Extractor ducting size - help please

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

  1. Rob H

    Rob H New Member

    Hi,

    My builder installed under floor ducting to run from my downdraft hood to the garden, about 7 metres. Now we're at the point of installation I'm very concerned. He has used 100mm Square section pipe. The hood requires 150mm, and the run is long and has a couple of links to look up with the box.

    Everything is telling me this is going to be a problem - is it best to link up the ducting and see how well it works, or just give up, curse the builder, and buy a recirculation module and bitterly accept the low performance and charcoal filters? Is the narrow duct really likely to damage the fan??

    Thanks
    Rob
     
  2. Pollowick

    Pollowick Well-Known Member

    It is well undersized - 10,000 mm² whereas you require 17,700 mm² so performance will be well down especially over a 7m run. Will it damage the fan? Don't know, although it will certainly be working harder and the overall flow significantly lower than you might expect. Can you not get to the duct and replace it? There are flat ducts about 230 x 80 to 90 which would meet the needs.
     
  3. kitfit1

    kitfit1 Well-Known Member

    When a cooker hood is specked with a 150mm outlet, then 150mm ducting (round or rectangular) is what should used for it. In the hoods spec will also be it's maximum ducting distance and when looking at that distance any bends in the ducting reduce the effective distance by 500mm for each bend.
    Even if your builder had used 150mm ducting, a 7m run would be a tall ask from all but the most powerful domestic cooker hoods. Using a 100mm ducting over that distance will result in the hood being very very noisy and pretty much ineffective.
    Did the builder have the hood specs before installing the ducting ?
     
    KIAB likes this.
  4. Kev shorter

    Kev shorter New Member

    I think this highlights the problems which can occur when more specialised work is carried out by people that do not realise the effect of using what's easy as opposed to what is req. Personally I would get it sorted .inline fans are available which might help on such a long run.
     
  5. KIAB

    KIAB Well-Known Member

    Undersize ducting is inefficent & the hood will be noiser when in use.

    And as Kit as already said you want a very powerful hood for the 7mtr run.
     
  6. Mr Rusty

    Mr Rusty Well-Known Member

    If poss change the ducting. However,the small duct will not damage the fan - it will just spin with no flow. The enemy of flow with any turbine type blower/exhauster is back pressure. We install pneumatic transport systems, and even a 5kW 3ph exhauster will have zero flow at 1/2bar back pressure - it's how we avoid getting involved with the pressure regs - centrifugal fans are self limiting with the flow/pressure curve dropping away rapidly as back pressure increases. As kitfit says this is likely to be very poor flow - too small duct with bends - but try it and see how it works.

    As Kev says inline fans are available. It might be a bodge to some extent but a secondary suction inline fan on the outside end might be enough to reduce the back pressure and get everything flowing. You'd perhaps have to experiment, and workout some on/off trigger (wireless switch?) but in theory it could be a robust and effective solution. Better than recirc anyway. Depends if you are keen cooks or not - if you are recirc is carp.

    Do try it first. You might have poor flow, or it might be enough. You might have to run at the next speed setting up. We rarely run our traditional hood over setting 1.
     
  7. Rob H

    Rob H New Member

    Thanks for all the responses - unfortunately it's all under concrete so there's no fixing it - I'm annoyed because it took one Google search for me to find out what the duct size should be - builder did have the specs, just lazy. General opinion seems to be to give it a go - bit concerned about burning the motor out though.

    Thanks again!
     
    KIAB likes this.
  8. rogerk101

    rogerk101 Well-Known Member

    If it really does struggle, you could help it along by adding another fan (inline one) on the outlet of the duct. That way, there'll be a fan at the front and one at the back of the flow, both helping each other. This of course assumes that you can get a wire through to power the 'outlet' fan from the same switch that operates the inlet fan. I guess you could run the wire down the ducting, but it will end up getting pretty mucky with all the cooking grease that will build up inside the ducting.
     
  9. KIAB

    KIAB Well-Known Member

    Thanks for update, hope it all works out.
     
  10. Pollowick

    Pollowick Well-Known Member

    Under concrete! Where did he run the ducting?
     
  11. KIAB

    KIAB Well-Known Member

    Not uncommon to run ducting under concrete or suspended floor to a outside wall with a downdraft hood.
    Just make sure it has a slight fall to allow any moisture to escape, & under suspended, better to insulate, to stop warm air condensating in ducting..
     
  12. rogerk101

    rogerk101 Well-Known Member

    As a family, we hardly ever fry, and even then the inside of my overhead extractor fan ducting is disgusting after a few years of extracting cooking fumes. Imagine what a downdraft one's ducting looks like after a few years ... and to think it can never be cleaned ... yuk!
     
  13. Rob H

    Rob H New Member

    Thanks - it's a new extension so we had the opportunity to run the duct under the floor (couldn't run through the ceiling unfortunately due to the direction of the joists and a big steel.) The builder seems to take on things he doesn't understand to unfortunately, even though it seems obvious how big the Duct needed to be. It won't be insulated, in with the underfloor heating.

    I'm increasingly wary - as client I should be able to have some faith in things being done properly.

    What I have seen over the past few weeks is how a builder can waste the time of the other contractors/fitters, due to plans not being followed/bad plumbing. Frustrating for everyone.
     
  14. Rob H

    Rob H New Member

    You can clean the filters on the downdraft in the dishwasher... Downdraft obviously not ideal, and more expensive, but will be increasingly common for open plan spaces where chimneys don't fit/work, or flush ceiling extractors can't work.
     
  15. kitfit1

    kitfit1 Well-Known Member

    @ Rob H, can you post up the exact model number and make of the hood you will be using.
     
  16. xifwercs

    xifwercs New Member

    I installed my AEG cooker hood straight out of the wall, it was recommended at 150mm but came supplied with a reducer to about 125mm I think. the manual referred to the reduced air flow using the smaller size so I don't think it would damage the motor, the flow rate wouldn't be great though to say 100mm is usually reserved for bathroom extractors.
     
  17. Rob H

    Rob H New Member

    Hi - it's a Siemens LF16VA570B modulair
     
  18. Mr Rusty

    Mr Rusty Well-Known Member

    You will not damage the motor whatever. As i said earlir backpresure will just kil the flow. The fan will just spin. Its not a positive displacement pump so the only thing that suffers with too small duct is performance.
     
  19. Rob H

    Rob H New Member

    Ok - thanks a lot. Really responsive forum!
     
  20. kitfit1

    kitfit1 Well-Known Member

    Before i went to work this morning i spent some time searching Siemens PDF's for the maximum venting distance for this hood, and then some more time tonight.
    It's obvious from all the data they have released, they have no intention at all to let anyone know the maximum ducting distance for the hood you have.

    Based on past experience, that's about normal for Siemens. My advice would be to bite the bullet and just use it in re-circ mode. There is no way on this earth that that hood will ever duct 7m even at 150mm.
    At least in re-circ mode you will know when to change the carbon filters.
     

Share This Page