Range hood vent: rigid w/ elbows vs semi-rigid

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Too Ambitious

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There is an old wall cap for a range hood vent in my house. Seems like the range location was moved when the kitchen was redone, and they installed a ductless hood.

I am installing a new ducted hood and would like to use the old cap.* It's at most a 5' run of duct total, but if I used rigid I would need to use 3 90°s (2 of them back to back) since I need to jump over a joist bay.

It needs to go up into the cabinet, 90° over to the right, 90° up into the joist bay, then 90° out to the vent cap.

Should I go with rigid w/ 3 90°s, or a continuous run of semi-rigid?

(I know ideally I'd just go straight out the back of the hood through the wall w/ a 10x3-1/4 rectangular duct, but this is just a kitchen facelift rather than a full reno. I don't want to make another penetration in the exterior wall especially considering it's brick.)
 

wwhitney

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The path you describe with (3) 90s, up, to the right, up, and then out, can be done with (1) 90 and (1) 45 (say). Going backwards from wall cap, you'd use a 90 between the joists, with the inlet pointing down and to the left (so 45 degrees off plumb), lined up with the vent hood as far as the distance off the wall. Then you'd use a 45 to turn down vertical directly over the vent hood.

This assumes that you have clearance in the joist bay to rotate that 90 so the inlet isn't straight down. And it assumes you can fit that path through the cabinets without too much disruption.

If you use the adjustable elbows, you can get easily get anything from 0 to 90 degrees. So if 45 is too shallow (hits a joist), you could roll the 90 down closer to plumb and use a smaller angle lower down. Regardless, once you have those elbows adjusted to where you want them, they are 4 different segments, so you want to seal (with foil tape or duct mastic) each of the 3 internal joints, as well as the inlet and outlet joints.

Cheers, Wayne
 

Too Ambitious

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The path you describe with (3) 90s, up, to the right, up, and then out, can be done with (1) 90 and (1) 45 (say). Going backwards from wall cap, you'd use a 90 between the joists, with the inlet pointing down and to the left (so 45 degrees off plumb), lined up with the vent hood as far as the distance off the wall. Then you'd use a 45 to turn down vertical directly over the vent hood.

This assumes that you have clearance in the joist bay to rotate that 90 so the inlet isn't straight down. And it assumes you can fit that path through the cabinets without too much disruption.

If you use the adjustable elbows, you can get easily get anything from 0 to 90 degrees. So if 45 is too shallow (hits a joist), you could roll the 90 down closer to plumb and use a smaller angle lower down. Regardless, once you have those elbows adjusted to where you want them, they are 4 different segments, so you want to seal (with foil tape or duct mastic) each of the 3 internal joints, as well as the inlet and outlet joints.

Cheers, Wayne
I see what you’re saying. The only way to make this work would be to partially cut away the corner of the cabinet (top and side, not the back). This wouldn’t affect the integrity of the cabinet box, would it?
 

wwhitney

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This wouldn’t affect the integrity of the cabinet box, would it?
Not enough detail to answer the question. A scaled elevation showing everything (including cabinet side walls and the joist you have to get around) should suffice, assuming nothing unusual is going on in the dimension perpendicular to the wall.

Cheers, Wayne
 

Too Ambitious

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Not enough detail to answer the question. A scaled elevation showing everything (including cabinet side walls and the joist you have to get around) should suffice, assuming nothing unusual is going on in the dimension perpendicular to the wall.

Cheers, Wayne

FD55EBA7-15E0-4022-89E8-603C689B8BF0.jpeg


The joist bay w/ the outlet is directly above the slim cabinet to the right. I have the elbow laying in the opening right now.
 

wwhitney

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In that situation, I could see either trying to drill the corner of the cabinet, or just going with a 45-45-90 solution so that you pass through the sidewall of the cabinet only.

To drill the corner of the cabinet, you could make up a drilling guide that you temporarily mount in the corner, e.g. out of a diagonally ripped piece of 4x4. If the cabinet seems unsturdy afterwards (not sure what the cabinet construction is), you could use some diagonal blocks in front of and behind the corner opening to reinforce the top to side connection.

To go through the sidewall only at a 45 degree angle, it's probably easiest just to drill two overlapping circles and connect them to make a racetrack shape.

Cheers, Wayne
 

Chuyue

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I'm just a DIYer. I'd think semi-rigid flex would be much easier to install. Does it really make a whole lot difference in terms of pressure drop if rigid one is used considering only 5ft duct run? I'd go with flex having one size bigger if pressure drop is a concern but then two reducers will be needed at each end to fit with existing outlet and hood.
 

Too Ambitious

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I'm just a DIYer. I'd think semi-rigid flex would be much easier to install. Does it really make a whole lot difference in terms of pressure drop if rigid one is used considering only 5ft duct run? I'd go with flex having one size bigger if pressure drop is a concern but then two reducers will be needed at each end to fit with existing outlet and hood.
Yeah I’m coming to that realization. I’m a little stubborn and like to do things the best way, but this is turning out to be more trouble than it’s worth. Maybe if it weren’t a retrofit…
 

Too Ambitious

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I ran the semi-rigid this morning. I'm now torn on this. Considering a hybrid approach. Rigid in the cabinet, and semi-rigid to make the complex turn around the corner.

Just want to minimize noise & grease build up.

edit: Never mind. I'm going back to the 3 elbow approach. Just flushed $20 down the toilet on that semi-rigid duct. Maybe I'll find a use for it some day. Robot costume???
 
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Too Ambitious

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Wasn't easy. Isn't pretty. But it's done. (Well, almost. I still need to fasten it to the hood. Need a second set of hands to help plumb it. Also replace the wall cap, insulate, and close up the ceiling as best I can.)

Just want to give you all the sincerest thanks. I really appreciate that this place is friendly to DIYers.

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