Venting multiple extractor fans through one duct.

Discussion in 'Builders' Talk' started by GrumpyGit, Jan 18, 2018.

  1. GrumpyGit

    GrumpyGit New Member

    Is there any simple way to connect multiple extraction units to the same bit of ventilation ducting?

    I'm helping with some work in a flat which has a extractor fan in the bathroom that vents into a length of ducting that runs through the kitchen and out through the external wall via a louvered flap valve.

    The flat can get a bit damp, so we want to add an extractor hood over the cooker and a tumble dryer vent. The rules for the building prevent us from cutting holes in the outside walls, so we'd like to connect the two new items to the existing duct work. However if we do this we need to make sure that when the tumble drier is running it's pushing air outside, and not back into the bathroom or down the extractor hood.

    So, we're looking for some simple one-way valve mechanism for merging two ducts into one, a quick google reveals several high-end industrial versions cost hundreds of pounds, is there a simple domestic system available? or is there a better solution?

    Thanks
     
  2. Yes, there are back-flaps for such a task, but I've no idea how air tight they are, and you don't want either room's smells swapped around.

    Also plenty of Y-branches for this sort of thing.
     
  3. GrumpyGit

    GrumpyGit New Member

  4. Dr Bodgit

    Dr Bodgit Super Member

    Last edited: Jan 18, 2018
    Deleted member 33931 likes this.
  5. Also plan the ducting runs, and don't necessarily go for the shortest route. Eg, I'd have thought that taking each duct separately for as far as possible up to the shred exit vent would be better than trying to 'tee' them further down.

    The idea is to try and prevent any pressure buildup inside the duct which would try and force expelled air back down the other one. So if you take each duct as close to the ext vent as possible before Y-ing them together, I'd hope that the presence of the ext vent right there would allow the air to be expelled as easily as possible with less risk of it trying to find alternative routes.

    Make sense?

    And also don't fit an excessively powerful fan. And, I guess, make the two fans of a similar air-flow rating?

    Jeepers, it ain't straight-froward...
     
  6. DIYDave.

    DIYDave. Screwfix Select

    Couple of threads on here, what, couple months ago about running just 2 fans on same ducting

    Both had big big problems doing it this way, although planning the routes and the actual work wasn’t good either - take a search for the threads (I ain’t doing it for ya) ;)

    4 fans on 1 run of ducting - there’s gonna be a lot of vapour in there that will be waiting to condense back into ‘liquid water’, especially in uninsulated pipe and cold pipe runs

    Think of separate ducts per fan
    Solid duct improves performance
    Insulating ducting
    Condensation traps
    Careful planning
    Possibly combining ducts just prior to exit
    Careful planning and more
    Don’t buy cheap fans, false economy

    Probably other stuff but I’m the meantime, search the forum :)
     
  7. Joe95

    Joe95 Screwfix Select

    Those shutters are bloody good. You have to make sure the right part is at the top, but it does stop almost all air. I've used them when extractors do not have built in shutters.

    TLCs ducting is good stuff, and they have a fair range too.
     
    KIAB and Dr Bodgit like this.
  8. It's good to haver a review of them Joe, so that's reassuring.

    I have to say that I think I'd still only use them as a very last resort; not only can they not be fully air tight (tho' from what you say they sound pretty good) but if a kitchen is ducting through one, how long will it be before the inevitable light coating of grease makes the flap reluctant to open?

    Also it's a compromise solution to the problem - it doesn't tackle the route issue. If the OP really must go this way, then fine.

    Don't forget that air flow is like water flow is like electrical flow - it don't all just take the easiest path, but some will always travel down alternatives if there is a way. And you wouldn't want any of what I produce on the throne coming out yer kitchen vent.

    GG, is this a kind of DIYish job - are you helping a friend or is it a paid job? If the latter, then go with the flaps. But if the former, then what I would do in such a case is to (if it's possible) 'split' the ext vent outlet into two.

    See these Y-branches:
    FD44960.JPG
    well it's quite possible to contrive a centre panel (plastic or metal) to split it down t'middle. Jobbie jobbed. With this fitted right at the vent exit, it should keep the flows nicely separated.

    Yes, this would mean more air resistance, but that can be addressed - if needed - by a slightly more powerful fan.


    If you are going the shared 'flap' route, then I think care needs to be taken to get both fans running at a similarish flow rate?
     
    Last edited by a moderator: Jan 19, 2018
  9. GrahamSP

    GrahamSP New Member

    I have found that the shutters are good to start with, but if they have any moisture to deal with, the spring rusts.
    If you want/ can, place two fans for each section.
    One at the air intake and the other past the Y point. If this is done for both runs then a: you don't need powerful fans and b: you won't get any back draught. One fan pushing and the other pulling.
     
  10. Ibrahim Khalifa

    Ibrahim Khalifa New Member

    Hey, folks

    a question about ducting. Currently I have had ducting installed where I have two fans running off one duct I have an understair WC and then approx 2meters down within the kitchen I have the kitchen extractor hood Teeing off the same duct, I am aware that each fan ideally needs to have their own ducting, but since the work is already done is it a major issue or are there any additional sections/components needed to prevent back flow as I'm concerned if the WC fan is in use and the kitchen hood is not there may be issues there and vice versa. I can get photos / video if required Thank you
     

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