Replacing branch line to trunk from 8" to 10"

[Lifespeed]

I have an existing 8" sheetmetal branch line off an (approximately) 18" trunk from my air handler. I need to replace it to 10" to better serve the 1/3 of the 1450sf house this line handles. Currently the air temperature is consistent throughout the house except for this under-served area.

My question is regarding the cone-shaped sheetmetal interface cut into to the side of the trunk that is already existing. It appears to me as though I would simply remove the 8" duct, trim the cone back a couple inches so the cone exit matches the 10" duct, and proceed from there to my upsized wye and larger registers.

Does this sound right? Are these cones, boots, or whatever the interface that is cut into the side of the trunk is called size-specific at the outlet, or are they generally trimmed to fit a range of duct sizes at the outlet? If I need different sheetmetal to interface to the trunk I would like to purchase it before proceeding.

 

[Dana]

I don't know of any duct reducers that are specifically designed for that sort of adjustment, but there is no reason it wouldn't work. It may take a bit more effort to air-seal the duct than replacing it with one made for the new duct sizing, but fiber reinforced duct mastic can cover over even the crudest of hacked sheet metal.

 

[Sponsor]

 

[Lifespeed]

I'm wondering if I should replace the "cone" on the side of the trunk with one that specifically has a 10" outlet instead of 8". Except these really appear not very size specific, and the cone shape allows the installer to choose a range of exit sizes simply by cutting the cone closer or further from the trunk. I'm very much the type to do things correctly, but it doesn't appear that replacing the adapter to the boot to upsize the exit 2" is appropriate.

I'll see if I can post a photo this weekend. Still waiting a few more days for all the sheetmetal to arrive.

 

[Dana]

Most duct reducer cones have a short section of straight section on each end sized appropriately to make it easy to attach the duct, often times with a lip to snug-up and seal the duct up to:

Cutting it back a few inches wouldn't have the supportive east-to-connect straight section or the lip to know when it's fully seated, but there isn't any reason you couldn't make it work if you're handy working with sheet metal in the field.

 

[Lifespeed]

Most duct reducer cones have a short section of straight section on each end sized appropriately to make it easy to attach the duct, often times with a lip to snug-up and seal the duct up to:
Cutting it back a few inches wouldn't have the supportive east-to-connect straight section or the lip to know when it's fully seated, but there isn't any reason you couldn't make it work if you're handy working with sheet metal in the field.

That is not the type of cone used to cut into the side of an 18" trunk line. That simply reduces a larger duct to a smaller one inline, which does not describe my configuration. Hence my comment about posting a photo. The existing cone doesn't have the nice, straight size-specific fitting. The vintage is 1969.

 

[Dana]

Conical duct take-offs usually have similar features at the output end, but I'll await your picture.