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Discuss Sealing washing machine drain to prevent smells in the Plumbing Forum area at

  1. PaulR

    PaulR New Member

    I have a washing machine that drains through a pipe connected under the sink, before the U-bend. When it drains, bad smells come up through the sink.

    I want to give it its own U-bend, like my dishwasher already has, but the dishwasher's drain pipe just slides in the top of a 40mm waste pipe, and isn't sealed. I'm concerned it will still smell bad if I use this approach.

    Is there any reason I shouldn't seal the outlet pipe to the drain, before a U bend, to prevent any smells escaping?
    Alternatively, can I connect both appliances to the same U-bend, and seal their pipes onto it?
  2. rpm

    rpm Trusted Plumber

    • Agree Agree x 1
  3. Ric2013

    Ric2013 Plumber

    • Like Like x 1
  4. rpm

    rpm Trusted Plumber

    Seems like a clicked on the wrong one - cheers Ric.
  5. PaulR

    PaulR New Member

    The arrangement above obviously vents through the sink it's under, allowing smells to escape. My question is can I route the appliance waste directly through a U-bend and away, with no venting at all.

    People seem to question the wisdom of doing this, mentioning siphoning, but I'm not really sure what that means in this context and why it would be a problem.
  6. steadyon

    steadyon Active Member

    The U bend is a water trap. Water passes through, and establishes a seal which prevents smells from coming back from the sewage system.

    You have two choices for either / both the washing machine and dishwasher:

    1. Use a single or double spigot on the waste fittings for the sink bowl. If you do this, its important that the hose rises above the spigot and then falls back to it, otherwise excessive water will be trapped in the hose.

    2. Use a trapped standpipe. Length of tube with a trap at the bottom. Appliance flexible drain goes into top of pipe (NOT sealed), bottom connected via trap to sewage system.

    If you use method 2 you must be sure the waste pipework can handle the simultaneous discharge of the two appliances and the sink. The same applies to method 1, but in this case, if too much water is discharged it will come back into the sink. In method 2, two much water equals a spillage on the floor.
    • Like Like x 2
  7. rpm

    rpm Trusted Plumber

    As proven when you fart in a bath. :D
    • Like Like x 1
  8. rpm

    rpm Trusted Plumber

    Syphoning draws water out of the pipe therefore making it an open pipe which air & odours can travel through.
  9. PaulR

    PaulR New Member

    Thanks for the input. What I'm asking is can I use solution #2 above, but instead of simply sliding the appliance drain pipe down into the standpipe, can I seal it in place so no smells can escape? Or would this somehow interfere with the correct operation of the U-bend? And if everything is sealed, would I need a U-bend at all?
  10. rpm

    rpm Trusted Plumber

  11. steadyon

    steadyon Active Member

    You can use solution 2, but don't seal appliance flexible drain pipe to the rigid stand pipe.

    If you did seal it you would run the risk of syphoning the water out of the machine, and / or out of the trap of any other appliance / sink connected to the same run of drain pipes.

    If you did seal it without a trap (U bend) you run the risk of sewer smells getting back into the washing machine.
  12. Ric2013

    Ric2013 Plumber

    IF the washing machine itself is that smelly, then the smells will come into the house anyway via the (usually) open overflow on the detergent drawer. It is far more likely that the hot water being discharged from the machine or sink is warming the gunk in the drain and sewer and this comes into the house via the open pipe. If it is the machine that smells, it needs cleaning (use a proprietary product, or run a 60°C wash empty, with detergent only).

    A washing machine standpipe or connection above the sink trap both allow the water to flow down, but not the air in the waste pipe/sewer to rise back up.

    If you seal the machine waste into the standpipe, there is a risk that the machine will not be able to stop draining due to the siphon effect set up by the flow of water. The seal created by using an appliance nozzle as my and rpm's link earlier is not a problem, as it goes onto a waste pipe via a fitting designed for this purpose (and probably the air that can get in via the sink waste or overflow connection prevents a siphon being set up).

    By all means experiment if you wish and, in your specific circumstances, a workaround may or may not work. But a trapped washing machine standpipe with machine hose NOT sealed, or an appliance nozzle thing are the tried-and-tested methods...
    • Like Like x 1
  13. Best

    Best Trusted Plumber

    If you have an overflow as part of your sink, do check it is clean as when you run water down waste pipe, the air will push up through overflow giving bad smell if it is not clean.
    • Agree Agree x 1
  14. PaulR

    PaulR New Member

    Thanks for the advice, I feel like I understand the situation, particularly around syphoning, a whole lot better now.

    I've realised my machine is connected to the sink drain pipes in such a way that dirty sink water can flow into the washing machine outlet and fester there for days before the washing machine is run. I'm pretty sure that's what's causing the smells, so moving to a conventional standpipe set up will likely fix the problem.
    • Like Like x 1
    • Agree Agree x 1
  15. PaulR

    PaulR New Member

    Just wanted to add I discovered something called an air admittance valve, which allows air into drain pipes, but stops smells getting out.
  16. YorkshireDave

    YorkshireDave Active Member

    NO. Please do it properly as already advised. You will NOT regret it.
    • Agree Agree x 1
  17. PaulR

    PaulR New Member

    Can you expand on this? Isn't this what the component linked above is for?
  18. Stoney Ground

    Stoney Ground GSR

    Didn’t believe you so just done an investigation

  19. Ric2013

    Ric2013 Plumber

    Letting air in, as you say. But too many words now and thissis and thesees and thoses. Can you do a sketch of what is there and what you are planning to do as I don't understand why you think you need an air admittance valve?
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