Question: Why would the 6" duct (represented by the red X) produce almost zero airflow?

I have a small piece of tissue attached to it to see that it's barely blowing anything. However, the blue arrow, which is a vent that's further down the path, is getting plenty of airflow.

All of the blue arrows represent good flow.

And...as I write this, I just though of something.

The bottom blue arrow leads to a bedroom that's attached to the bathroom and it's at the end of the main HVAC duct, which is much larger in volume than the 6" branch feeding the bathroom.

In essence, both ducts are feeding the same space, and since one is larger, would it be safe to say that the larger duct is causing static pressure that's basically squashing the 6" duct's output?

There are no return ducts in this house, just the single intake for the A/C.

It would seem that if two ducts that are different sizes feed the same space, one would overpower the other, or there'd be some sort of way to predict the flow out of each depending upon the volume of the branch...

I'm wondering if the only way to equalize the flow is to ensure that the output volume on both is the same by adjusting the registers.

I also suspect that the the 6" take-off is too close to the A/C unit's output. It's a roof mounted unit and the take-off for this branch is right next to where the A/C unit feeds (vertically) the main HVAC duct (horizontal.)

Current Problem

  • Turning vanes
    – Mazura
    May 31, 2021 at 12:14
  • Are you using Flex or metal round duct to supply the room ?
    – PowerTech
    Jun 2, 2021 at 15:08
  • The house was built in 1973 with 6" round tube. Jun 5, 2021 at 23:22

2 Answers 2


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This configuration can cause large pressure losses, because the turning vanes disrupt the uniformity of airflow in the main duct, which in turn causes a high pressure drop at the fitting. Branches should be installed with a 45° entry or a radius branch fitting, as shown in Figure 2.3 below.

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Turning Vanes: Necessary Component, or Efficiency Reduction Device? buildingengineer.wordpress.com


That duct should have a damper in it, and it appears it is working well. You need to find the damper and open it and adjust it to the air flow you want. It is probably a metal arm in the range of 3" and should be pointing 90 degrees from the way the duct is running, that is off. Try turning it, it should go in either direction.

  • The 6" round duct does not have a damper. It's a straight shot with a 90 degree angle to the bathroom. If I close the register to the bedroom completely, the 6" line to the bathroom starts to flow. May 31, 2021 at 1:00
  • ...so close the damper to the bedroom until you get the balance of flow you desire...
    – Ecnerwal
    May 31, 2021 at 1:51
  • Something does not sound correct. Check your air returns, if air goes in it has to come out. In your case probably the doors, they would have to be open or vents through the walls to allow air flow. Partially closing the register would be OK and possibly the only way you could adjust it. You may have to do this through your home to balance everything. Years ago especially with gravity furnaces they use to use transient windows above the doors into the room to accomplish this.
    – Gil
    May 31, 2021 at 3:05

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