I've just had our kitchen re-done.
We've got a new kitchen extractor hood (Zanussi zfv919y), which has a 150mm duct outlet, with a set of flaps (anti-backdraft?) where it exits the hood.
The old extractor had a 100mm duct, so there was a 100mm hole going through the wall.
The installer placed a 150 -> 100mm reducer at the top of the hood, and ran 100mm concertina ducting from this to the wall. This hole is slightly to the side of the outlet on the new hood, as the hood & oven have moved 100mm along the wall as things were rearranged in the refit. This means there's a slight sideways bend rather than a straight right-angle to the duct before it exits through the wall.
We discovered when we tried using it that the suction was poor – on closer inspection, the reducer has blocked the flaps from opening.
Several options appear to present themselves:
We remove the flaps, and keep the rest as is: 150 -> 100mm reducer, then 100mm concertina ducting. (What purpose do the anti-backflow flaps serve? Are they to prevent fumes, or cold wind, or insects, or what?)
We extend the 150mm duct with a section of rigid pipe, to allow the flaps to function, then use the 150->100mm reducer and concertina pipe
We get the installer back to cut a new 150mm hole in the wall above the hood, and use a 150mm duct.
What's the best solution?