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Which of these two options would provide the greatest airflow. Hood fan is a 300CFM Vent-a-hood. 6" flange from the hood. All ducts are 6"

I have a major constraint which is a gabled roof connecting to the homes roof right above where the range hood is; straight up isn't an option. Straight out 90 degree would be putting it in a sun room, also not an option. So these are my two options. Both have their pros and cons installation and aesthetics wise, but I am wondering which of the two would be better for air flow.

Thanks in advance.

Options for ducting

Jon
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  • Even if you get the correct answer, It may also be helpful to know whether the difference in air flow would be noticeable, and if not, perhaps it shouldn't be the main driving factor for the decision. – TTT Jul 07 '22 at 17:17
  • Option 1 is a difficult install, at least to me, because it involves cutting into the roof and shingles. Option 2 is easier to install, but will be a bit of an eyesore having an insulated duct running through the open sunroom rafters. I would go through the trouble of Option 1 I suppose, if it doesn't greatly impact hood performance. – Jon Jul 07 '22 at 17:22
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    Any chance you could use two 45 degree elbows in option 1 instead of the 90s? That would make it a clear winner. – TooTea Jul 07 '22 at 18:35
  • Are the ducts metal or plastic? Smooth rigid ones, I presume? – TooTea Jul 07 '22 at 18:39
  • metal smooth rigid. Unfortunately no way to do two 45s . If you saw the physical constraints I’ve got here you’d think I was lucky to have the two options I have! – Jon Jul 08 '22 at 03:48

2 Answers2

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4' of duct verses 13.5' and the first option has more vertical distance. The winner is option 1. The duct provides the most resistance to flow, more than the extra 90 degree bend.

JACK
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  • The extra 90 degree bend in option 1 adds as much flow resistance as 30 feet of straight duct. – Mark Jul 07 '22 at 20:49
  • The manual for the hood fan says that each 90 adds as much flow resistance as 5 feet of straight duct, I dont know if that is model specific or a blanket statement they put in all manual. – Jon Jul 08 '22 at 03:44
  • @Jon I got the 30 feet from here: https://www.southwarkmetal.com/tech-tip-2-equivalent-lengths/. I've seen similar numbers on other sites as well. – Mark Jul 08 '22 at 04:36
  • @Jon I've seen the 5' number or similar valves on many hoods. The air being vented from hoods will be lighter than the air in AC systems. – JACK Jul 08 '22 at 12:05
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Every elbow will create air turbulence and reduce air flow.

You option 2 would work better.

However if you decide for Option 1, make sure you use round elbow, not square to reduce the turbulence and friction

friction

source: turbulence

Ruskes
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