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TJanak
12-17-2011, 08:21 PM
I need to install 3.25 x 10" duct for the new downdraft cooktop. Do I screw the joints together, or is there some other way the sections join? I read something online about some kind of clips but that sounds kinda flimsy.

Should I tape or use some sealer/mastic?

This will go up an exterior wall. Should I try and use fiberglass batts between the sheathing and duct? Obviously I'll have 1/4" of space to work with.

What about condensation? This will run under the cabinet for a couple of foot and then up the wall and out the roof. There is no where for condensate to drain. Should I just insulate well and hope for the best?

Anything else a ductwork/HVAC newbie needs to know?

Thanks

mtcummins
12-17-2011, 11:29 PM
I am certainly no HVAC pro, so take my advice with a grain of salt...

Generally rectangular ducts have a sort of clip together attachment built into them. This should hold fairly well, but if it feels a bit loose, you can add some sheet metal screws (however, it should be noted that you should NEVER use sheet metal screws in a dryer vent, as the screws inside the pipe will catch all kinds of lint and clog up your vent over time). I tape all seams with either duct tape or foil tape.

Can you not vent this to the side wall that you're going up, rather than going all the way up to a roof? Seems easier and more efficient for heating/cooling purposes.

Cramming fiberglass into a 1/4" space will give you basically the same insulation value as just leaving the space empty. Compressing it like that drastically reduces its insulating value - you want it loose and fluffy in the bay. You could use a thin rigid insulation board or a duct wrap or something, but with that little space, you won't get much benefit. You're better off ducting to the side wall if possible so that you avoid this problem. There are restrictions on how/where you can sidewall vent (distance from windows etc) on a water tank or furnace... I'm not sure if these apply to a cooktop vent... hopefully someone else can chime in on that.

ballvalve
12-18-2011, 01:19 AM
Short pop rivets are standard, and or a TOP quality aluminum HVAC tape, perhaps topped with duct tape.

hj
12-18-2011, 07:27 AM
"S" cleats on the long sides, and "drive cleats" on the short ones, the same as if it were for a heating system. It would take a "lot" of pop rivets to make it even somewhat air tight.

jadnashua
12-18-2011, 02:57 PM
Because the fumes coming off the stove may have grease in it, you don't want the seams to leak. There are some mastics designed to seal flues, but I don't know if they are approved for this application. I know your fan probably has a grease filter, but over the years, some will likely get by and into the duct. This can be a fire hazard, but is more common in commercial kitchens. I'd call the manufacturer of the vent and ask them their thoughts.

TJanak
12-19-2011, 05:37 AM
Didn't know about the "S" and drive cleats, after looking them up that sounds like a good, secure connection. Looks like I will use foil tape and/or mastic over the joints. Just have to get a good approved mastic.

Thanks

ballvalve
12-20-2011, 11:32 AM
I bent a million s's and drives in my youth. Now its all about tape and laps. A good aluminum tape really is incredible stuff.

s's and drives give an internal lip like a rivet or much worse a screw, and do not do the corners. So no good solution. Be sure to insulate well to try and keep the grease in suspension.

Built a community center in Alaska. Sub forgot to insulate the duct from the hood though the attic... plans actually didnt show it. Condensation filled the hood and burned the place down a year later.

hj
12-22-2011, 05:24 AM
quote; Condensation filled the hood and burned the place down a year later.

Huh? Condensation is water which seldom burns.

ballvalve
12-22-2011, 11:51 AM
That was the story. No details. It wasnt my contract luckily , I was just the gunner, doing the detail mechanicals and keeping the drunken indians working. But the electricals in a big hood can short out and arc and start a fire.

Actually I had installed a phone dialer and detectors, so the place stayed up. Fire burns slower when its -45 outside. Try hanging steel siding at that temperature. Great fun. lived in a motley mobile where the diesel furnace NEVER shut off. Couldnt put a drink on a windowsill, would freeze in a few minutes. Odd to see all the cars in a market lot parked with motors running.

Dana
12-22-2011, 01:49 PM
Condensation is anything that enters the draft hood in vapor or gas form, and liquifies or solidifies when the wall of the duct is below the temperature at which that material is in gas form. In a whiskey still a lot of that condensate is ethanol, in an oil-fired heating appliance flue it has all SORTs of goodies.

But in range hood vents the flammable culprit is usually fats/oils caking into a thick film of grease on the cold duct walls, then dripping a flammable stream of liquid oil into the range top flames at some point when it's warmer, going off like a flame-thrower. Grease fires are less likely in a downdraft cook top, (it can't drip up against gravity) but it's still better to keep the grease from building up in the duct by insulating it in cold climates.

TJanak
12-22-2011, 02:55 PM
Dana,

How would I insulate the 1/4" gap between the duct and exterior sheathing in the wall? There is brick outside of sheathing. Also, what would be ideal to use in the attic space?

I have ideas but often times my ideas are unconventional.

Dana
12-23-2011, 10:19 AM
I'm not sure if there are any code restrictions on what can come in contact with a range hood duct. A flexible low expansion foam designed for windows & door framing (eg. Dow "Great Stuff Window & Door" ) would probably make the nicest air seal for your 1/4" gap in the exterior sheathing. Fiberglass is barely more than an air-filter in that type of application, and most caulks would likely fail (and quickly.)

R8 batts (designed for 2x3 framing) wrapped and taped around the ducts would be enough. Alternatively kraft faced R11 or R13 batts would be fine (but overkill.) If you use unfaced batts you'd have to give it an air-barrier wrap of Tyvek or something for the insulation to perform, on the other hand it's not a very high-performance application- it just has to have sufficient R to let the walls of the duct to come up to temp quickly when the blower is running.

TJanak
12-27-2011, 05:39 AM
Thanks! Looks like I've got a project for this week.

BobL43
12-27-2011, 08:13 AM
quote; Condensation filled the hood and burned the place down a year later.

Huh? Condensation is water which seldom burns.
must have been firewater:p

DonL
12-27-2011, 08:30 AM
must have been firewater:p

Sounds like Dana was on the right track.


We have that Firewater here in Texas, Because of Fracking.

I wonder how much a bottle would be worth.

Who knows it may become the fuel of the future...

BobL43
12-27-2011, 08:38 AM
Sounds like Dana was on the right track.


We have that Firewater here in Texas, Because of Fracking.

I wonder how much a bottle would be worth.

Who knows it may become the fuel of the future... At what cost to the environment or safety? Take a drink of water, light up your Marlboro, and POOF the majik dragin

DonL
12-27-2011, 09:07 AM
At what cost to the environment or safety? Take a drink of water, light up your Marlboro, and POOF the majik dragin

lol


The Gov allows Fracking®, Marlboro©™ smoking, So its got to be good.

As long as you pay your Sin Taxes and Fees it must be safe for you and your environment.


I like Dragons...

dlarrivee
12-27-2011, 09:49 AM
Short pop rivets are standard, and or a TOP quality aluminum HVAC tape, perhaps topped with duct tape.

Where did this little gem come from?

Duct tape on top of other tape? Duct tape doesn't last.

Rivets?

ballvalve
12-27-2011, 11:16 AM
Shopping at harbor fright? RATED duct tape lasts, under or over RATED aluminum tape. Not the crap from home depression and Asia. Short pop rivets are gems too. Seems like no one uses them much anymore.

those million miles of flex ducts are held together only with duct and aluminum tape. Maybe some mastic for the few honest and careful installers.

tape for real men: http://www.grainger.com/Grainger/NASHUA-Duct-Tape-6A061?Pid=search

http://www.grainger.com/Grainger/SHURTAPE-HVAC-Tape-10A411?Pid=search

And a 300 degree tape:

http://www.grainger.com/Grainger/NASHUA-Extreme-Temperature-FSK-Foil-Tape-10A994?cm_sp=IO-_-IDP-_-RR_VTV70300505&cm_vc=IDPRRZ1

dlarrivee
12-27-2011, 11:28 AM
I use foil tape, and a LEED compliant mastic, 1/4" screws... I've never heard of using rivets on small stuff like this...

I have used a pin welder to insulate larger duct.

I have never used 2 different tapes...

ballvalve
12-27-2011, 12:30 PM
I remember the pin welders. have one somewhere. But aluminum tape over duct tape seems to be double endemnity.

With a GOOD drill bit, pop riveters are fantastic for many things, and the inner projection is much less. Ask Boeing!

dlarrivee
12-27-2011, 12:36 PM
I don't have a problem with the concept, I just don't see it being used where I work.