Suppliers of cooker hoods with 120mm outlets don't sell 120mm ducting!

Discussion in 'Kitchen Fitters' Talk' started by Busterhymen, Jan 14, 2022.

  1. Busterhymen

    Busterhymen Member

    I've been looking at 60cm cooker hoods and found that many have 120mm outlets, recommending not to reduce else cause a reduction in performance. Reducing from 120 to 100mm is a 31% loss of cross-sectional area.

    Looked at Wickes, screwfix, toolstation and no 120mm flexi ducting to go with their 120mm outlets! There's lots of 100/150mm ducting.

    Now, I have managed to find some 120/125mm ducting (inc B&Q), but why don't the suppliers of the hoods sell 120mm ducting to go with their hood recommended size?

    Is this an anomaly that others have noticed or am I missing something?
    Last edited: Jan 14, 2022
  2. BiancoTheGiraffe

    BiancoTheGiraffe Screwfix Select

    Most people just use a reducer, going to make very little difference to performance

    FUNDIMOLD Active Member

    In the process of installing a new hood. No duct was included (150mm) with the Smeg unit I bought online so electrician had to phone around local outlets. No joy. Finally sorted one at nearest Screwfix (£4) but lost a day because round trip to SF impacted his schedule necessitating delayed completion of installation. Interestingly we discovered the now defunct original unit of our new build house was lacking the ducting anyway! Ten years of greasy fumes floating around in the chimney! Got to love Redrow quality control.:rolleyes:
  4. Busterhymen

    Busterhymen Member

    I didn't expect the 120mm ducting to be included, I just expected it to be freely available. All the suppliers have 150mm as standard.
    Is it possible that your previous hood was using the carbon filter recirculation / non extraction method, although I don't think it's as effective as extraction.
  5. CGN

    CGN Screwfix Select

    CEF electrical suppliers sell 5” ducting.
    Astramax likes this.
  6. Busterhymen

    Busterhymen Member

    Thanks. I'm not looking for any now. As said - I've managed to find some. I'm just pointing out that I'm surprised that many suppliers don't seem to sell the right size ducting to go with their own hoods. I just find the mismatch a bit strange.
  7. CGN

    CGN Screwfix Select

    yes it’s a pain. You’ll find 4 and 6” easily enough.
  8. koolpc

    koolpc Screwfix Select


    "Did someone mention duckting'?" :p
  9. Busterhymen

    Busterhymen Member

    I actually bought both 100mm plastic flexi & 125mm alloy flexi (to give me options).
    I now realised that the suggestion of reducing to 100mm may be more restrictive than I initially thought, so I'm glad that I bothered to search for the 120mm (actually 125mm aluminium).
    The internal corrugation in the white plastic flexi-duct, means that when a 3m pipe stretched to fit 2.5m, it has an internal of about 75mm (using a 100mm plastic flexi).
    This represents 75^2/120^2 = 0.39 of the intended cross-section, (61% reduction in cross-sectional area), compared to the 120mm specified (without corrugation). Having said that, I don't know how to calculate the effect of a 61% blockage and the turbulence/resistance caused by the corrugations, but the hood instructions do warn against using a smaller than specified diameter.
    Out of interest, do installers usually cut and stretch the wrong & undersized white flexi stuff tight to limit the reduction in internal diameter caused by the corrugation?
    I'm sure that the lower/wrong diameter ducting works, and if you don't measure the flow of both pipes (or be clever enough to calculate), you'd be non of the wiser on the negative effective that an undersized ducting has. I expect someone's got a chart with the numbers?
  10. kitfit1

    kitfit1 Screwfix Select

    All hoods have an optimum designed ducting size. That size does vary between one make and another make. To get the throughput and noise levels as designed, only the correct ducting size will work. Anything smaller will restrict the flow and increase the noise.
  11. Busterhymen

    Busterhymen Member

    I received a call back from the tech dept of a manufacturer today and I have come to a similar conclusion plus some additions.
    They calculate the optimum ducting size and 'round up' to the closest commonly available size (using a table - 100, 120, 150).
    It could be that a hood required 102 and another needed 120, and they specify 120 for both. Using a 100 on the one that required 102 would have little impact, but using 100 on the one that required 120 would have a major negative impact.
    They are also aware that many retailers do not stock 120mm, and suggest to increase to 150 instead of reducing to 100.
    They said that a hood may appear to work with a lower than recommended size duct for years, but It will reduce the performance, increase the noise, draw a higher current, reduce the longevity and invalidate the guarantee. None of this is not my opinion, it's simply repeating what I was told by the tech dept of a hood manufacturer.

    Thanks to everyone for their opinions/suggestions...

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