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Odour when toilet flushes

  • 08-06-2025 09:09PM
    #1
    banchang
    Registered Users, Registered Users 2 Posts: 1,281 ✭✭✭


    IMG_9465.jpeg IMG_9467.jpeg IMG_9466.jpeg

    Background: we decided 2 years ago to re-tile bathroom floor. & tiler insisted on the toilet bowl & washhandbasin being removed & replaced on top of new tiles by his plumber. Ever since then we’ve noticed a smell when we flush toilet which wasn’t there in the 25 years before that. There is definitely no blockage in the pipes as there’s only 2 of us in house & there is no evidence of blockage when flushed.

    Another plumber came out last week to quote to replace our immersion tank in hotpress in the same bathroom, & I mentioned this to him & he looked & said that seal on waste is perfect, & reason for smell was due to the recent re-plumbing, which apparently is now set up such that whenever the toilet flushes, it sucks the water out of the washhandbasin U-bend, thereby allowing odors to come up through the washhandbasin drain & overflow openings. 

    I have to say I was amazed to hear this. How could a plumber set it up this way knowing this could be a consequence? 

    Before I ask my new plumber to reconfigure to fix this issue, can I pls check that this diagnosis is plausible & likely correct ? Pictures hopefully explain setup.



Comments

  • #2
    Lenar3556
    Registered Users, Registered Users 2 Posts: 2,044 ✭✭✭Lenar3556


    The diagnosis is certainly plausible. Quite possibly the plumber witnessed suction on the basin when the toilet was flushed. Note that this scenario would likely give rise to an ongoing smell issue after the toilet is flushed, until the basin trap is refilled by running the tap for a few seconds. It would be worth validating this yourself.

    It’s not obvious however that removing the sanitary ware and refitting would have brought about any great change which was likely to introduce this issue? Was the waste pipe from the basin re-routed during these works or did it always tee into the toilet in that way?

    The arrangement isn’t ideal, and your plumber may be able to identify a better way of accommodating the basin waste independently of the toilet. Is this on an external wall? It would also be worth having him check that the external sewerage vent pipe is present and unobstructed.

    (Incidentally removing and refitting sanitary ware on top of new tiling is usually a much better job than tiling around them.)



  • #3
    Victor
    Registered Users, Registered Users 2 Posts: 78,736 ✭✭✭✭Victor


    For the moment, every time the toilet is flushed, ensure the wash basin tap is run afterwards.

    How are the bath / shower connected?



  • #4
    mickdw
    Registered Users, Registered Users 2 Posts: 23,922 ✭✭✭✭mickdw


    That arrangement is pretty much guaranteed to create the issue you have re odour.

    Toilet flushes, water flow pulls the water trap out of the hand basin leaving an open path for dangerous sewer gases to enter the bathroom.

    An air admittance valve into the line would likely solve the problem.



  • #5
    banchang
    Registered Users, Registered Users 2 Posts: 1,281 ✭✭✭banchang


    Thanks for all comments. I don’t know how it was previously plumbed, but smell wasn’t there previously. Bath to left of toilet appears to have a separate waste outlet.

    Someone mentioned to me that I need something called an ‘anti-vacuum trap’ on the sink, & that would fix the issue ?



  • #6
    Explosive_Cornflake
    Registered Users, Registered Users 2 Posts: 2,186 ✭✭✭Explosive_Cornflake


    Yeah, you can get anti syphon traps, e.g. https://www.screwfix.ie/p/mcalpine-anti-syphon-bottle-trap-white-32mm/37314

    But check if any of the ubends are clearing out first. I'd be tempted to just clear the current ubend in the sink. There will be some absolutely foul stuff in there from washing teeth and hair etc. It could just be less of a volume of water in that you had previously, and flushing the toilet is now enough to syphon it. Air will take the easiest path back in.



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  • #7
    banchang
    Registered Users, Registered Users 2 Posts: 1,281 ✭✭✭banchang


    https://www.boards.ie/discussion/comment/123528113#Comment_123528113

    thanks will do !



  • #8
    Victor
    Registered Users, Registered Users 2 Posts: 78,736 ✭✭✭✭Victor


    https://www.boards.ie/discussion/comment/123528113#Comment_123528113

    You can get €2 wire spring sink-rodding brushes in Tesco in the 'random stuff' section.



  • #9
    Lenar3556
    Registered Users, Registered Users 2 Posts: 2,044 ✭✭✭Lenar3556


    An air admittance valve and/or antisiphon trap may resolve the issue, but I would rather see the basin piped out separately and vented. It may be easily done if it’s on an external wall. It’s a more comprehensive solution.



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