Allowed path of range hood vent

[DonnV]

I want to replace a downdraft gas range with an electric range and so need to install a new range hood. A path directly up through the roof would be quite difficult. The only feasible path for the vent is up 5 feet into the attic, ell, horizontal 10 feet (with a backdraft damper), another ell, then down 2 feet to exit through the "ceiling" of our covered front porch. The exit would be more than 3 feet from any window and can be directed away from the house. I've searched this forum and every other source I can find and don't see anything that would indicate that this violates code, but always a good idea to appeal to better minds. Am I on the wrong track here? This is in Washington State. Any comments appreciated. Thanks!

 

[Mr. Inspector]

Need to know:
is this for a house?
duct size?
What is the CFM of the fan?
Flex duct or smooth-wall duct?
What does the manufacturers installation directions say about max length and max change of directions?

 

[DonnV]

It is for a residential home. Duct is 7" round smooth wall. CFM is 370. Manufacturer doesn't spec the vent run, but all sources I've found allow at least 20 feet of run when two ells are used. The only real concern I have is the downward leg to the exit. I've yet to find a diagram that shows that or source that discusses that.

 

[Mr. Inspector]

2015 IRC table M1506.2 says the limit to the duct length is 54' but you need to deduct 15' for each elbow. So using 2 elbows would limit the duct to 24' long.
So you should be OK at 17' long. I don't know what would be wrong in a downward direction.

 

[Genduct]

Sounds like a grease collector and a maintenance issue (Fire Risk)