Flexible ducting for kitchen extractor hood

1 Mar 2016
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United Kingdom

I have a Siemens extractor hood bought as part of a clearance, so no manual.
I need to get the PVC flexible ducting for it. I measured the extraction hole and believe it is 120mm in diameter. I can only find 100mm and 150mm flexible ducting in Wickes. Should I be getting 120mm size ducting or the larger 150mm ducting and and use some form of adapter on it?

Last edited:
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An adaptor is the usual way. 120mm down to 100mm should be sufficient for a domestic hood. The centrifugal fan will produce enough pressure to overcome the additional duct resistance quite easily. You can usually find extractor instructions on the internet if you want to be sure.

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a lot of people don't like cooker hoods due to the excessive noise/vibration. for the qiuetest and most efficient operation, go with the largest ducting, and rigid if possible. Personally I'd use the 150 with the 125->150 reducer if you can make a bigger hole easily. Another option is to convert to 225x90(ish) rectangular and feed through a square grid.

but as stem says a powerful hood will force a decent amount of air through any normal hole even if it doesn't match the makers ideal conditions, as it's only a short run.

bear in mind the size is measured as the internal diameter for the round ducting, the fittings have the external diameter to match.
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I'd also recommend a cowl vent on the outside, the cowl keeps most of the rain and wind out and they are more durable and quieter than most. There is one large anti-draught non-return flap inside.

Thanks. I guess it wont be a technical problem to position this so that it points sideways away from the neighbouring boundary right? The air will flow which ever way it's positioned.
I don't do that much duct installation, but when I do, I usually get it all from here. Using smaller duct will increase the air velocity slightly, but any additional noise from it travelling through the duct will be drowned out by the fan which is much noiser anyway.
That photo looks like a giant *ick sticking out of the wall! Might stick with the downward pointing cowal.

it doesn't all stick out, only the end, the rest stays in the hole you drilled...:rolleyes:
saw the neighbour opposite stick one out like that, prob for his boiler flue due to window distance.

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