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Community and Q&A

Poor airflow from too many bends?

kneewall | Posted in General Questions on

I have one register in my upstairs that has absolutely anemic airflow. It’s one of the farthest from the air handler, enters the upstairs via an unconditioned knee wall attic, and has what seems like a ludicrous path for the air to take before getting to the wall stack duct that takes it up to the 2nd floor. Photo attached.

The rectangular supply branches off into a round duct that goes 90° up and over the return duct, goes straight for a few feet then bends down 90° and turns 90° to the left, before a short straight run, finally turning 90° upward into the wall stack. 

The portion in the attic is a straight run of flex duct to the wall register, which I haven’t looked at in a while.

How significant of an effect could this odd duct-routing have on the low level of airflow at that register? I know more bends generally mean worse airflow, but does the way it doubles back have some negative effects independent of the bends? Could a more direct duct route improve upstairs comfort before tackling other factors with the ductwork?

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  1. Expert Member
    BILL WICHERS | | #1

    Yes, lots of bends can have a negative effect on airflow through a duct. My first thought here is why did they tap the round duct off where they did, when they could move it to the right a few joists and it would be a much more direct run (although possibly they were limited by the radius of those 90s).

    If you can straighten out the run and limit the number of bends and transitions, that will help airflow. Long runs can also be an issue though, and you usually can’t do much about the required length of the run. Worst case, you can add a duct booster fan to that duct run an interlock it with the blower in your furnace so that the booster fan comes on and goes off automatically together with the main blower.


    1. kneewall | | #2

      Thanks for the insights, and Happy New Year.

      Honestly, I have no idea what the thinking was. It seems like the route was dictated by a previous bathroom remodel, since it snakes around newer plumbing.

      One idea I've had to re-route it would be to tap off from where the register is in the attached photo, which would be much shorter and straighter. It would reduce headroom in that spot, but that's a worthy tradeoff. There's already a hole for the register, so it intuitively seems like an easy choice.

      Is that a good option (or even a viable one)?

      1. Expert Member
        Akos | | #4

        What I would do is put a side takeoff on the main trunk right bellow where the floor joists of the 3x14 duct chase are. From there it is a straight run to the riser with an 3-1/4 x 14 end boot. This would reduce the equivalent length by about 75% vs what you have now so a significant improvement.

        For a bit of extra flow, instead of cutting a full rectangular opening for the side takeoff, cut it only on 3 sides to form a flap that points into the airflow direction. Bend this flap into the flow to create a scoop.

        1. kneewall | | #7

          I appreciate the suggestions. I think I've resolved to make a change. As for your proposed run, you're talking about starting where I've circled, then going under the return and straight to the riser?

          1. Expert Member
            Akos | | #8

            For some reason, it looked like the duct was coming off the top of the trunk closer. I see now what the issue is, it needs to come from the supply one, jog around or under the return.

            If you don't mind going under, that is the simplest. If you are going under the duct, you want a top takeoff on the supply but mounted to the bottom of the trunk.

            If you want more headroom what you can do is use semi rigid aluminum ducting (looks like the dryer vent stuff). You still start with a side takeoff on the supply trunk (try to mount it near the top for easier routing), slightly over under one of the wider joist bays, to left seems to be easier. From there, run semi rigid duct with nice wide bends to get over the return to the riser. End boot on the flex to connect. Going larger say 7" or 8" if you have the room also help a bit.

            P.S. Whoever did the original ducting did a great job. You rarely see a proper reducing trunk and branch supply in residential.

  2. Expert Member
    ARMANDO COBO | | #3

    I find hard to blame any HVAC contractor when ±99.99% of homes designs don't plan for HVAC systems. Although there's plenty to blame HVAC contractors for oversized and ineffective systems, framing a house floor with dimensional lumber, I-joists, and beams without planning for ducts its a travesty... and that is the norm.
    Planning properly for good HVAC design is key, and framing homes with open web floor trusses designed with dedicated chases or furr-downs for ducts and no beams going thru makes it a lot easier. Maybe some day the industry will find that education, integrity and pride is a good thing.

    1. Jason_K | | #5

      This is something that's driven me crazy during my (ongoing) house build -- so much seems to be placed in the "we'll deal with it when we get there" bucket, even though all the relevant information is available up front and issues could be avoided with some early work.

      HVAC certainly falls in that bucket, as well as hot water routing. Knowing what I know now, I wish I'd pushed harder on this up's a shame I will :)

    2. kneewall | | #6

      I can see where you're coming from here. Of course, my house was built in 1960, so the science and engineering of HVAC systems have come a long way since then. Retrofitting will naturally come with more compromises.

  3. kneewall | | #9

    AKOS TOTH, I think going under will be simplest. What would be the best way to initiate a new branch without losing too much airflow? Would a bottom takeoff have any issues with that? From some articles I've read, and my amateur intuition, it seems like just having a new branch come off the bottom of the supply trunk would mean the air would have to turn 90° to enter. For the side takeoff, you mentioned cutting only 3 sides of the hole. Is there a similar method for a bottom takeoff?

    I may or may not try to tackle this myself, but either way, I'd like to know what good options look like, so I could evaluate whether an HVAC contractor is actually doing it the right way. The bedroom I'm trying to improve with this will be my baby daughter's, so I'm especially invested.

    Thanks again for all the helpful input.

    1. Expert Member
      Akos | | #10

      A bottom takeoff does add a bit of extra equivalent length (45' for bottom, vs 15' for side) so it is something but not a whole lot.

      The scoop I described works with either takeoff orientation, you have to cut and form it to point into the air flow.

      If you are paying for somebody to do this work, I think the simple and guaranteed to work would be a duct buster fan. This can be added to the ducting as is and won't reduce your headroom. One of the in-line duct fans with an ECM blower (ie CLS6) can be had for low cost nowadays. These are much more efficient and quieter than the older duct boosters and also come with speed or temperature controls.

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