Is this duct necessary?

[Rmundo]

I am in the process of finishing my basement and have part of the cold air trunk that extends beyond the drop to the air handler by about 26” (see pic). The closest return connected to it is about 6 feet to the right of the drop. It hangs down lower than the supply trunk that runs the length of the room on that side. Cutting this back and re-capping it would really clean up the soffit line and give me an extra inch of head room. I would like to extend the wall I built under the trunk straight up to the floor joist. Is this extension beyond the drop needed? Cutting it back would still leave about 12” beyond the drop. I thought I read somewhere that it was designed this way for something about the air flow or turbulence, but don't know where I read that. I had an HVAC company at my house for two days this week replacing my heat pump and adding supplies and a return for the basement and I never thought to ask them about it. Thanks for your help.

 

[Dana]

A fatter duct has lower velocity = lower turbulence, but square-throated turns are fairly turbulent no matter how fat the duct. Replacing an oversized sharp throated turn with a radiused ell of the smaller duct's size is usually better. Sometimes large sharp-throated duct turns will have internal curved vanes to reduce the turbulence.

There is also the issue of the transition- it's better to have a tapered transition between sizes, not a sudden sharp throated step size.

Better duct systems also use trunk take0ffs to branches that have a small 45 degree throat or coming in/out of the trunk at a 45 to reduce turbulence.

The difference in noise levels between doing it with tapers & radiuses can be significant:

But unlike the picture above, every seam and joint in the ductwork needs to be MASTIC SEALED if you really care about noise, effectiveness & efficiency.

 

[Sponsor]

 

[Rmundo]

Thank you Dana. I wish the builder of this house 26 years ago put that kind of though into the duct work design. From what I can tell there are no type of deflectors inside the rectangle return trunk or anywhere for that matter. So if the 26” extension beyond the drop to the air handler on the left in the picture of my duct was intentional to slow the turbulence, would it affect air flow or noise if I cut it back flush with the new wall I built? Just thought they may have run it longer to give some flexibility as to where they placed the air handler.

 

[Jadnashua]

Any good HVAC shop can custom make a curved piece for your transition to your exact dimensions.

 

[Dana]

Thank you Dana. I wish the builder of this house 26 years ago put that kind of though into the duct work design. From what I can tell there are no type of deflectors inside the rectangle return trunk or anywhere for that matter. So if the 26” extension beyond the drop to the air handler on the left in the picture of my duct was intentional to slow the turbulence, would it affect air flow or noise if I cut it back flush with the new wall I built? Just thought they may have run it longer to give some flexibility as to where they placed the air handler.

It's hard to ascribe any intentions to the original installer. Could be sheer laziness, lack of a specified duct plan, but it probably was not to reduce turbulence.

As long as cutting it flush to the studwall doesn't create a reduction from the main return duct to that point it's fine. Even a small radius (or even a short 45) on the throat (inside of the turn) of the new-improved duct will reduce turbulence & noise. A radiused outside of the turn also helps, but it isn't as important as the inside radius of the turn.

Be sure to mastic-seal all joints & seams of the new ductwork, and any other ducts you have easy access to. Duct mastic is messy, but it's cheap and more reliable than tape. Apply generously, obliterating evidence of seams or screws in the ductwork (typically 1/4" will do).

The mastic application on the picture below is a bit to thin- it might seal, but it may also miss and leak a bit in a few tiny hard to inspect locations:

Even pinhole leaks can create an annoying hiss, even when the total leakage isn't big enough to be consequential for efficiency.

If using tape to seal joints & seams, use a temperature rated HVAC tape (eg: Nashua 324A is commonly found at box stores), and wipe down the surface with a mild solvent such as isopropanol (rubbing alcohol) to remove any oily residues that will affect long term bonding of the adhesive before applying the tape.