Issue with remote vent for range hood

6 years ago

I have read a fair amount of posts on other messages that are similar to my situation but none were right on point so I was hoping for a view from someone who may have experience with this. I am doing a remote vent to provide for both more power and, hopefully, much less noise in the kitchen when in use. The issue (one of the issues) is that the master bath is directly above the kitchen. It is not possible to vent out of the side, has to be roof. Therefore, to circumvent the bathroom the duct will have 4 elbows (I know, not ideal). Hood duct will elbow at the top of the hood immediately to the right for about 7', then elbow again and go up for about 10', elbow back to the left for about 4' and then elbow one last time straight up and out for about 3' to the roof. Also, I am using a wolf hood and wolf remote blower, which calls for a 10" duct. I only have room in the walls and ceiling to run an 8" duct. Like I said, not ideal. To try to compensate for the winding run and 8" duct I am using a 1,200 CFM blower, figuring I will need more power to get the needed venting from it as I am sure a lot of waste will take place. Only other option may have been to go w/ the 1,500 CFM blower, assuming I would need even more power to compensate for the smaller duct and winding run. Noise is not an issue, I don't care how loud this will be on the roof. Also, I figure if I did a local blower vs remote I would have the same problems since the duct run can't be changed. Maybe this would be worse with a local motor. Should this setup work to vent the kitchen or will I end up w/ a kitchen that is basically not vented at all?

Many thanks if you a have a view.

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Comments (9)

User
6 years ago
Move the range to a better spot with less obstacles. An outer wall for instance,
HU-6868130
Original Author
6 years ago
Thanks but moving anything is not an option, more interested to get an idea if this will end up working. It is an MDU, MEP and the rest is done. There is no moving anything. Is what it is but was hoping to get a feel if anyone has a view if it will end up being a bust and not working. Had very little RE to work with and hated idea of a recirculating vent so took a shot.
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worthy
6 years ago
last modified: 6 years ago
Each 90º bend adds the equivalent of 35 feet of ducting. Reducing the duct sizing means substantial pressure loss, no matter how high-powered an exhaust fan you use.
HU-6868130
Original Author
6 years ago
Ha. So I am running the equivalent of about 150’ of duct. Great.
strikeraj
6 years ago
In order to figure out the effectiveness, on top of cfm rating, you need more information on the pressure ratio of the blower, as your setup has too much pressure drop over the long duct and the bends.
kaseki
6 years ago
I'm sure we all agree that this is a less than ideal configuration.

As it happens, I have (ca 2008) Wolf/Broan blower fan curves. What is the hood entry aperture area of the hood you intend to use? With that and some guesses about baffle loss and duct loss as a function of flow rate, we can choose a blower. I hope this hood is not too large, as high flow in a small duct may cause the duct velocity to exceed the recommended limit of 2000 ft/min -- effects the impingement of grease onto the duct walls.

Blade tip turbulence noise and motor vibration noise is propagated down the ducting. It will be less at the hood then with the same blower in the hood (although the larger Wolf / Broan blowers won't fit in a hood, I think). Recommend angling the duct in the attic so a Fantech silencer can be added to the path for a slight increase in duct length.
HU-6868130
Original Author
6 years ago
Appreciate the thoughtful, detailed response. Frankly, all I can say is the model is the Wolf 34" Pro Hood Liner - 19" Depth, PL341912 and the 1200 CFM blower that goes with it. Online they seem to say that while you should limit elbows and the length of the run, you can have up to 4 elbows and to keep the run to 50'. I have 4 elbows and the total run is maybe 25' (not accounting for loss as one commentator pointed out above).
strikeraj
6 years ago
using Kaseki's curve, assuming a equivalent duct diameter of 8" and equivalent duct length of 150', you will get max 700cfm from the 1200cfm blower, and that is not accounting for the loss due to hood filters. If we add that in, you are getting 500-600 cfm at your hood.
kaseki
6 years ago
We probably should assume 0.75 inches, w.c., for the baffles. There is at least one site on the Internet that calculates pressure loss for combinations of duct length and elbows. I think strikeraj estimate is reasonable for the present.

What actual CFM do you need? I argue for 90 ft/min times (34 x 19 / 144 sq ft.) = ~400 CFM

So to the level of analysis so far, the 1200 CFM blower should be adequate with some margin.