How far can i run hood ducting?


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Old 03-24-09, 08:25 AM
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How far can i run hood ducting?

I need to run ductwork for my stove hood. I've never done it before so i dont know the "rules". Once the duct gets to the ceiling (will be about 4feet from the bottom of the vent or so) it needs to 90 to run parrallel to my floor joists for 12 feet and then 90 again to run another 12 feet to get to an outside wall.

Is that too far to run 6" ducting?
 
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Old 03-24-09, 05:20 PM
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Obviously the best route is a straight vertical to a roof cap. Each 90 degree bend slows the air flow. Yes, you can do it, but I would be looking at a larger cfm hood fan than you are probably figuring on.
 
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Old 03-25-09, 09:22 PM
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do not forget to figure in the rise of the pipe along the way. You need rise on your horz. run for proper ventilation .
 
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Old 04-20-09, 02:52 PM
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I am looking to do about the same thing with pretty similar runs! Plan on using microwave with built in 300 CFM fan. I think instructions say that it is OK to have run up to 120 equivalent feet of 3 1/4" x10" or 6" round duct. If you are using a transition adapter (to go from 3 1/4" x 10" to 6" round) that counts as 5'. The straight runs simply count the number of feet. For each 90degree 6" elbow, you have to count them as 10'. And the wall cap for 6" round duct counts as 40'! So if you have 4'+12'+12' runs with 2 90 degree bends, a transition adapter and a wall cap, that would equal 93equivalent feet--28+5+10+10+40. I plan to use 3 1/4" x 10" duct as it fits in joists better (to give rise) and it will fit in the 2x4 wall that it will have to go in once it hits the wall where it will eventually exit. I wish I could help more, but now you are at same point as me, wondering if your run is going to be OK. my equivalent feet is more because a 3 1/4" x 10" 90 degree elbow counts as 25'. My run ends up 112 equiv feet. Buy the numbers, 93 and 112 are less than 120. I am concerned that once I make the second 90 and start up, part of that vertical run is in interior wall. But once it gets above a shed roof line, it will then be exterior and since the duct will take up most of the stud bay, I won't be able to really insulate it. Therefore, I am concerned about condensation! I was hoping someone else would have more to offer.
 
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Old 04-20-09, 03:14 PM
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actually, another place in instructions said OK to have up to 140 equiv feet. and another model of microwave hood, also with 300CFM vent, says up to 140 equiv feet is OK.

In my application, I suspect best to limit the length of run that is in unisulated wall. So I guess I should make the 90 and exit maybe a foot or so above the shed roof (need to allow some space for snow accumulation!) any input welcome
 
 

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