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Discussion Starter #1 (Edited)
I'm about to get deep into a basement renovation project that I started work on last year and left alone for a while. The basement was very poorly finished by the previous owners and I've ripped out a lot of what was there (and more will get ripped down shortly). One major source of indecision I have is on how to treat the supply and return trunks that run along one of the exterior walls. I had originally decided that I would box them in as-is, but then my gears started turning.

The house is fairly old and is ducted a bit strangely (but does stay perfectly comfortable year-round). The main (and pretty much only) return is right by the front door. The 8x24" trunk runs from the furnace room, close to the exterior wall, to the gigantic intake vent in the floor. The more interior 8x12" trunk is a supply and feeds a small 2nd floor bedroom, 2 bathrooms, and one of two living room vents. Another very short 8x12" trunk within the furnace room (not pictured) feeds the rest of the house.



Here you can see the return swoop up into the giant floor vent.


See that 6" supply duct popping up and heading away between the joists on the left side? That goes up to a vent in the 2nd floor hallway AND to a vent in a small bedroom which already has another supply. I'm thinking whoever installed the system screwed up and these vents were actually supposed to be returns, since there are no other returns on the 2nd floor. I'm thinking I could easily change that run to connect to the return trunk.


To the furnace room, I replaced the crappy bifold louvered furnace room door with an exterior door and added a layer of 5/8 drywall with green glue to that wall. As you can see I had a little fun trying to fit a full-height door between the existing ducting.(I'm fully aware of the air supply implications of sealing off the furnace room and am dealing with that with outside supply air)


The big return trunk takes up a lot of headroom and is a soundproofing burden. It also inhibits me (but perhaps doesn't fully prevent) installing an egress window in that wall. My big IF, right now, is what IF I moved the big return vent all the way from the front door to the other side of the living room, making it so the big swoop is within the furnace room (practically right above the return drop) and that whole trunk disappeared. The downsides are: I'd have to patch/match some very old oak flooring, I'd probably hear more furnace noise in my living room tv area, I would no longer have that return trunk to tap off of for the 2nd floor and basement returns, and the new return vent would also be very close to the fireplace. Having the supply trunk there is pretty unavoidable (in fact, I think I'll need it to add a couple basement supply vents) so it's not like I'd win my whole ceiling back. Would any effort at simply making the ducts narrower and wider be worth the trouble? I guess I could move the ducts closer together.

Anyways, since a lot of folk here deal with these kinds of augmentations (I've read through a few ducting threads already) I'd love to hear your thoughts.

This space will be used as office space at first, then probably become a bedroom one day, or possibly a second family room type area. I plan on building it with bedroom in mind. I'll definitely be putting drywall/gg up against the subfloor.

Thanks!
 

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Discussion Starter #2
A bit more information: I haven't fully decided what I'll do for the ceiling, but it'll probably be a drop something. I might end up making my own tiles, who knows. I'm not sure I can accept not being able to access my utilities - I change/add wiring way too often ;)

I hope to have most sound issues snubbed by my drywall + gg + insulation against the subfloor, and boxing in of ducting. Whatever ceiling I add beyond that hopefully won't need to have superior soundproofing qualities.
 

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Make sure you have combustion air for your furnace now that you've sealed up the room.

Nothing prohibiting you form moving the return.. They usually put them central to the house so it draws air relatively uniform from the rooms.

As you noted, noise will be a concern.

Tim
 

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Discussion Starter #4
Yes, as noted in the original post I'm already aware of the air supply implications.

I spent some time walking through the basement today, looking at where i would route various supply and return lines if I did or didn't move the trunks. Putting headroom aside for a moment, I'm thinking moving the truck line(s) would probably cause more sound problems than it would solve. Instead of having to box-in the trunk I'd end up with some very short ducts popping right off of the furnace and going directly into adjacent rooms. I'm thinking that furnace noise and noise shared between rooms will actually be worse this way. I'm starting to see the long trunks as an asset which I can use to obfuscate and diminish sound over a distance of varying duct size/type.

Comments?
 

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Discussion Starter #6 (Edited)
Thanks for the link. I do already plan to use insulated flex duct on each leg. I've already done so on other parts of the house and the difference is remarkable. Even just stretching out a length and trying to talk through it with a friend gives you a good idea of what it does.

I don't think I need to go as far as building dedicated mufflers as suggested. In a way I feel that I would already have that since there would be no constant cross-sectional path from one room to another. Sound would first travel through a 6" round duct, then through a much larger trunk (3-6 times larger), then another 6" duct. In many cases you add a 3x14" rectangular duct into the line.

Anyways, the main question is whether I want to consider moving the trunks to save headroom and possibly improve soundproofing. As mentioned, I'm leaning toward keeping the runs as they are since I feel like shortening them would likely make the sound problems worse, and at best I'd only be getting back SOME of my ceiling, not all.

One compromise I could consider is kicking the smaller 8x12" supply to be right next to the return, rather than having that big space between them.

Then, if I'm not moving them I need to box them in. The question then is what is the least-bulky build style. Do I need to worry about framing/drywall touching the ducting (squeaks?). Should I wrap the ducts in something first? How would I box in the tops of the trunks if they're installed against the joists?
 
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