Rangehood performance bottleneck

For my new home I have installed a powerful rangehood with maximum Airflow 1860m³/hr and flue outdoor.
When I turned on the first time I feel that it is not powerful enough, then I tried to disconnect the duct exiting the rangehood above and the rangehood instantly becomes very powerful, of course as there are no airflow restrictions.
Then I climbed up the attic and check the plumbing, the diameter of the duct is only 125mm and using a flexible duct, and exiting through the eaves the duct needs to do a U-turn, and the small eaves vent with the insect grille impedes much of the airflow.

So any suggestion to improve the performance?

I am thinking to connect a Y connector and split the duct and install one more eaves vent to have two exit points does that work?

I think that would work as it would increase the ducting capacity.

Increase the size of the ducting to 150mm if possible and maybe change it to round metal ducting. All those little ridges in flexible ducting slow down the airflow considerably.

Stewie

It will improve the performance for sure, but would it be enough for your needs? It's hard to say without seeing the whole system... Try to arrange a consultation with a local specialist, or take the risk and realize your own idea hoping for the best.

I have two options:
Option one is to replace the 125mm flexible duct coming out from rangehood with 200mm perhaps semi rigid, (but I don't know where to get it, bunnings only sell up to 150mm). Then connect a junction that split the 200mm duct to 3x 125mm flexible duct and exit throught the eaves.
[url=https://www.ebay.com.au/itm/114145392291]https://www.ebay.com.au/itm/114145392291[/url]

Due to the spacing constraint I can only install 125mm to do a U-turn and exit the eaves.

Option two is to use a 200mm duct and do a 90 degrees turn in attic to exit through the colorbond sheet vertical wall.

Option one is easier to install as only involves cutting the eaves plasterboard, but will have three openings.
Option two seems less restrictive airflow as shorter and more direct route, but involves cutting colorbond metal, and I dont know how to mount the vent on the colorbond sheet.

Doing option 2 allows you to use vents that have small louvres. They are gravity operated in that when the range hood is not use the louvres or flaps are shut. As soon as the range hood is turned on the air coming out opens these flaps. No insect screen or other restrictions needed.
Like this one https://www.ebay.com.au/itm/Gravity-Ext ... 0677.m4598 but without the screen.

Stewie

I've seen this mounting plate on ebay https://www.ebay.com.au/itm/264513656954
It's a bit small, but it's the largest they have, the mounting area is 202mm x 218mm, its a bit small if mounting the gravity vent to that plate for the guide holes, but maybe can mount to corner near the edge of the mounting plate, and drill 4 holes on the gravity vent near the opening and rivet them together to my colorbond sheet?

btw, the gravity vent you refered to, it titled 200mm, but actual diameter is 150mm so it wont fit, anywhere I can get gravity vent for 200mm diameter duct?

I am unsure but some of your better hardware stores may be the best bet. Aircon suppliers may have something along these lines too. What size ducting does the range hood manual specify? The reason I ask is that I installed kitchens for a living many moons ago and the biggest ducting I ever installed was 150mm. That size can certainly shift a lot of air with a decent range hood pushing it through. A lot of guys I know with workshops also use dust extractors that have 3HP motors and 150mm ducting so I would be surprised if going bigger would result in any change in air volume being extracted by your range hood. The size of your motor in the range hood ( plus a few other caveats too ) would be the biggest limiting factor.

Stewie

The unrestricted airflow is 1860m³/hr and can fit 200mm hose to it, and it comes with a multi size reducer for smaller duct diameter.
Of course the larger the duct the less restrictive. So I decided to go with 200mm. It's an expensive rangehood and I dont want to waste the potential of it.

So the last question comes to how to mount the gravity vent to the vertical corrugated metal sheet...
I dont think the mounting plate works on my previous reply, its too small

A flat Colorbond sheet folded 90º at the edges and then cut around the curves is probably your best bet larger than the gravity vent 50mm all round then you cut out and fix the gravity vent to that. We have had to do the same thing making up flashings around roof penetrations for sewer waste vents, chimney flues etc. Silicon all round to weatherproof everything of course.

Stewie

Can I ask what sort of rangehood it is? I install kitchens and know that the rated air flow can be optimistic.
If the back of the rangehood only has a 125mm spigot then that's all you need to run. If a reducer has been installed somewhere along the ducting then I'd remove that and run the same diameter duct all the way from the rangehood to atmosphere.
Normally in a metal roof all the large penetrations are installed at the time of sheeting. For an exhaust vent a roof cowl would be installed and flashed in.
As a retro fit you can punch through the roof deck and use a Dektite to seal it up. You just need to be able to make sure that whatever you use to penetrate the roof can be mounted in a rigid manner. You will normally need to fix to a roof member to achieve that.
It will need a waterproof cap and you can install an inline backflow device to prevent drafts and bugs from entering when not in use.

It's a chinese rangehood... the brand is Robam. https://www.robam.com.au/products
Normally I don't like chinese appliances but for rangehood they are superior. Since chinese cooking generates a lot of fumes...

I've uploaded a pic of the roof. As you can see there is a silver duct hanging out, that became my eaves vent for my rangehood now. There's a tight U-turn to get around the beams the airflow is quite restricted.

Now I am planning to install an additional gravity vent to the vertical corrugated sheet close to the eaves.
I never plan to puncture the roof for a roof cowl, there's no reason to puncture the roof if you can avoid it.

What is the size of the duct off the back of the rangehood? Can you attach a 200mm diameter duct?

yep I've measured it and can fit a 200mm.


is a semi rigid duct huge difference to flexible duct if the flexible duct is fully extended? or just marginal?

Just use 200mm flexible ducting. Many of the external motor rangehoods which are the most efficient use flexible ducting.
Pull it reasonably tight so it's opened up and install something like this through the metal sheeting.

So where do I find or make "this", the flange of the gravity vent is 200mm and its rigid plastic, a duct joiner won't be able to cover the flange of the vent right?

What I showed there is a back draft wall vent that are used with Qasair rangehoods. They would be available through any of their sales representatives in any state.
You can google them to find one in your state.
It would be a matter of cutting a 200mm diameter hole in your wall then inserting the vent. You could silicon it in place and seal it. Then attach your duct to the vent and attach the other end to the rangehood.
You've then got a clear 200mm duct all the way