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Discussion Starter #1
I'm looking to add a jump duct (aka transfer grille) in the second bedroom to help with airflow. I don't want to install it over the doorway frame due to sound and light leaks and there's no going over because there's no attic access. There is an adjacent closet so running it through there seems like the best option.

The duct will start above the closet door, turn 90 degrees then empty into the hallway where there are return vents on the other side. Ideally it'll connect directly to the return vents but that would require installing duct over stairs which makes this far more difficult.

See diagrams and pic below.
Blue = new duct
Red = new frame and drywall to conceal duct
Green = existing drywall

Top view

f084355aeab248a79612dad8090fc65e.png

Pic of inside of closet

dfc64d8b97a44cf1b1a8135e64b98f8d.png

Questions:
1. Given how close the walls are, can the two register boots be connected together directly if the measurements work out?
2. If the boots are too far from each other, can metal uninsulated ducts be used since this is in the closet and not going through the attic? Something like https://www.homedepot.com/p/Master-Flow-3-in-x-2-ft-Round-Metal-Duct-Pipe-BCP3X24/100129266
3. After the new duct is installed, it'll get framed out and new drywall will be added to cover it. Before doing so, does the existing drywall need to be removed down to studs or can it be left as is? See pic above marked with green x's
4. Other ideas or options?
 

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The Home Depot link you provided shows 3" round duct, definitely not worth the effort with that size. I would think 7 or 8 inch round would work better.
As far as options go, you could cut transfer grilles in the bedroom to closet wall and closet wall to hallway (4 grilles needed) and use the closet itself as a "plenum". No drywall or framing needed. Or you could just box in the area shown in the closet and install a grille in the hallway and bedroom (2 grilles needed), no ducting needed as the box becomes the return duct. Might be some code issues to consider if you think you will have it inspected.
 

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Discussion Starter #3
The Home Depot link you provided shows 3" round duct, definitely not worth the effort with that size. I would think 7 or 8 inch round would work better.
As far as options go, you could cut transfer grilles in the bedroom to closet wall and closet wall to hallway (4 grilles needed) and use the closet itself as a "plenum". No drywall or framing needed. Or you could just box in the area shown in the closet and install a grille in the hallway and bedroom (2 grilles needed), no ducting needed as the box becomes the return duct. Might be some code issues to consider if you think you will have it inspected.

I picked the first duct I saw as an example but that's good to know +7" is better for this case.



Great idea about using the closet as a plenum! Not having to frame and drywall definitely makes it easier. Is there any chance it's not as effective compared to a dedicated duct or the box plenum option? Would forcing the air to go through the closet cause it to lose too much pressure for it to be pushed into the hallway?
 

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Probably not enough difference to notice. Most important thing is to size your return grilles properly. Don't know your supply air cfm to the bedroom but let's say 150 cfm. From the chart below (supplyhouse.com) look at the face velocity on top, keep it at 400 fpm or less, find your cfm and corresponding grille size. For this example a 10x8 grille will work. However you have 2 grilles per opening so I would probably go up to 12x8 grilles. How many supply registers are in the bedroom area and what size round pipe feeds them? Include bathroom too.

https://s3.amazonaws.com/s3.supplyhouse.com/product_files/43177-Engineering.pdf
 

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Use the biggest grill the existing wall framing will permit (could be a 20x20)! The more return area you have, the more effective this "virtual" return will be ("virtual" because it's not connected directly to the return ducting, like the other bedroom appears to be).
 

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Discussion Starter #6
Probably not enough difference to notice. Most important thing is to size your return grilles properly. Don't know your supply air cfm to the bedroom but let's say 150 cfm. From the chart below (supplyhouse.com) look at the face velocity on top, keep it at 400 fpm or less, find your cfm and corresponding grille size. For this example a 10x8 grille will work. However you have 2 grilles per opening so I would probably go up to 12x8 grilles. How many supply registers are in the bedroom area and what size round pipe feeds them? Include bathroom too.

https://s3.amazonaws.com/s3.supplyhouse.com/product_files/43177-Engineering.pdf



There's one supply in the bedroom. The register grille is 10x4 and the duct is 6" diameter (the round end of the boot). There's also a supply in the bathroom of the same size however we keep both that register and door closed in an effort to get more air into the bedroom.


I'm not sure I follow the chart but as long as the return grille is bigger than the supply grille, it should work right?
 

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Actually, keeping the bathroom door and supply register open would help cool the bedroom. So, assuming an average of 100 cfm per supply you have a potential flow of 200 cfm, the bathroom probably doesn't have that much but best to be conservative. Looking at the supplied chart and using 4 grilles I'd go with 12x10 or 12x12 grilles, or any of similar size that will fit your framing. Note that the larger grilles have a lower face velocity and pressure drop. How does the bedroom cool if you leave the hallway door open, better? No change?

4RA.png
 

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Discussion Starter #8
Actually, keeping the bathroom door and supply register open would help cool the bedroom. So, assuming an average of 100 cfm per supply you have a potential flow of 200 cfm, the bathroom probably doesn't have that much but best to be conservative. Looking at the supplied chart and using 4 grilles I'd go with 12x10 or 12x12 grilles, or any of similar size that will fit your framing. Note that the larger grilles have a lower face velocity and pressure drop. How does the bedroom cool if you leave the hallway door open, better? No change?

View attachment 605993

This bedroom is actually fine with cooling and it's the other bedroom with the problem. I'm hoping that by adding these ducts, I can get some of the cool air to recirculate back into the house. Might not be much but putting square holes in drywall is an easy change.


Keeping the bathroom door open isn't really an option due to having a toddler who sometimes roams around on her own at night.
 

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Discussion Starter #9
Is there a way to seal the drywall edges and keep out the dust and dirt from between the walls from blowing into the return airway? I think something like this which can connect the the opposite grilles of a wall but not as expensive (this is $50 at home depot).



transfer.png


Normally this would be done by the register boot but most of the boots I've seen are shaped to connect to a round duct. I suppose I could just buy one and cut off the round side? Or buy some metal and bend it myself?
 

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Easy enough to make some sleeves from some aluminum flashing or similar. I think the bigger concern is that you are doing this to possibly improve the cooling in the other bedroom. I'm pretty sure you won't see much improvement there. But the bedroom in question will improve. As far as under cutting doors. personally, I don't like the look. How effective is the undercut door as a return path? Below is some "science" discussing this method.

https://www.energyvanguard.com/blog/can-door-undercuts-work-as-return-air-pathways
 

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Easy enough to make some sleeves from some aluminum flashing or similar. I think the bigger concern is that you are doing this to possibly improve the cooling in the other bedroom. I'm pretty sure you won't see much improvement there. But the bedroom in question will improve. As far as under cutting doors. personally, I don't like the look. How effective is the undercut door as a return path? Below is some "science" discussing this method.

https://www.energyvanguard.com/blog/can-door-undercuts-work-as-return-air-pathways

I understand this change won't make much of a difference for the other bedroom and planned on making a separate thread about that.



I don't like the undercut door look either. Aluminum flashing is exactly what I need. Thanks!
 

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Great idea about using the closet as a plenum! Not having to frame and drywall definitely makes it easier. Is there any chance it's not as effective compared to a dedicated duct or the box plenum option? Would forcing the air to go through the closet cause it to lose too much pressure for it to be pushed into the hallway?
Not quite sure 57's comment was fully understood here. What I read his comment to say is that your sketch shows you framing and walling up a soffit in the closet ceiling to cover the elbow duct, but that you don't really need a duct as the walled box creates a duct. Not that you're using the entire closet as a plenum.
 
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