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Thread: airflow

  1. #1
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    airflow

    Service techs test systems, how many of you tell design and sales 6 inch flex duct does not move 100cfm. I fight this constantly. RNC design says 16 6" runs move 1600 cfm. My static is .89 IWC. 4 ton system, 14 inch trunkline. The big problem, installers own the company and they believe the old adage"industry standard"

  2. #2
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    Quote Originally Posted by markk2 View Post
    Service techs test systems, how many of you tell design and sales 6 inch flex duct does not move 100cfm. I fight this constantly. RNC design says 16 6" runs move 1600 cfm. My static is .89 IWC. 4 ton system, 14 inch trunkline. The big problem, installers own the company and they believe the old adage"industry standard"
    Is that your available static or the friction of the duct. The friction loss also takes into consideration the entire duct system from the longest supply branch and the longest return run, filter, wet coil and register and grille.
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  3. #3
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    Check out Man. D ,16 six inch flex ducts could move 1600 cfms ,if every thing else in the duct system was large enough,which in youR case is likely not the case.
    Last edited by dash; 02-22-2008 at 09:16 AM.

  4. #4
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    How much is your return static.

    Those 16 - 6" supplies won't move 1600 CFM if the return can't draw 1600.
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  5. #5
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    Static is .4 supply, .48 return without a filter, about 280 feet of 6" flex, 13 runs, 4X10 floor registers, 20X20 Shoemaker return grill. 4 ton heat pump trips out with high discharge temp. Install the 20X20X4 inch filter and static goes to 1" total. The runs average 20 feet with a few at 30 feet. Theres no way this system will deliver the proper airflow and no way it will deliver comfort as far as I can see.

  6. #6
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    1200 CFM --- tops

    Probably more like 800 or 900 CFM.

    What's the make and model number of the fan coil?
    Perhaps you should have read the instructions before calling.

  7. #7
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    Quote Originally Posted by markk2 View Post
    Static is .4 supply, .48 return without a filter, about 280 feet of 6" flex, 13 runs, 4X10 floor registers, 20X20 Shoemaker return grill. 4 ton heat pump trips out with high discharge temp. Install the 20X20X4 inch filter and static goes to 1" total. The runs average 20 feet with a few at 30 feet. Theres no way this system will deliver the proper airflow and no way it will deliver comfort as far as I can see.
    You're correct,lot's of rework to do.

    Good time for a load calc., as it's likely oversized as well.

  8. #8
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    Jan 2008
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    The fan coil is a YORK MA016 with an FC48 coil. Do you ever feel like your talking to a brick wall? My brick wall is designers and installers that act like you attacked there kid when you question the ductwork. Even with proper testing, they refuse to believe they could be wrong. I read the Manual D, again, just to prove it, but they still insist its an equipment problem. I can't seem to convince them "it all starts with airflow" They are more concerned with"but that would cost more" So do 5 callbacks!

  9. #9
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    Quote Originally Posted by markk2 View Post
    I can't seem to convince them "it all starts with airflow" They are more concerned with"but that would cost more" So do 5 callbacks!

    If this is the case "it all starts with airflow" and your not even sure of the size of the equipment yet how can it start with airflow? you have to know size to get cfm to even know were to start, this is an example of new construction were size has not even been determined yet.
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  10. #10
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    0.9 IWC on high = 1033 CFM

    Where do you live? I hope it's not in my territory.
    Perhaps you should have read the instructions before calling.

  11. #11
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    Quote Originally Posted by jrbenny View Post
    0.9 IWC on high = 1033 CFM

    Where do you live? I hope it's not in my territory.

    If it can cool the house at that low air flow,it's surely oversized.

    We see many systems like this ,where some "cowboy" used rules of thumb,insists Manual j undersizes,etc.,etc..

  12. #12
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    Your between a rock and a hard place.

    Until they admit the its screwed up. Your stuck with call backs on it.

    Next thing you'll tell us, is that its a 1600 sq ft house.
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  13. #13
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    You must have missed the part about 1600 CFM, 4 ton heat pump. The size was determined in advance, the problem is the misconception of "industry standard" being 100 CFM per run no matter the size of the room, the type and size of the register and return grill and the length of the run. Being service I can't convince them the static numbers don't lie and possibly they should reread the Manual D.

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