Flat - inconsistently smelly sink, what to do?

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Hullo,

Asking for a friend. She has a flat she moved into a few months ago - middle of the block, so neighbours above, below and to the sides. Every now and again, but certainly not every day or all the time, her sink starts to really pong.

It smells like sewer waste. I've experienced it - I guess that it could be rotten food waste, but it really smells like sewerage. She can put the plug in the sink to literally no effect. She has been using various products for the smell - which I've advised might probably only be a short-term solution and now she's read about bicarbonate of soda and white vinegar, so she wants to try that.

I disassembled the sink's s-bend(?) for her and pulled it out - it looked kinda clean to me, and it wasn't dry either. Also, when I took it apart I found that it didn't actually smell that much. It might have been a good day. It's definitely the sink that smells when the smell come along.

So I couldn't figure it out, but I'm no Plumber... I said I'd so some research on her behalf. I was convinced it must be something some of the other neighbours are doing at some point, with the irregular times it happens, but I don't know, really.

The sink seems to drain in good order - so I don't think there's a blockage, but would using a winding metal snake thing be any use? What about getting someone in to shoot pressurised water down it? What do people usually do beyond, unwisely, punting a load of scalding hot water and bleach down the sink for temporary relief?

I remain confused as to how the smell can be present at all if the s-bend has water in it. It's possible I don't understand the theory behind the design.

I would appreciate any advice and guidance, even if it is just to say "we can't say from what you've told us, you need to get someone in who can see this for themselves to diagnose"... also, as it's a flat, I suggested that the Management Company should at least be made aware, and maybe it becomes their responsibility... because she's not smelly, and she's not putting smelly stuff down her own sink.
 
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I dunno how to respond to that...

Is that an off-the-cuff remark or a serious suggestion as to problem determination? I'm not being funny (you might be) I just don't know!
 
It's serious suggestion - the smoke will go right up the vent to the roof like a chimney. And show up any air leaks from pipes in the flat.
 
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Serious suggestion, sorry it was so brief.

If you didn’t notivmce the smell so much when you took the trap off I wonder if there’s an open joint somewhere downstream.

A smoke bomb should show if any air is escaping the pipework. If you don’t see the smoke you can usually smell it.
 
Ah, I see. OK, this seems like the realm of professionals, that's for sure...

Maybe I'm better advising her to approach the Management Company for the entire building and try to lay it at their door. I was really just wondering if there was anything obvious I had missed, or something else I could try (or suggest she try).
 
You could take the trap off and fill it with warm bath water to see if the plastic smells funny when it’s warm.

I saw that once on a plastic gully that stank but have no idea why.
 
Have you checked for old standpipes or open inlets (ex washing machine or dishwasher)? Like the thing on the left at the back:
IMG_20180921_154434733.jpg
 
Does your friend have a washing machine ,or dishwasher ,where the waste pipe/s from them join into the sink waste ? If so ,the smell may be from their flexible wastes. And only apparent when in operation.
 
Thanks all.

Firstly - there's definitely no old redundant pipework, as far as I know, the flats are quite new - and (to follow on to the next question) yes, there's always been plumbing located close by for both a washer / dryer and dishwasher... but I've stayed at her place for a few days and she can use neither of these appliances over that time, and has told me they've not been used in the last week or so, but then the smell will come at a time all of its own. I cannot help but think it's from outside this specific flat and something to do with the others, but I have no evidence.

Lastly, I don't know what a "hep 2 o valve in the pipe run" really means, but that's OK - I'm starting to think this is not a DIY job for me. :)
 
Check how the dishwasher and washing machine are connected to waste pipework.

a) If it's via a standpipe (or two) then check that the trap(s) are not dry
b) If it's via spigots to the sink waste then check that the waste pipes from both appliances rise to the height of the underside of the sink - many are installed so that the waste pipework allows for waste sink water to flow back along the appliance waste hoses.
 
Also; are glugging noises ever heard, after letting out a sinkful of water or at any other time? Any slight wavering on the surface of the water in the toilet bowl long after it has been flushed?
 
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Does your friend have a washing machine ,or dishwasher ,where the waste pipe/s from them join into the sink waste ? If so ,the smell may be from their flexible wastes. And only apparent when in operation.
Good point. probably worth using a dishwasher cleaner bottle and for the washing machine run it on the hottest wash with no powder/capsule in it ( and watch how much foam comes from an empty machine ! )
 

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