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Thread: Fpm

  1. #1
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    Fpm

    Hey everyone.

    1) Is there a calculation to keep my FPM the same through the whole duct system.

    If so can you help me out with it?

    I seem to keep it the same most of the way with my duct calculator.

    But at some point things start to change.

    2)
    Or is it okay for it to change as long as my FPM dont go less then what my FPM state for that individual run at my desired friction rate?

    3) also i heard try and keep my FPM at what my grill will allow?? Not sure if thats correct






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  2. #2
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    Quote Originally Posted by Cool_man View Post
    Hey everyone.

    1) Is there a calculation to keep my FPM the same through the whole duct system.

    If so can you help me out with it?

    I seem to keep it the same most of the way with my duct calculator.

    But at some point things start to change.

    2)
    Or is it okay for it to change as long as my FPM dont go less then what my FPM state for that individual run at my desired friction rate?

    3) also i heard try and keep my FPM at what my grill will allow?? Not sure if thats correct






    Sent from my LG-M257 using Tapatalk
    FPM of the grille has no bearing on the FPM your duct work should have.

    Just stay within the range your duculater list.
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  3. #3
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    Quote Originally Posted by beenthere View Post
    FPM of the grille has no bearing on the FPM your duct work should have.

    Just stay within the range your duculater list.
    Okay i wasn't sure if there was an extra equation i can use maybe to keep that fpm maintained constant.

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  4. #4
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    CFM/sq ft=FPM

    EG: 580 CFM through a 14x8 duct, ((14x8=112sq in)/144=.77 sq ft), 580/.77=753FPM

    Your duculator should show the same FPM though.
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  5. #5
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    Quote Originally Posted by beenthere View Post
    CFM/sq ft=FPM

    EG: 580 CFM through a 14x8 duct, ((14x8=112sq in)/144=.77 sq ft), 580/.77=753FPM

    Your duculator should show the same FPM though.
    Okay were did you get 144?

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  6. #6
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    Sq inches to sq feet conversion.
    "If common sense was so common, everyone would have it."

  7. #7
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    FPM changes as the ducting changes.

    I'm guessing what you are trying to do is figure CFM.


    Quote Originally Posted by Cool_man View Post
    Hey everyone.

    1) Is there a calculation to keep my FPM the same through the whole duct system.

    If so can you help me out with it?

    I seem to keep it the same most of the way with my duct calculator.

    But at some point things start to change.

    2)
    Or is it okay for it to change as long as my FPM dont go less then what my FPM state for that individual run at my desired friction rate?

    3) also i heard try and keep my FPM at what my grill will allow?? Not sure if thats correct






    Sent from my LG-M257 using Tapatalk
    If you were a real tech, you'd solder a relay on that board and call it good to go.

    I do a triple evac with nitro to remove non condensables.

  8. #8
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    Get yourself some ACCA books and you'll understand you're asking the wrong question.
    Last edited by btuhack; 11-03-2018 at 09:42 PM.

  9. #9
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    Quote Originally Posted by BBeerme View Post
    FPM changes as the ducting changes.

    I'm guessing what you are trying to do is figure CFM.
    I was trying to see if i can keep the same fpm as my duct system starts and ends in design.

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  10. #10
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    To what end? Or, said another way, why?


    Quote Originally Posted by Cool_man View Post
    I was trying to see if i can keep the same fpm as my duct system starts and ends in design.

    Sent from my LG-M257 using Tapatalk
    If you were a real tech, you'd solder a relay on that board and call it good to go.

    I do a triple evac with nitro to remove non condensables.

  11. #11
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    Quote Originally Posted by Cool_man View Post
    Okay were did you get 144?

    Sent from my LG-M257 using Tapatalk
    As said earlier. 144 sq in to a sq ft.


    Look at your duculator. You should be able to see the FPM your duct size is capable of, and determine if your staying close to what you want. But keep in mind that its better to have a lower FPM than a higher one the further a way from the air handler/furnace you are.
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