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Discuss Some advice regarding 125mm hole for cooker hood fan please? in the Electrical Forum area at ElectrciansForums.co.uk.

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  1. happyhippydad
    Online

    happyhippydad Member Electrician's Arms

    Location:
    Gloucestershire
    Morning all..

    I am about to quote for a cooker hood installation including core drilling for the vent. The fan is 125mm.

    I have checked upon the sizes of core drill bits and 127mm is the closest fit. I cant see the 125mm ducting getting through that due to the outer diameter of the ducting being greater than 127mm. Does it fit through?

    How much of an impact would it have on the motor if I used a 125mm-100mm reducer for the ducting?

    Could I cut a 152mm hole and use a larger vent on the outside wall, also using a 150mm-125mm reducer for the ducting?

    Cheers.
     
  2. Pete999
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    Pete999 Trusted Advisor

    Location:
    Northampton
    Business Name:
    None
    Doesn't the instructions on the Fan/ducting tell you what size hole is required?
     
  3. Murdoch
    Offline

    Murdoch Electrician's Arms

    Location:
    Woking
    Have you measured the outside diameter of the ducting? Surely that's 125mm, not the inside!
     
  4. Wilko
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    Wilko Electrician's Arms

    Location:
    Berkshire
    Business Name:
    Wilko Electrics
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    Last edited: Aug 29, 2017
  5. happyhippydad
    Online

    happyhippydad Member Electrician's Arms

    Location:
    Gloucestershire
    I've just measured some rigid 100mm ducting I have Murdoch, the outer diameter is 103-104mm. I'm pretty sure with ducting (unlike conduit) the specified size is for the inner hole, stand to be corrected though? Flexible ducting, although more malleable has an even bigger outer diameter relative to its inner diameter.

    I also remember drilling a 107mm hole for 100mm rigid ducting and it 'just' scrapes through.
     
    Last edited: Aug 29, 2017
  6. happyhippydad
    Online

    happyhippydad Member Electrician's Arms

    Location:
    Gloucestershire
    The customer has the instructions Pete but I cant see them being much use in this case as they will just say drill a 125mm or 127mm hole. I'm trying to preempt any problems after scouring the internet and seeing that this is a common problem with 125mm ducting.
     
  7. Midwest
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    Midwest Electrician's Arms

    Location:
    Oxfordshire
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  8. HandySparks
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    HandySparks Trusted Advisor

    Location:
    Hampshire
    Business Name:
    Neish Electrical Services
    Worth a careful measure-up. It's not exactly the same thing, but I've definitely installed a "125mm" room ventilator using a 127mm core drill.
     
  9. egg67
    Offline

    egg67 Electrician's Arms

    Location:
    Surrey
    Did one not long ago, 125 DIA from fan instructions said drill hole at 130 DIA, all I did was fit a reducer to the cooker hood, add 5mm to ducting size and it will fit
     
  10. happyhippydad
    Online

    happyhippydad Member Electrician's Arms

    Location:
    Gloucestershire
    So you used a 125mm-100mm reducer and then drilled a 107mm hole?
     
    Last edited: Aug 29, 2017
  11. egg67
    Offline

    egg67 Electrician's Arms

    Location:
    Surrey
    No l used a 105 but that was because I had one, 107 will do the job just as good, you just do not want it too Tight on the ducting as it makes it hard to get it in, the cover will hide the hole bit off sand and cerment and rember to drill from both sides not just from one
     
    • Like Like x 1
  12. happyhippydad
    Online

    happyhippydad Member Electrician's Arms

    Location:
    Gloucestershire
    This seems like the best option but if the the motor is designed to remove air through a 125mm hole will it not be straining to remove through a 100mm hole?
     
    • Agree Agree x 1
  13. Pete999
    Online

    Pete999 Trusted Advisor

    Location:
    Northampton
    Business Name:
    None
     
  14. Pete999
    Online

    Pete999 Trusted Advisor

    Location:
    Northampton
    Business Name:
    None
    Shouldn't think it will make any difference
     
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    • Agree Agree x 2
    • Disagree Disagree x 1
  15. egg67
    Offline

    egg67 Electrician's Arms

    Location:
    Surrey
    No the moter is designed so that it can push air a certain Length from hood Manufacturers allow for bends so all will be fine, if using flexible duct make shore you pull it out straight first and then fit it
     
    • Like Like x 1
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