So, evertime we use the washing machine, there is a stagnant water smell when it drains and pumps the water into the drains. The washing machine pipe emptied into the waste pipe under the sink before the u-bend. When I disconnect the washine machine drain pipe it smells. We do wash at 60c once a week and 90c most months.

So my question is, if the stagnant water in the drain pipe is making the stink, what should I do about it? Should I move it to join the waste pipes after the u-bend? or is it likely to be something to do with the drain pipe and the water collecting there?

Is there something in the u bend being disturbed? Have you checked it is clear. Failing that You can get a direct connection ‘nipple’ to put the end of the washing machine hose on to, rather than draping into the waste tube.

I’m assuming when you say “before the trap” you mean upstream? If so, any smell from the washing machine waste pipe might be coming up through the sink waste. If you move the wm waste connection downstream of the trap you may replace the smell with a not dissimilar smell from the drains (finding its way up the wm waste pipe and through the wm) 🤔.
You may be able to reduce the smell with soda crystals washed down the sink waste with hot water and/or using a washing machine cleaner.

The undersink u-bend isn’t that big and is close to the surface. I suspect the pump action is disturbing the u-bend water enough to allow smell to come up from beneath it, or just completely replacing the relatively OK sink u-bend water with stinky water.

I would attach it underneath via a direct connection with some kind of non-return valve to stop it interfering with the u-bend water on its way out.

I had this, disconnected u bend and cleaned it, it was disgusting. Now drain wm down every 3 months and clean filter. Run an antical tablet on hot wash.

Ditto dishwasher, I ve bought Miele appliances and reckon they ll outlast me.

I reckon problem was eco washes not getting hot enough to prevent grease build up.

I think that there are several plumbers on stw who should be able to answer this, but I believe that the problem is that the outlet on your washing machine is so powerful that it is syphoning the water out of the trap thereby allowing smells from the main drains to enter.

And that the solution is an anti-syphon trap.

Anti-syphon trap, that’s the correct term I was hunting my brain for.

I do find it uncanny how stw has a knack of throwing up questions I’ve been thinking about posting on but not getting around to! I’ve been meaning to ask about this for ages! Also a mildewey smell when the machine gets left when we go away for a week. It seems to have been remedied by doing more frequent 90degC washes (towels each week). But watching with interest. I’m mid-30s (and have been doing all my own washing since leaving home for uni before you say) and it seems like this has only started being an issue for me in the last couple of years. Not sure if it’s connected to using method washing liquid (that’s perhaps the only thing that’s changed).

Top tips: white vinegar, it’s brilliant. Make your own fabric conditioner by adding an essential oil such as lavender (avoid citrus ones) to white vinegar. Ideally use a dark coloured glass bottle and keep out of sunlight.

Every 2 months add 500 grams of soda crystals to the drum and select hottest wash. You’ll need to do a cold rinse once or twice immediately afterwards.

One month after this and then alternate months, add half a litre of white vinegar and select hottest wash. Again, one or two rinses will be needed afterwards.

Never had pongy drains, washing machine or dispenser drawer. Pipes are kept clean by the vinegar.

I do find it uncanny how stw has a knack of throwing up questions I’ve been thinking about posting on but not getting around to! I’ve been meaning to ask about this for ages!

Ditto.

I have a similar setup, the drain hose is plumbed directly into the sink drain. It’s on a T-junction (a ‘vampire tap’ if you like) between the sink drain and the U-bend (which is a actually bottle trap, not a U-bend).

My question was, is this normal / correct? As above, I was under the impression that this sort of closed system either needed a vent (which I suppose the sink is, so long as the plug isn’t in) or some sort of anti-syphon device which I can’t see but don’t really know what one would look like.

Thanks all,

There is some food for thought there. The wierd thing is that it only smells during the pumping, but the stagnant water in the drainage tube has #air access# (for want of a better term) to the sink plug hole…

But i suppose it gets all chruned up when it pumps into the u-bend.

Is there any reason you cant have 2 u-bends with the drainage pipe emtying in between??

Reminds me, just put ours on the ‘cleaning cycle’.

The important part missing here is the rise and fall of the washing machine (also applies to dishwasher) flexible waste hose. It’s a flexi hose so people (installers also) assume they can wang it to the drain any which way, but there must be a fall from the outlet of the appliance and a rise above the connection to the waste spigot – which should be before (upstream) the u bend. The sink provides an air vent, even when the plug is in due to the overflow. RTFM of the appliance and you’ll see the diagram but due to multilingual manuals they rarely convey how important the rise and fall is.

The wierd thing is that it only smells during the pumping

Because the outlet water is being pumped out with such force that it syphons all the water out of the U-bend/trap allow smells from the drains/sewers to come back up.

After pumping everything settles down and the U-bend slowly fills up with at least some water which stops the smells from the drains coming through.

I think. I am a carpenter – I don’t touch plumbing or any other trades!

Edit: Ignore me, goldfish seems to know what they’re talking about. And the username suggests a greater understanding of water related issues than me.

And to elaborate (I do seem to have a habit of replying to my own replies), the rise and fall of the flexi waste pipe should be forming a u bend, such that even if the bottom of your WM is a stinking cesspit, you’ll never know.

Because the outlet water is being pumped out with such force that it syphons all the water out of the U-bend/trap allow smells from the drains/sewers to come back up.

Siphoning would only happen if the WM waste were plumbed in (what I would call) downstream of the u bend. I’m assuming it’s plumbed upstream, as it should be, and as per the diagram I posted above that has the caption “diydata.com”. That could be a big point of confusion amongst all of us replying.

The important part missing here is the rise and fall of the washing machine (also applies to dishwasher) flexible waste hose.

Yeah, I’ve seen that marked on a sticker on the backs of machines. It’s definitely got the rise – it comes up and over between a hole in the cabinet walls. The drop I’d have to pull the machine out to check, I know something is tight back there but can’t remember what offhand.

Thanks.

Edit: Ignore me, goldfish seems to know what they’re talking about. And the username suggests a greater understanding of water related issues than me.

I’ve been trained to use lots of big words to sound clever, but I’m not a plumber so I could def be wrong.

My username is coincidental 😂

Thanks Goldfish – I suspect you have said to what the problem may well – the drainge pipe does not have much rise and fall. It is a Bosch machine with the outlet for waste water on the opposite side to the sink and there does not appear to be much slack to make the pipe have the proper rise and fall. I will have to slide it out and see if the pipe is stuck or not. I did not install the machine nor the sink. Its probably time to hve a proper look behind knowing what I know now.

Just had another look and it is not caught on something, but rather just too short as the sink cabinet is flush against the wall and only has a space for the pipe to pass underneath it. This means it cannot rise above the spigot into the sink waste pipe.

I am planning sometime in the next year to replace the sink unit, but until then could I just add an extension to the waste pipe?

My username is coincidental

no it’s definitely goldfish24

but until then could I just add an extension to the waste pipe?

Can you post a photo? Not necessarily for me to answer the question but because these things are generally easier to solve if the problem solver can see the problem.

I know that if I am trying to figure out a practical solution to a problem it’s often easier if I can see the problem rather than having it described to me, especially by a lay person.

The thingy underneath the sink is called an appliance trap. There are also twin appliance traps if ypu also have a dishwasher connected in to the waste.
You can extend the WM drain, 17mm swaged connectors are available.
The grey drain pipes come supplied with a crook that you screw to the wall above the inlet of the appliance trap. To give a fall so you effectively have a trap between the WM and the drain.
The smell is most likely to be the first slug of water thats been sitting in the pipe finding its way up into the sink. Hot wash or a slug of bleach into the pipe work occasionally should help.

Every 2 months add 500 grams of soda crystals to the drum and select hottest wash.

Just for clarity, you mean baking soda, not caustic soda right?

500g of caustic soda/drain cleaner into boiling water will certainly clean your pipes out, and you lungs if the drain is vented!

thisisnotaspoon

Just for clarity, you mean baking soda, not caustic soda right?

cinamon_girl said, soda crystals, not caustic soda
Could be wrong, but I don’t think Soda Crystals are the same thing as caustic soda….

What Are Soda Crystals?

Here you go and it can be bought in Tesco and Wilko. Has multiple uses and the website is full of cleaning pron tips:

https://www.dri-pak.co.uk/cleaning-products/soda-crystals-1kg/

Edit: the American term baking soda, known as bicarbonate of soda in the UK, is not the same as soda crystals. Caustic soda is not soda crystals.

According to the link “Great for quick service restaurants”, which seems a strange claim to make and highlight.

Why aren’t they just as great in a multiple of other situations including restaurants with very slow service?