pretty well. They're not exactly a thing of beauty- on a par with using transom grilles or partition-wall grills. Jump ducts with ceiling grilles may be a bit less visually intrusive.

  5. #5
    DIYer, not in the trades LLigetfa's Avatar
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    A 4 inch duct feeding or exhausting a room is 12.5 sq inches, a 6 inch is just over 28 sq inches. Since it is a forced supply, the passive return or make-up needs be greater. A 1 inch gap under a 34 inch door is 34 sq inches, probably adequate for return or make-up IMHO.

    I say, give the hamster freedom to roam!

  6. #6
    In the trades Dana's Avatar
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    The max impedance spec Tamrack is working against for passive returns is 250cfm (min) @ 3 pascals pressure difference, which surely won't be met with a 1" door cut on a 34" door.

    Jump ducts are typically executed with 10" diameter flex duct, or 78-79 sq-in, which is roughly the same as a 2" door cut. Bigger rooms/volumes use 12" flex for the jump ducts. See the duct sizing and flow discussion, particularly the jump duct sizing in figure 3 here:

    http://www.buildingscience.com/docum...em-sizing-pro/

    See also: http://www.buildingscience.com/docum...return-systems

    While a 1" door cut would be much better than a tight fit, it would still be sufficient impedance to drive outdoor air infiltration.

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