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Extractor fan ducting - flat channel vs round efficiency

Discussion in 'Electrics UK' started by MasterMark, 1 Apr 2019.

  1. MasterMark

    MasterMark

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    Hi all,

    I'm getting different answers when speaking to ducting suppliers and looking online, so thought I'd check here.

    I have a "blind" bathroom w/ shower. I'm using a pretty powerful inline fan (TD Silent 350). The run to the outside is long (about 5m), and the builders installed low profile/flat channel ducting of 110mmx54mm. The bathroom still steams up pretty quickly, and given I think I have just about enough space to change the ducting to the round 100mm type, I was wondering whether this would be materially more efficient, or not worthwhile? Have read much about pressure loss (which seems far greater for the flat channel ducting), but not sure whether this would be relevant in my scenario.

    Any suggestions would be much appreciated, thanks!
     
  2. Taylortwocities

    Taylortwocities

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    A hole is a hole. Work out the aperture size for 110x54 versus a 100mm circle (sorry, CBA to do it for you!)
    You can then see if there is any real difference that would throttle the flow rate.

    I'd first look very carefully at the duct runs, any output flaps, etc to see if there;'s something slowing the airflow.

    One thing that is forgotten is that to get air OUT of a room, you need to let at least as much air IN. If your doors are tight fitting, you may need to make a 100mm dia hole somewhere so that the air can come from somewhere. If not, your fan will be trying to make a vaccuum.

    EDIT, The other thing to try, is to take the output duct pipe off the fan. Then let the air extract into the loft for a couple of goes. If the room is still steamy, then the duct isn't your problem.
    Also; The TD350 is more than capable of pushing 5m of conduit.
     
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  3. reds42

    reds42

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    I would think round ducting would improve the situation a bit, particularily with that fairly long run. Bends will introduce lots of drag so keep them to a minimum.

    Another thing to maybe check is how easily air can get into the bathroom to replace whats being extracted. Is there a big enough gap under the door or other means for the air to get in the room?
     
  4. MasterMark

    MasterMark

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    Thanks, Reds42. Good point re getting air into the room, frankly hadn't really thought of that - was focused on keeping the bathroom door shut so as to not let the moist air escape into the bedroom. There's a bit of a gap under the door, but not huge.
     
  5. Taylortwocities

    Taylortwocities

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    Wish I had mentioned the tip about air input..…:cautious:
     
  6. bernardgreen

    bernardgreen

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    A = pi * R²

    pi = 3.14

    3.14 * 50 * 50 = 7854

    110 * 54 = 5940

    So 100 mm round will have approx 50% larger cross sctional area than the 110 * 54 rectangular.

    Avoid flexible corrigated ducting as it invariable results in turbulant air flow which significant reduces the effective cross sectional area of the duct.
     
  7. MasterMark

    MasterMark

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    Thanks, @bernardgreen - but, for my understanding, how does that 50% greater internal internal area available for extraction translate into real world results? i.e., all other things being equal, what % improvement in extraction rate can I expect? Eg per the TLC stats in the link below, using 100mm rigid ducting with the TD250, over a 5m run, leaves 135 m3/h extraction. What kind of numbers do you estimate you'd be able to achieve using the 110x54mm ducting?

    https://www.tlc-direct.co.uk/Technical/Ventilation/Ducting.htm
     
  8. bernardgreen

    bernardgreen

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    If nothing else changes then the 50% increase in cross sectional area will allow the fan to push approx 50 % more air than is being moved through the existing duct.



    TYPICAL and by no means assured, there is no mention of the air pressure difference between inside the room and outside at the end of the ducting.

    The fan has to overcome this presssure difference if it is to move any air.

    Very often these "typical" values are measured in a test set up in a room ( not venting to the outside ) . The air pressure at the input to the fan is the same as the pressure at the far end of the ducting.

    Without a means to get replacement air into the bath room the pressure in the room will until the pressure difference is so high that the fan cannot move any more air
     
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  10. ericmark

    ericmark

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    In my wetroom with door closed, fan on and underfloor heating on it takes around 1/2 hour to dry room, with door open it takes 15 minutes to dry room. Clearly while using the room water vapour will fill the room, if the fan was strong enough to clear it the room would be too cold, it is the speed it clears after use that matters.

    Also heating in the room, in my house standard bathroom we have a problem with black mould build up, but in this house no such problem, I have looked at what is different, and it is temperature and humidity of the replacement air, sucking air from hall which has cool moisture free air dries out the room far faster than drawing air from the much warmer landing and likely the landing air has higher humidity.

    House with wetroom humidity is showing below 50% most of the time, house with bathroom showing above 50% most of the time. The main thing is house with wetroom has vents in every room and chimneys in three rooms, where other house has one flue brick which is only vent to outside, seems in 1954 houses were better designed than in 1979, with single glazed windows the metal frames had small holes to outside so moisture in the house would condense on windows then be collected in trough and run outside.

    Although we see the mould in the bathroom, as that is likely the most humid room, the whole of the house is likely too humid unless you fit a heat recovery unit. Opening a bathroom window is far better than any fan simply because it allows air in as well as out, you can buy heat recovery units where the heat exchanger is built into the duct through the wall, but other than that the problem is when cooling air the moisture can drop out before it leaves the ducting so although it would seem a good idea it does need careful designing to ensure you don't get water pools inside the duct.
     
  11. MasterMark

    MasterMark

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    Thanks @bernardgreen and @ericmark , very helpful on both counts. Seems like the pressure build-up may be contributing to the issue. So if i replace the existing flat ducting with the 100mm round, and leave the door open a crack to let some additional in to be recirculated, that should get me to the best possible result. Alas, no windows given building construction, so it's never going to be perfect, but will at least yield an improvement.

    Thanks again!
     
  12. ericmark

    ericmark

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    Would it be possible to have second ducting with out a fan so replacement air drawn from outside?

    Humidity is relative i.e. linked to temperature, so air inside at 50% has likely more moisture than air outside at 75% because outside air is cooler, drawing replacement air from outside better than drawing it from inside, normally it would cause too much draft but with length of ducting less likely to be a problem, clearly some closable shutter wanted.

    I assume reason for fan is mould?
     
  13. FrodoOne

    FrodoOne

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    7854 / 5940 = 1.322
    So 100 mm round will have approx 32% larger cross sectional area than the 110 * 54 rectangular.
     
  14. bernardgreen

    bernardgreen

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    I need a new calculator.
     
  15. MasterMark

    MasterMark

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    @ericmark - technically, yes, it's possible, but a bit of a faff and then becomes a cost issue as well (creating new hole in external wall etc).

    Re mould - I keep the fan running a decent length of time after finishing showering, so in the 18 months since installing with the 250 power fan, no mould issues. But to reduce risk of this, and also super fast steaming up of the shower room (eg mirrors totally steamed up within 3 or 4 minutes), thought I'd upgrade to a more powerful fan and replace ducting. The fan is also currently unsightly (resting above a bedroom wardrobe), so will need to be boxed in (though still accessible for maintenance). But I really want to do the job just once to the best possible level without overkill.
     
  16. Alec_t

    Alec_t

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    Since for a circular-section duct with a given pressure difference between its two ends the flow rate is theoretically proportional to the fourth power of the duct's diameter, I would expect a lot more than a 50% increase in flow rate.
     
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