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?

Photo above hood

  • 1
    The flaps are back draft dampers since there is only 18” to the wall why not fix this correctly so the fans can do there job, undersized plastic flex sounds like a super grease trap.
    – Ed Beal
    Commented Jun 29, 2021 at 13:38

1 Answer 1


What's the best solution?

The best solution would be to remove the old 100mm duct and replace it with a newly installed 150mm duct that is inline with the outlet of the new range hood. This would give you the best performance for your new range hood.

That may be somewhat expensive depending on your specific situation so the next best would likely be to redo the "mess" that has been created immediately above the hood with angles and reducers that interfere with the operation of the range hood parts. I'd look at replacing the first few feet of the old duct with a properly aligned and sized new duct and then place a 150mm to 100mm reducer away from the hood so that it doesn't interfere with the flaps.

  • To clarify - the whole duct is only a few feet - there’s about a foot of twist to get from the top of the hood round to the hole in the wall, then about 18” or so through the wall and it’s out.
    – Dan W
    Commented Jun 28, 2021 at 12:47
  • Thanks for your reply – I've added a photo if that helps. The wall is an external cavity wall.
    – Dan W
    Commented Jun 28, 2021 at 12:52
  • Also check the manual for the extractor. IIRC ours specifically said 125 or 150mm duct (even came with an adapter between those sizes). The kitchen fitter was going to use 100mm and wasn't impressed when I insisted on respecting the manufacturer's instructions. I'm glad I did though.
    – Carl
    Commented Nov 25, 2021 at 19:08

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