extractor ducting

[davrob]

I'm installing a cooker and hood on an internal wall and plan to run rigid rectangular ducting (220x90mm)along the top of tall wall cupboards and out through an external wall. The height of the cupboards will mean the ducting venting at an awkward point outside - just into the rendering on a half rendered house! Any reason why I can't drop the ducting down (using two 90 degree bends) just before it exits? I'm thinking it might just weaken the effectiveness of the extraction.

[Taylortwocities]

The less bends the better but cooker hood fans are pretty powerful so u should be ok

TTC