Best way to install ducting for a cooker hood?

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23 Feb 2013
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I'm fitting a new cooker hood with a 150mm duct outlet. I want the fumes to be vented outside the house and there is an available 100mm vent in the wall about 4 metres away via the loft space. My question is about the best way of reducing the size of the ducting between the cooker hood and the vent. Given that it has to transition from 150mm down to 100mm, where is the best place to install the reducer? Should it be near to the fan at the cooker hood end, or near to the vent at the external wall? Does it make much difference? I'm thinking that if it is near to the fan then it will be noisier, but if it is away from the power of the fan it may be less efficient in shifting the air. Anyone here know about these things?
I would run as much as possible in the 150mm and then transition to the 100mm in as smooth a way as you can manage right at the outlet, i.e. don't have a brick wall transition. My reasoning is that 150mm has a lower air resistance that 100mm so you're better having much in this size as possible. Ideally you'd upgrade the vent as it will construct the extractor as they are of the LPHV type but at least this way you always gave the option of upgrading the vent or adding a second 100mm one if it doesn't work well enough.
The general rules for extraction and venting pipes are keep runs short as possible, have few bends,use wider pipe for most of it then reduce the last bit, and flexible pipe should be only 10% of run.
4 m is quite a long run for a kitchen extract particularly if there are many bends .Could you use a tile vent once you reach the loft space or put in a new closer wall vent? 150mm pipe isn't cheap so doing the extra work may pay .
Nearly all of the extractors I have to install require a 150mm or 125mm duct all the way outside to qualify for there warranty.
Best check the instructions to cover yourself.
Think about how you can easily take it apart. Over time, fats and oils will condense inside the pipes even with a cooker hood filter. We rarely fry anything but every so often I pull my extract duct apart and de-gunge.

(its a big issue for commercial kitchens, less so for domestic but still needs doing)

Not sure about sizing in your case. I guess the hood pushes the air through, unlike many extractors where something at the far end sucks air through.
By going from 150Ø to 100Ø you are all but halving the outlet area. Add this to the long run and you will dramitaclly restrict the airflow. It is also likely to be noisy too.
I would advise biting the bullet and getting a 150Ø outlet. As others have said keep the run straight, and as short as possible.
I designed ductwork systems for over 20 years, we allways tried to keep duct sizes as large as possible (except dust extraction which is a whole diferent ball game)