Roof extractor vent terminal - airflow rating

Hello,

I've built a single storey kitchen/diner rear extension on my 3 bedroom semi-detached house. We have our hob on a kitchen island, with an extractor fan mounted above the hob. We have constructed a 'drop down' box out of MDF the same size as the island that hides, amongst other things, the motor for the extractor fan.

This is our fan:
https://www.siemens-home.bsh-group.com/uk/productlist/LF959RB50B

The extractor fan has a 150mm air outlet and supposedly shifts 730m3/hr of air. Currently it is connected to 150mm soft foil extractor flu which travels upwards, makes three 45-90 degree turns, and eventually vents out of the side of our house through an "air brick". Total length of the flu is around 4m.

The amount of air actually being extracted is very poor. No doubt hindered by the length of the flu, three turns and an air brick.

I'm thinking of cutting a hole in my flat felt roof and venting the extract straight upwards. Technically this would be 'amost straight' as the pipe would need to bend around a RSJ, but it would probably be two 30 degree turns with a total pipe length of 1m.

I'm looking for some suggestions on how to do this.

The highest rated flat roof, 150mm vent I can find is only rated for 200m3/hour:
https://www.roofingsuperstore.co.uk...-roof-vent-terminal-150mm-diameter-black.html

I sense this might still be much better than the performance i currently have, but I don't have a way to measure current air flow. I'm guessing my current "air brick" offers less than 200m3.

Does anyone know of any similar vents rated to higher amounts?

It also has a "below roof" depth of 608mm. That is a little long, so I would intend to trim 150mm or so off. Is that problematic?

Really grateful for any suggestions on the vent in particular, but if there are any pitfalls in cutting the hole/patching up afterwards, I'm all ears.

Many thanks!

 

We fitted one for a customer of ours recently and it extracts through the wall. The flow is directed through 90 degrees almost immediately after the fan and is the only bend. The total travel distance can only be 450mm or so. I used duct tape, straightened out all concertinas, stretched hose and generally tried to make an unhindered path for the extracted air, as is possible.

At full bore it will blow your hair back and cause you to squint your eyes to protect them from the rush of air.