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Thread: Duct question

  1. #1

    Duct question

    I want an opinion on this proposed change.

    This is a 3 ton system.
    Currently this system has an approx. 10 foot 14" flex supply line with a 90 degree bend midway that is kinked at the bend going to a distribution box and then splits to two trunk lines.
    It reads .7 pressure just above AHU.

    Proposed change:
    Change to approx. 3 foot of 14" flex supply line that tees to the two trunk lines - no distribution box.

    Would this change reduce the pressure since a 90 degree bend and kink would be eliminated but there would no longer be a distribution box?
    What is the side effect of removing the distribution box and being only a tee?

  2. #2
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    a 14" is good for 700 cfm...coming up a little short

  3. #3
    It's not practical to increase the size of the line.
    It already has 14" flex and it pushes 1200 cfm through it.
    The plan is to reduce the pressure on the AHU by removing a 90 degree bend and the associated kink.

    The question has more to do with the distribution box and it's effects. The two trunk lines would come off at 90 degrees
    from the line in both setups.

  4. #4
    OK, how about if I clarify the question.
    Due to the house construction, I'm stuck with that 14" flex line attached to the air handler and the ductwork. My tech is trying to resolve the pressure issue. I'm concerned if the fix may be worse than the problem. The red lines indicate the changes.

    Name:  ductchange.JPG
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  5. #5
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    Any way to leave the 14" thats there and add the new one also?

  6. #6
    Interesting thought - I'll have to ask about it - I expect there won't be enough space though.

    The flow out my vents is quite good although the flow on the right hand side is a little lower than those on the left. It's not causing any real problems though.
    Only the high pressure is the issue being addressed. The tech claims that leaving that exisiting box in the line might increase the flow on the right hand vents - which would be a good thing.

    Do you think the original proposed change would be better (lower pressure on AHU) than what's there considering the removal of a sharp 90 degree bend and a kink?

    The concern I have is that tee - since it would have no box like the old setup. What does that box do if anything the way it's setup?

  7. #7
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    Quote Originally Posted by freddyrheem View Post
    It already has 14" flex and it pushes 1200 cfm through it.
    I doubt it, how did you verify cfm?

  8. #8
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    Quote Originally Posted by freddyrheem View Post
    Interesting thought - I'll have to ask about it - I expect there won't be enough space though.

    The flow out my vents is quite good although the flow on the right hand side is a little lower than those on the left. It's not causing any real problems though.
    Only the high pressure is the issue being addressed. The tech claims that leaving that exisiting box in the line might increase the flow on the right hand vents - which would be a good thing.

    Do you think the original proposed change would be better (lower pressure on AHU) than what's there considering the removal of a sharp 90 degree bend and a kink?

    The concern I have is that tee - since it would have no box like the old setup. What does that box do if anything the way it's setup?
    Its hard to say without looking at it, but I think you very well could make the problem worse with the change. Air moving fast and making turns with nothing to "guide" it is what drives up static. Maybe you have room to leave what is there and add a 10" or so to the right?

  9. #9
    How is the air being "guided" now? I'm pretty sure that box is just an empty box with two lines coming off it?

  10. #10
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    Quote Originally Posted by freddyrheem View Post
    How is the air being "guided" now? I'm pretty sure that box is just an empty box with two lines coming off it?
    Well, its not. But the box lets the air slow down which helps a lot. The air is going fast through the 14" flex duct, but it is somewhat guided by the radius in the turn. With the change you would have it moving fast and slamming into the duct and turning both ways.

  11. #11
    The box is not radiused - unless it's inside - doubtful. It was some sort of hack job conversion when the air handler was moved some 18 years ago.I think it was just cut and capped and a collar put on for the flex line.

    So you think moving the box or creating a new box would be better than the simple tee?

  12. #12
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    High static pressure is caused by "Resistance"... Has anyone confirmed duct velocity or CFM? Sounds like you need to start from the beginning. Confirm duct sizing, velocitys and if the volumes you are looking for can be handled by the duct system.

    Good luck!

  13. #13
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    Quote Originally Posted by freddyrheem View Post
    The box is not radiused - unless it's inside - doubtful. It was some sort of hack job conversion when the air handler was moved some 18 years ago.I think it was just cut and capped and a collar put on for the flex line.

    So you think moving the box or creating a new box would be better than the simple tee?
    Unless there is something Im missing which is quite possible since Im not looking at it, I would:

    1. Have it all replaced with a vaned tee properly sized. This would be the best approach.

    or

    2. Add another "jumper" duct as large as possible where the red is on your sketch. This would be the value approach.

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