Supporting round duct through a wall

Fabrik8

Fabrik8

Overcomplicator
Joined
May 27, 2015
Location
Huntersville
Yet another of my strange questions about best-practices for building stuff....

Bathroom fan ducts exiting house wall to wall caps, two of them next to each other (for two different bathrooms).

There will be a mounting block for the wall caps, and the wall caps slide into the rigid duct. The rigid duct will be supported at the mounting block. The rigid duct will be fairly short (through the 2x6 stud wall, and then coupling length), and then a piece of flex duct will connect from there to the inline fan.

So: How do I support the other end of the rigid duct where it exits the wall on the interior side? I want to do 2 points of support, so the exterior wall block would be one point, and I want another point. Basically I don't want to only support the duct at the wall cap side, and have to worry about breaking the sealant because of relative motion if the flex ducts needs to be serviced or reattached.

Should I just make a plywood/OSB plate that spans across 2 studs and has a hole for each duct?


Less important background info:

Cosmetics aren't important; I'm making a shallow mechanical space behind the back of a linen closet. I'm assuming the finished back of the closet will satisfy the vapor barrier requirements, etc., and the insulation will be in the exterior wall studs per normal practice. Both fans will be in the mechanical space.

The duct runs will be really short, as one bathroom shares a wall with the closet (cathedral ceiling from the first floor) and the closet is inside the other bathroom. Maybe 5-6 feet total duct length from each shower to the wall caps, including the inline fans.

I could actually make the ducts all or mostly rigid, but I know the flex duct would be quieter (but slightly more flow restriction). Could use rigid and put a flex coupler between the shower and fan also...

Going with mostly rigid seems like a pain in the ass to couple together with such short duct runs...



The fans aren't in or above the shower enclosure, so happily no requirement for GFCI protection.
 
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You're overthinking it

51LzZvXRFBL._SX355_.jpg


Spotlight_ducts_4C_PR0116.png
 
Either wood or sheet metal. If you plan on drywalling over in interior surface where the twin hard ducts come through, a flat sheet metal span between studs, bored for the 4" rounds can be secured to the studs themselves with roofing tacks and won't stick out past what could be sheetrocked over.
I run into a parallel scenario all the time building custom dryer vent hard runs to wall connections.
 
@CasterTroy support spacing on 6-10" flex duct is typically more like 10-15ft, though, with a 1-2ft sag between supports.
 
I am probably overthinking it...
CasterTroy said:
You're overthinking it

Spotlight_ducts_4C_PR0116.png
Click to expand...

That's what I want the flex duct in my crawlspace to look like, I've seen that picture before. That's on the list, along with replacing all of the deteriorating flex duct. I can't see it in the crawlspace, but it will make me happy knowing that its done properly in the dark under under the house.
 
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shawn said:
@CasterTroy support spacing on 6-10" flex duct is typically more like 10-15ft, though, with a 1-2ft sag between supports.
Click to expand...

You have lower standards than I do :flipoff2:

Support duct as per SMACNA "HVAC Duct Construction Standards,” Tables 4‑1 through 4‑3 and Figures 4‑1 through 4‑8

Flexible ductwork shall be supported at MIN 5'-0" intervals (regardless of sizing)
 
XJsavage said:
Either wood or sheet metal. If you plan on drywalling over in interior surface where the twin hard ducts come through, a flat sheet metal span between studs, bored for the 4" rounds can be secured to the studs themselves with roofing tacks and won't stick out past what could be sheetrocked over.
Click to expand...

It won't be drywalled, because the rear of the closet will take care of that. That's why I was thinking of plywood or OSB scraps. I've got a router with an circle jig, so I can friction fit if I really wanted to.


The fans will mount to the studs, at least that's the plan at the moment.

I think it's the least interesting thing I can do with a secret compartment behind a closet though. It would be better if it was guns, stacks of drug money, or a dead hooker wrapped in plastic.
It just isn't exciting to say "I have a secret compartment in my house, and it's full of... ...bathroom fans".

XJsavage said:
I run into a parallel scenario all the time building custom dryer vent hard runs to wall connections.
Click to expand...

That's actually the other weekend project, I have a new wall cap and semi-rigid hose kit to install. The drywall is already busted up, so at some point I'll have to repair it and close out the opening properly. Right after I get a laundry room door that isn't slightly warped and leaky.
 
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Fabrik8 said:
That's actually the other weekend project, I have a new wall cap and semi-rigid hose kit to install.
Click to expand...
Be careful with semi rigid hose. Sections from the back of the appliance to a wall intrusion is fine under 4'. Never use anything but hard 4" pipe/elbows inside a wall cavity, and make damn sure it can't ever come loose and spray hot dry air and lint into a closed wall cavity next to live electrical.
 
XJsavage said:
Be careful with semi rigid hose. Sections from the back of the appliance to a wall intrusion is fine under 4'. Never use anything but hard 4" pipe/elbows inside a wall cavity, and make damn sure it can't ever come loose and spray hot dry air and lint into a closed wall cavity next to live electrical.
Click to expand...

The dryer wall cap has a 8 or 9 inch long section of rigid tube permanently attached, which I why I got it. That should make things easy and will move the connection to inside the room for safety. And it's a Seiho cap, so it looks really cool.

Putting house jewelry on the house while fixing things makes me feel happier about the amount of stupid shit that needs to be done to the house.
 
CasterTroy said:
Support duct as per SMACNA "HVAC Duct Construction Standards,” Tables 4‑1 through 4‑3 and Figures 4‑1 through 4‑8

Flexible ductwork shall be supported at MIN 5'-0" intervals (regardless of sizing)
Click to expand...

I didn't say what SMACNA specifies... I said what is "typically" done. ;)
 
I'm going to support the flex duct under the house according to the minimum, and then as much extra as it needs to be well supported and look pretty like the picture above. Webbing is cheap and quick to install.

You guys keep arguing about minimum standards though.... :D
 
Speaking of dryer wall caps, mockup is done. Blew everything apart and primed it all, assembly and sealing tomorrow. Too bad I wasn't able to move it down about 3/8" and not have to notch the upper piece of siding. Minor aesthetic details, no biggie. Whoever cut the hole for the original wall cap was a dumbass; the inner and outer holes were about an inch apart horizontally. Didn't want to do a lot of drywall work so I just moved the hole on the outside when I cut the siding.

Gotta fab up some drip flashing tomorrow morning.

WallCap.jpg
 
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