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    tableclocks's Avatar
    tableclocks Posts: 404, Reputation: 2
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    #1

    Jul 1, 2020, 08:07 PM
    hvac duct size
    i know my duct is to small 20 x 8, it runs one way from the furnace 23 feet, is 20 x 12 ok for my 100,000 btu, 12000 cfm furnace
    if it was running two ways, 12 foot each way, what size should it be ??? thank you for your help and time, richard
    talaniman's Avatar
    talaniman Posts: 54,290, Reputation: 10853
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    #2

    Jul 2, 2020, 04:39 AM
    https://www.hunker.com/13418400/how-...20duct%20size.

    A duct sizing calculator may then be used to determine appropriate duct size.
    A lot of measurements to take and plug in.
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    #3

    Jul 2, 2020, 07:12 AM
    will the 12 x 20 be better then the undersize 8 x 20
    trunk line runs one way from the furnace, 23 foot to heat one floot house is 35 x 35,
    i have a 8 x 10 to the main LR, 6 rd, 6" to the other RMs, one 8" rd
    soooo, is the 12 x 20 ok???
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    talaniman Posts: 54,290, Reputation: 10853
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    #4

    Jul 2, 2020, 07:29 AM
    What are your total square feet so the CFM can be calculated?
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    tableclocks Posts: 404, Reputation: 2
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    #5

    Jul 2, 2020, 10:25 AM
    1300, one floor
    talaniman's Avatar
    talaniman Posts: 54,290, Reputation: 10853
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    #6

    Jul 2, 2020, 11:16 AM
    Sorry I should have asked for cubic feet which includes the height on the one floor.

    Maybe this chart is easier to use

    https://hvacdirect.com/hvac/pdf/Fiel...zing-chart.pdf

    My guestimate is 22x12 or 20x12 depending on the height we are talking about.
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    tableclocks Posts: 404, Reputation: 2
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    #7

    Jul 4, 2020, 10:21 AM
    about 35 ft x 34 ft x 8 ft ceiling
    lr is 12 x 16,one 8 x 12 reck,,, 2 BR one 8 " rd, other is 6 " rd
    kit and din rm is 11 x 35,with 0ne 8" and 2-6" rd, Bath rm, one 6" rd
    looks like i have about 1150 cfm at the registers, main trunk is 20 x 8,
    getting ready to make ie 20 x 12
    realy, thank you for your time and your thoughts,
    i will consider and of you ideals and thoughts
    line is insulated with the silver back 1 " stuff
    THANK YOU, THANK YOU
    the 8 x 14 reck is at the end of my 23 ft run
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    tableclocks Posts: 404, Reputation: 2
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    #8

    Jul 5, 2020, 07:29 AM
    about 35 ft x 34 ft x 8 ft ceiling
    lr is 12 x 16,one 8 x 12 reck,,, 2 BR one 8 " rd, other is 6 " rd
    kit and din rm is 11 x 35,with 0ne 8" and 2-6" rd, Bath rm, one 6" rd
    looks like i have about 1150 cfm at the registers, main trunk is 20 x 8,
    getting ready to make ie 20 x 12
    realy, thank you for your time and your thoughts,
    i will consider and of you ideals and thoughts
    line is insulated with the silver back 1 " stuff
    THANK YOU, THANK YOU
    the 8 x 14 reck is at the end of my 23 ft run
    talaniman's Avatar
    talaniman Posts: 54,290, Reputation: 10853
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    #9

    Jul 5, 2020, 11:22 AM
    Before you go buying stuff can I please ask what you are trying to accomplish here as you sure have a big furnace to begin with.
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    #10

    Jul 5, 2020, 02:23 PM
    i think i need more air flow from the furnace, my main trunk line goes one way 23 foot and it is only 20 x8, think i need about 20 x 12
    rheem 100 btu, 80%
    i have all the stuff i need at very little cost
    my return is 24 x 16,, 3 ton air, thats where i think i need more air movement
    talaniman's Avatar
    talaniman Posts: 54,290, Reputation: 10853
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    #11

    Jul 5, 2020, 04:20 PM
    Let me know how it works out. Good Luck.
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    #12

    Jul 5, 2020, 06:21 PM
    thanks, after all the reading, do you have a opinion?
    do you think 20 x 12 is better then 20 x 8 ??, thank you
    talaniman's Avatar
    talaniman Posts: 54,290, Reputation: 10853
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    #13

    Jul 6, 2020, 01:25 AM
    What works best with your system is what you want, and as far as duct work goes too small is as bad as too big, and location and conditions play as big a role in matching ductwork to furnace efficiency as insulation, as maintenance. They all work together and you do have a rather large furnace for the house you have. LOL, I suspect that you replaced a smaller one with this one you have now, which makes the existing duct work way to small. There must have been some noticeable changes in performance and efficiency or COSTS that prompted this project, and I was curious if that was the case. How old is the house?

    Plus I tend to be nosy and all up in peoples business concerning such things. 8D
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    #14

    Jul 7, 2020, 04:22 PM
    can we get to the bottom lie, is 20 x 12 better then 20 x 8 ??
    for my 100, 80% btu furnace, 3 ton air with the registers i have listed, about 1300 cfms coming out, one floor
    i thank you for your input !!!
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    talaniman Posts: 54,290, Reputation: 10853
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    #15

    Jul 8, 2020, 05:03 AM
    20x12 should be fine for your furnace, 22x12 and 24x12 works as well for such a huge furnace, if you have the room for it. To be honest the furnace is oversized for the house.
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    #16

    Jul 8, 2020, 07:49 AM
    thank you, my wife is either hot or cold, so .....thank you
    talaniman's Avatar
    talaniman Posts: 54,290, Reputation: 10853
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    #17

    Jul 8, 2020, 08:54 AM
    Quote Originally Posted by tableclocks View Post
    thank you, my wife is either hot or cold, so .....thank you
    LOL, know exactly how that works, but I open and close vents and doors.

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