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convert downdraft duct to hood duct

9 years ago

I have a downdraft cooktop /w duct that turns sharply "U" under the cooktop and up the same wall cavity. I am interested to get a range hood now because the downdraft is really inadequate for the way I cook.

The wall cabinet above is flush with the soffit, the outside vent and under the cooktop that I can see is all 6" round ducts. Does anyone know if this downdraft stuff turns into a rectangular shape inside the wall?

What I need to do is to find this 6" duct and connect it to the new range I am going to cut open behind this wall cap and above, but if you had seen something similar during remodel, please give me some ideas what to expect, I am not doing a full remodel so I want to limit the amount of demolition needed.

Thanks!! PO.

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Comments (6)

kaseki
9 years ago
By careful measurement of wall distances over a path from one side to the other of the duct wall, it should be possible to determine the thickness of the wall. If it is a standard 2 x 4 stud wall, the cavity is 3.5 inches; if a less standard 2 x 6 wall, then the cavity is 5.5 inches. Neither of these will accommodate a 6-inch round duct, so transition to rectangular is likely. The third possibility is a double wall built to accommodate the round duct. It should be possible to distinguish these even with modest error in measurement.

kas
PoorOwner
Original Author
9 years ago
Thanks..
you seem to be very knowledgeable about kitchen ventilation. Would you know if the round duct that are horizontal inside the soffit, how are they attached to studs or anything in place? for example, if I remove the outlet, would it be possible to pull out the ducting if I cut the duct from inside? I know a lot of ducts in attic are just holding up by the rigidness of the duct itself, but I am not sure about ducts between floors.
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There will be some repair work that will have to be done to the wall to run the duct work if you install a vented hood. Without too much more work the stud could probably be moved. A header would need to be installed to carry the load of the stud, just as would be done with any doorway or window. So I guess it comes down to what amount of work the vented hood is worth. I definitely agree that the downdraft would be better than the non-vented hood. The other thing to consider is the fact that adding 2 more 90 degree turns and the extra ducting is going to diminish the effectiveness of the fan. With that length you may need an in-line fan installed to boost the power. Your appliance provider should be able to help you figure out what you need for the amount of pipe you have. Jesse Pender
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@bcarlson78248 - can spray foam cover the entire 6" duct (i.e. without anything else to fill the gap) @energy_rater_la - the exterior is fiber cement siding. I have posted some pics of the duct/wall cap. I have removed the the old cooktop and some of the metal vents. The one attached to the vent cap is remaining (I think foil tape was used to attach this to the wall cap). I don't see a way to remove this last metal duct without removing the vent cap from outside. I was thinking I will leave it as is (and cover inside with fiberglass/foam) - or see if I can cut using some tin snips to shorten the length. How about using a duct cap on the inside to close it off (like the one in http://www.homedepot.com/p/Master-Flow-6-in-Round-Duct-Cap-DC6/100396933). Outside wall cap: Metal duct left on the inside:
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PoorOwner
Original Author
9 years ago
Here is a pic of my kitchen and soffit. The "real" ceiling where the fluorescent lights mounts are maybe 10" higher inside the light box. I could always run it inside the cabinets and out in the worse case.
kaseki
9 years ago
I wouldn't be able to guess at the duct mounting. The sections might just be sitting on joists. A terrahertz radar imager, like those used by TSA in airports, would be useful here, but likely too pricey. Moderately priced borescopes for mechanics might work. Else you may have to plan on opening the sofit.

I fear you are going to have to do some physical snooping. If you take down some cabinets you can ravage upwards and rearwards and then hide the damage before the cabinets are returned to their places

kas
homepro01
9 years ago
Poorowner,
Why not get a hood which has a back vent? From the picture, it looks like the cooktop is against an outside wall. I would just close up the downdraft ducting and put a short duct behind the new hood and an external vent cap. That would be the simplest and cheapest option in my option. Am I missing something?

Broan, Zephyr and Kobe make back venting hoods.

Homepro01
PoorOwner
Original Author
9 years ago
The cooking area backs to interior, otherwise would be easy like you said
I am going to run the inspection camera to see inside

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