Extractor ducting size - help please

Discussion in 'Kitchen Fitters' Talk' started by Rob H, Jun 20, 2018.

  1. kitfit1

    kitfit1 Screwfix Select

    Ahhhhhhhhh, just looked at his postings..........................dangerous would be a generous description i think. The ban hammer is long overdue.
    Pollowick likes this.
  2. Pollowick

    Pollowick Screwfix Select

    So Kat, what happened? Did he give you a shovel and tell you to did it yourself or admitted he was wrong?
  3. JZW

    JZW New Member

    Hello, just read this thread and was looking for similar advice.

    I have a Siemens extractor hood and bathroom fan venting through separate flexible 100mm duct running over a support beam and then connecting to 5m-long rigid 100mm ducts (supposed to be 125mm but my builders ignored me) running along the kitchen ceiling to the external wall. This has all been installed by my builders who I now realise don't know enough about ventilation.

    I have since learned that the 3 or 4 90 degree duct bends around the support beam plus the 5m-long ducts will leave the fans ineffective. Inline fans have been recommended, but as my builders don't know anything and have asked me to work out a solution, I'm wondering if anyone can give me any more information or at least recommend where I should place the inline fans. I've tried the council and multiple duct and fan retailers to no avail.

    Thank you!
  4. KIAB

    KIAB Super Member

    Extractor hood wants to be 150mm diameter ducting fot quietness & efficiency.
  5. kitfit1

    kitfit1 Screwfix Select

    Can you tells us the exact model number of the hood installed ?
    Without knowing the model number, i can tell you that 5m is at the limit of even the most powerful of cooker hoods. Add to that 4 bends at 500mm (assuming the ducting was 150mm in the first place), that is a ducting length of 7m.......................no domestic cooker hood exists that will work over that distance.
    As we already know you have 100mm ducting, by normal calculation off 100mm only being able to move 25% of the air that a 150 duct would your current set up will only duct 1.75m at the most. I doubt it will even raise the flaps on the outside wall.
  6. JZW

    JZW New Member

    Hello, thank you both for getting in touch. Yes, I've been told the ducting is pointless!

    I'm now trying to work out what to do. I've been told inline fans are the answer, but I'm trying to find more information on which fan to use and at which point in the ducting I should install the fan. Any ideas where to look for advice? Or do you have any?

    Thanks again!
  7. kitfit1

    kitfit1 Screwfix Select

    No, Inline fans are not your answer. What would be a big help to your answer is what actual cooker hood you have ?
  8. rb_daman

    rb_daman New Member

    Hi all,

    I've got a somewhat related query.

    Our kitchen has about 1.5m between the hood and the wall where the vent is but the ducting is for 100mm only. The current hood is over 10 years old, a random make from B&Q or somewhere and very noisy (possibly because the aluminium flex pipe isn't taut and maybe it's been reduced from 125mm / 150mm from the hood as well). I've decided to buy a new, quieter, more powerful and energy efficient extractor because my mum's been having tinnitus for a while (hears a random humming noise from nowhere) but, the new hood is 150mm so wanted to check i'm doing the right thing for maximum airflow / noise reduction.

    The plan is to use the included aluminium flex pipe from the hood to the kitchen unit height, then use a 90 degree bend which also converts to a 220mm x 90mm rectangular duct over the top of the kitchen units (which will be somewhat visible hence opting for rectangular ducting rather than 150mm round pipe). It's then a 1.5m run to the outside wall where i will use a straight adapter from rectangular back to 150mm round for the vent cover (might need a bit more of the aluminium flex pipe to connect through the wall). Oh and I'll also drill some small holes around a 150mm template and 'chisel' out to make my 100mm hole in the wall 150mm.

    This all feels like a bit of a jigsaw so I want to make sure I'm not missing anything because I want to get it right for my mum. Any tips or tricks would be greatly appreciated!

    Many thanks in advance!

  9. kitfit1

    kitfit1 Screwfix Select

    What you are proposing is the correct thing to do, it will give you maximum flow with the lowest dB levels.
  10. rb_daman

    rb_daman New Member

    Thank you kitfit. Shame I cant actually find the right sizes I need at screwfix. I've gone ahead and bought from somewhere else instead.

    Warmest regards

  11. kitfit1

    kitfit1 Screwfix Select

    Just one bit of advice though. Make a template 150mm in diameter out of ply, glue it to the wall central with the existing 100mm hole and hire a 150mm core drill to drill the hole. The reason for this is that getting the flexihose through the hole will be much easier and very unlikely to tear it.
  12. GeorgeSOBrum

    GeorgeSOBrum New Member

    Hi all, I have been reading the posts above as I was looking for solutions to a similar problem here.

    the extractor position could not allow a short route to the outside so the route is from the extractor, 90 degree bend into a rectangular duct for approximately 2 meters, another 90 degree bend into a round 150 mm duct for another meter up and through a flat roof.

    The extractor is the most powerful Smeg extractor we could find when we put our kitchen in but it is very noisy and not very effective. An in-line fan along the ducting is not possible because there is no room, but what would be the case if I added a strong fan at the end of the duct outside? Would it reduce the back pressure?

    Thanks a lot!
  13. daviddavedavis

    daviddavedavis New Member

    Hi all

    I've got yet another iteration of the same question.

    I've got a 2 to 2.5 metre duct with two 90 degree bends in 5" solid, round ducting - I'm now struggling to find an extractor that isn't designed for 6" ducting. If I can't find one (and even if I can) after the first metre of 5", I am able to change to 6" ducting and a 6" exit grille ....or I can fit the 5" one I've already purchased.
    Would that help flow or would it affect backpressure/turbulence and be inadvisable? My main thinking is a 6" final outlet being preferrable.

    I know bigger to smaller would be a no-no , but not 100% on smaller to bigger.

    OR if anyone knows a manufacturer who does extractors designed for 125mm pipe - even better!

  14. daviddavedavis

    daviddavedavis New Member

    Found my answer - a company called luxair
  15. Jimbo

    Jimbo Screwfix Select

    Using a larger terminal and indeed larger everything is always preferable. The terminals create a lot of resistance to flow and hence increasing the free area at the terminal will increase the overall system performance. When running 4" inline bathroom fans, I always use 5" pipe and 5" grills for example.
  16. daviddavedavis

    daviddavedavis New Member

    Cheers for confirming, that's what I was thinking - I was going to put a 6" vent on a 5" pipe. In the end I got a cooker hood designed for 5" pipe so I used a 5" vent on the 5" pipe. It's all solid ,round piping and it's probably the strongest extractor exhaust I've ever seen. Makes your eyes water standing within a metre or two of the vent outside! I think that's more down to the power of the motor.
    Jimbo likes this.

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