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ducting for standard 250mm exhaust fan?

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  1. #1
    Senior Member ForeverYoung's Avatar
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    Default ducting for standard 250mm exhaust fan?

    In the roof cavity and thought I would add some flexible ducting to the standard 250mm exhaust fan in there.
    One like this:

    Arlec 250mm White Exhaust Fan - Bunnings Australia

    But can't find any 250mm flexible ducting.
    Now, I know most installs just have it venting to the roof cavity, but I would like to beg to differ
    What have other people done?
    I tried to find an adaptor 250mm to 150mm (which is the biggest ducting greenshed sells) but not sure that is a good idea anyway.

  2. #2
    Slow but rough Uncle Bob's Avatar
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    That type of fan probably won't move much air if you put a duct of any length on it.

  3. #3
    Senior Member ForeverYoung's Avatar
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    Quote Originally Posted by Uncle Bob View Post
    That type of fan probably won't move much air if you put a duct of any length on it.
    that's a good point.
    Air Extraction - cubic mtrs/hr 350
    but way beyond my aerospace aerodynamics mental gymnastics

  4. #4
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    I'd have to look closer at my 2 year old kitchen extractor fan research, and could if needed, but
    1. Flexible ducting isn't liked by the extractor fan manufacturers, esp over distances more than (say) 2-3 metres, and
    2. they love minimum diameters of 250mm or greater
    i.e. for good airflow, short runs, rigid ducting, bigger diameters is all really good. I had terrible trouble with manufacturers/distributors committing to a specification when trying to put a high powered extractor fan over a hot gas - wok - cooktop. And yes, yours is as smaller airflow issue. but the problem is the same.
    And I'm eyeing my daughters' (yes, plural) shared bathroom for an effective venting solution other than smashing the window.

  5. #5
    1K Club Member havabeer's Avatar
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    Quote Originally Posted by ForeverYoung View Post
    that's a good point.
    Air Extraction - cubic mtrs/hr 350
    but way beyond my aerospace aerodynamics mental gymnastics
    uncle bob is actually, take that m3/hr rating and half it to get a more accurate idea of how much air you're ACTUALLY moving


    so....

    if you have a 3x3x2.4m bathroom thats 21.6m3 of air in that room
    a bathroom you should look at 15 room exchanges of air per hour
    so you need to be moving 325m3/hr to get 15 air exchanges in the hour
    your linked fan is probably moving 150 if you're lucky due to the fan its self, ducting, vents/grills, how much air can get into the room etc etc

    https://www.fanmaster.com.au/air-calculator/
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  6. #6
    Seasoned DIY droog's Avatar
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    Any of the HVAC supply outlets will have 250mm ducting, if you want an easy way to adapt it to the exhaust fan just use a standard bucket with the bottom cut out.

    https://www.metalflex.com.au/product...-metre-3212057

    Either that or buy one of the ducted exhaust fans, although they are usually just an outlet to 100mm duct so no good if you want to move a lot of air.

  7. #7
    Senior Member ForeverYoung's Avatar
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    Quote Originally Posted by Compleat Amateu View Post
    1. Flexible ducting isn't liked by the extractor fan manufacturers, esp over distances more than (say) 2-3 metres, and
    ....
    It's 3m

    Quote Originally Posted by Compleat Amateu View Post
    ...
    And I'm eyeing my daughters' (yes, plural) shared bathroom for an effective venting solution other than smashing the window.
    I'm a sucker for good punctuation.
    Sounds like, just wait for them to leave home.

    Quote Originally Posted by havabeer View Post
    uncle bob is actually, take that m3/hr rating and half it to get a more accurate idea of how much air you're ACTUALLY moving


    so....

    if you have a 3x3x2.4m bathroom thats 21.6m3 of air in that room
    a bathroom you should look at 15 room exchanges of air per hour
    so you need to be moving 325m3/hr to get 15 air exchanges in the hour
    your linked fan is probably moving 150 if you're lucky due to the fan its self, ducting, vents/grills, how much air can get into the room etc etc

    https://www.fanmaster.com.au/air-calculator/
    Well its over 28m (punctuation police?)
    It is actually a laundry/bathroom. Shower; washing machine; dryer; toilet.
    But I solved it by moving the dryer to the verandah - I am sure others have done that, but not admitted it.

  8. #8
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    Flexible ducting is ok for air con but I have reservations about using it for kitchens/bathrooms due to build up of steam/lint/grease, much prefer metal ducting
    If you have a sheet metal shop within distance you could get it made up but if you are going through the roof with it you will need a H cowl on top
    and that will cost
    Or a chinamans hat will suffice but not the best in blowing rain.

    Or use 100/150 pvc but again depending on what you want to spend.

    Most new homes I have seen lately use 100 pvc from bathrooms with a pvc grill on the wall face or eaves.

  9. #9
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    Pacific Air have 250mm semi-rigid ducting:

    https://www.pacificair.com.au/ductin...pandable-250mm

    Also, I’ve used their low profile roof vents with no issues so far - no leaks during the recent Sydney downpours. Installed on a tiled roof in well under an hour. They also suit tin roofing but I have no experience with this. Available in 3 colorbond colours online or from the green shed:

    https://www.pacificair.com.au/roof-v...grey-125-150mm

    They also do vents for larger duct diameters (at least 180mm) but on tiled roofing this may require cutting battens and other avoidable complications.

    They’re pretty responsive if you give them a call: 0397587844

  10. #10
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    Any open bladed fan will not push much air against back pressure, what they will do is become an efficient noise generator. The type of fan shown in the OP will struggle to meet its rated flow volumes even into an open ceiling void. Best to use and inline fan kit like: https://www.fansonline.com.au/ventil...aust-kits.html
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  11. #11
    Senior Member ForeverYoung's Avatar
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    Quote Originally Posted by John2b View Post
    Any open bladed fan will not push much air against back pressure, what they will do is become an efficient noise generator. The type of fan shown in the OP will struggle to meet its rated flow volumes even into an open ceiling void. Best to use and inline fan kit like: https://www.fansonline.com.au/ventil...aust-kits.html
    Yes, based on everyone's comments I came to that conclusion.

    It's probably 1.8m to the ridge and there is a 150mm cowl/tophat thingy venting to the outside; not connected to anything below of course.
    So I guess the bath/laundry exhaust (and also the toilet cubicle one for that matter) get a bit of indirect circulation draw.

    I was in the roof cavity as I was pulling out a woodheater flue (in the bathroom? you might ask that, but this is not the thread to answer) and thought about ducting the exhausts because down the track I might turn the roof cavity into storage - cross that bridge later me thinks.

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