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Old 02-23-2013, 05:03 PM   #1
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Putting The Squeeze On Me!


I'm replacing a downdraft cooktop with a cooktop and pop-up vent combo. We didn't like the all-in-one downdraft cooktop and because we can mount the blower in the basement, it will be a lot quieter and won't take up so much of the under-cabinet space.

The duct exits the vent box at the bottom and comes with a transition piece that goes from 2x16 to 3-1/4x10.

Our house has 24" high 2x4 trusses that are 16" O.C. That means there's only 12-1/2" of space between truss joists. In the pic below you can see the cutout for the 6" duct for the old downdraft. It was towards the front of the cabinet. The new cutout will have to be against the left side of the joist, in between the truss and HVAC duct running parallel with it. The conduit with the ty-wrap on it sits atop a can light I had to remove to access the old duct.


After doing careful measurements, it looks like the 3-1/4x10 end of the transition piece, when it penetrates the floor, will be about 1/2" or more into the truss joist. And there's no touching the joists! At least that's what we were told when the house was being built.

This is what I have to install, with the bottom of the factory transition barely protruding through the 3/4" plywood floor.

With the conduit where it is, there is no room to run the 8" elbow any lower. I feel like I'm working in a sardine factory.

As I was looking at it, I was wondering if I could span something across the space where I'd have to notch for the duct (if I did that), screwed & glued or with plates or some other form of bond. But that makes me nervous.

The other option, and the one I feel much better about, is to make a 2x16 extension to get past the top 2x4 on the truss and then make the transition. Problems with that are:
  • the factory transition piece will be cocked and not seat fully
  • the 2x16 will push all the ductwork down, and it's already tight
  • factory requirements require 8" round as soon after the transition as possible
  • maximum "effective" length is 60' and we are now at 30'. As an example, the factory says a 3-1/4x10 90 elbow has an effective length of 15'. If you transition to 8" round in that elbow, it goes up to 25'. I have no idea what a 2x16 duct adds.
Bottom line is if we modify the ductwork, it will be UGLY at best and I might have to crush some of the 8" round to get it to fit inside the truss joist space. There's no going below the trusses. We have a drop ceiling in place there.

If it were you and this was your house, what would you do? And how confident would you be in your decision?

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Old 02-24-2013, 04:13 AM   #2
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Putting The Squeeze On Me!


Well, I wouldn't even think about notching the truss.

Have a fabricating shop make a new piece up for the factory duct extension to go to 2 1/4 by 12, or 2 1/4 by 14. And then get a transition or have then make one from that size to 8" round.

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Old 02-24-2013, 05:01 AM   #3
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I agree-----have the tin work made up for you-----
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Old 02-24-2013, 01:11 PM   #4
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Putting The Squeeze On Me!


Yeah, the idea of taking a sawzall to notch even a sliver of the truss made me cringe. I guess I'll have to figure a way to solve the dimensional quandary with the duct alone. I haven't the faintest idea who to go to to get that done. All the HVAC contractors I know do large commercial jobs and probably wouldn't want to do something this small.

I'd like to continue the cabinet (33"x2-1/4" on the bottom) down through the floor and come straight out the side of it with 8" round. Then I could get past all the obstacles and put the 8" round at the height I need it at to connect to the blower. The factory literature doesn't give any effective lengths for oddball sizes and they warn if you exceed the 60' mark, you'll have problems.
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Old 02-24-2013, 03:18 PM   #5
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Putting The Squeeze On Me!


There will be a fab shop somewhere near you that will find the time to make up what you need.
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