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Ductwork For A Popup Vent

1379 Views 10 Replies 3 Participants Last post by  JulieMor
We're planning for a new cooktop with popup ventilation. We'll have to start with 3-1/4x10 duct and run it down into the basement. Then a 90 elbow and a transitional piece to 8" round. From there it goes to the blower unit (600CFM) then to the outside where we install a wall cap.

Do we need heavier gauge pipe than what's sold at big box stores?

Should we tape the joints? I have some that's reinforced and some plain. Or should we use duct seal?

Are the transition piece and cap big box parts or will need to start searching elsewhere? I've found the transition online for about $30 but have yet to find the cap. Any ideas where to look if HD doesn't carry them?

Thank you,

One other question, for the wall cap we want it with a damper that actually works, in other words, one that closes and stays closed when the blower is off and doesn't flap in the wind or allow critters access. Any suggestions on a brand?
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These are the CAD drawings I did in wireframe and realistic. If they help at all...

Looking down:

Looking up from the basement:
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Thanks beenthere. I was reading the fine print last night. If I did that first, I wouldn't have had to ask. Isn't that they way it always is? But it's always reassuring to get a second opinion.

I found I have a maximum of 60' from the popup to the wall cap, to meet mfg. requirements. A 90 elbow that's 3-1/4x10 to 8" round counts for 25'. A 3-1/4x10 90 elbow counts for 15'. A straight 3-1/4x10 to 8" round counts for 7'. An 8" elbow is 7' The wall cap depends on the mfg.

I searched the Internet and just when I was almost at the end :laughing: I found what I think is the best option.

They are American Aldes wall caps - (top) heavy gauge G90 galvanized steel, (middle) 314 2B finish stainless steel and (bottom) solid copper type Ca 110. The specs say the damper has a magnet to keep it closed when not in use. Aluminum is $62.89, SS $237.50 and copper $231.75. So aluminum it is.

The design on the above tells me it creates less air drag that the standard design where the top of the outside section is tight against the wall.

I'm redesigning the duct run to change over to 8" round as soon as the 3-1/4x10 passes through the floor. That will create a total of 11' effective length on the fittings. The way I drew it up it would have been 22'. Then there's another 14' of straight run, plus the wall cap. So it will easily clear the 60', even if the wall cap adds a bunch.

And yes, the fine print says to tape the joints. Do I get my tinner's card now? :whistling2:
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I made THAT piece myself (above the coil housing). Can I have my tinner's card now? Huh? Can I? :no:

Maybe I should just go to bed...:sleep1:
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Gee, only four months later and the vent was finally ready to be finished. We had a problem with the first pop-up we got and had to wait for a replacement to arrive. And a hundred other things got in the way too. But looming deep inside me was a serious aversion to tackling this job because I knew it was going to be a real bear, considering the mfg. requirements and the existing conditions.

This is what I started with: The hole on the right is where the old 6" duct came down. My new space to work with would be on the left of the joist. The 6" duct was removed but there wasn't enough space for the new 8" to run in the same space so I had to drop it down to the bottom of the trusses.

Three days ago I began the project. The pop-up cabinet had a 2x16 opening in the bottom so I had to make a 2x16 piece long enough to penetrate through the floor. Then install the included 2x16 to 3-1/4x10 transition piece. Because of existing ductwork I had to mount the fan motor almost immediately. So I had to fabricate a 3-1/4x10 kick to get it over to where the 3-1/4x10 to 8" round transition piece would be. With the drop ceiling grid, the ductwork, the conduits and only 12-1/2" of space between the joists, I was a sardine. I couldn't see some of what I was working on.

Yeah, I know, it hurts your eyes, but this was the best I could do, considering my frustration level and the fact Murphy kept popping his head in, screwing up my plans.

When I went to put the fan motor up into the ceiling I found the smallest dimension of the housing was 13", 1/2" larger then the space between the joists. So I had to drill out the 8 spot welds that held the bell end on and chisel it free from the housing. Then I drilled 8 holes for 1/4-20 screws that would be used to remount the bell end once I had the housing into the joist space.

Another look at the fan motor & ugly transitioning.

This is looking away from the fan motor towards the exterior wall. The ducts above prevented me from installing the fan motor further down line.

The final transition before exiting to the hood outside.

I'm not proud of the workmanship. I really wanted to do a better job but I never expected this thing to be so hard. Three frustrating days and one brand new $150 Festool hose with a gaping hole that I melted open when I pushed a 500w quartz work light into the hose. I almost lost it when I saw that.

Anyway, it's done and it works. I just wanted to share this with you guys because I know you know what I was up against. Don't be too hard on me. :turned:
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Thanks guys!

You're right Bill, those lights ARE nothing but trouble. The only time I ever really appreciated them was when I used one to heat a slice of pizza while on the jobsite. It actually worked very well. But maybe it's time to look into LED.
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