What ducting size for kitchen extraction hood?

17 Jan 2015
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United Kingdom
I plan to run rigid white ducting approx 7m in the ceiling as part of a renovation project in which my ceilings are open and builders can make allowance. The run will be more or less straight from an open plan area through a wall and another room and then through an external wall.

I have used some spare 130mm white rigid ventilation ducting for this purpose. I know I would need to check the extraction hood manufacturer's recommended sizes but as I am yet to purchase or know which one I will buy, I wanted to check here if anyone knew what is the typical size to go for, for a medium to high end extractor hood with an externally mounted motor.

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Editing because i was thinking about bathroom extractor.
Foxhole is correct.
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Am I likely to see much pressure degradation or performance issue with a 130mm ducting v 150mm?
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Yes it makes a massive difference depending on the extractor.

If you can accomdate 150mm then do so.

My downdraft extractor was infective with 130mm ducting over 3m (builder didn't use the specified 150mm). Once we switched to 150mm the "chopping" noise was gone and the extractor performed properly.

7m is also a tall order for anything but the most powerful residential extractors. You might even need to consider units that are 150mm or square duct equivalent for it to not be unbearably noisy and to go that distance.
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Unfortunately our dropped ceiling will not allow for more than 130mm even though we have ceilings open because we've already at a tight squeeze.
Do you know any extractors which will accommodate up to 130mm?

130mm is still quite large compared with normal extractor ducting I presume?
You can get rectangular duct, in many sizes - 230w x 90h is common and might do it.

The enemy of flow with any blower/exhauster is back pressure. So you either need to 1. increase the pipe size, 2. reduce the bends 3. add secondary suction inline fan; which might be enough to reduce the back pressure and get everything flowing.

Otherwise the fan will be self limiting with the flow/pressure curve dropping away rapidly as back pressure increases and will just make noise with no suck!

If you don't get a beefy enough extractor for the 7m run, which should be listed in the specs for the extractors maximum ducting distance - then you are better off just running in a re-circ mode, at least then you will know when to change the filters.

Also, its easy enough to dry test setup on-site before you install; you might find the performance, whatever you decide; acceptable or not.

Westin Stratus recommended. The tech team there should also be able to assist you.
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Thanks, some useful information there.

I've never come across Westin before, but then again I dont buy extractor hoods often. Are they regarded as high end or mid market and how about cost wise, middle of the road or high?

Any other names you suggest?
Also how do you dry test the setup? Are you kitchen installer by any chance, you seem to know a thing or two about this!
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