Smelly bathroom, bath plug hole

[Mr Whippy]

New build ~ 2021 vintage.

Bath sealed in with tiles.

Chunk of wall boxed off with all pipes/cistern in it.

Soil pipe in corner in main boxed off area.


Can hear gurgling etc from bath plug hole when running sink or hear lots of noise from it when flushing loo.

Smells from plug hole.

If I run bath tap for a couple of seconds, gurgling noises stop and all ok… smells stop too.

For about 18hrs.

No sign of leaks on ceiling below.

I can’t understand how a u bend could dry out in such a short time unless the u bend is literally split… but each bath drainage cycle it’s likely or u bend top up it’s dumping enough water to let smells out (not crazy amounts like a totally dry one would)

But surely even a tiny leak like that would show up?


Only access is behind a tiled-on bath panel, so a bit of a task to take a look.

It’s been seemingly ok years sooo…


I’ve previously had low profile u-bend things (like a musical reed in a pipe) that clog up as they age, so wondering if it’s that kinda setup and has failed somehow but internally so it’s not leaking externally?


Soil stack runs through ceiling to a roof tile. Was wondering if wind direction could be causing it to pump u-bend a bit and drop its level a few mm under the bend and let smells past?
Clutching at straws here as it was seemingly fine for a years till now.


Yet another new build gripe. Everything tiled in. It should be against building regs to have it inaccessible but here we are.

[smokey mow]

It’s typical symptoms of siphonage which is the water being pulled out of the smaller or shallower traps of the basin and bath when the toilet is flushed or the bath emptied.

If you hadn’t said the SVP went through the roof I’d have suggested it was an AAV stuck closed.

Edited by smokey mow on Thursday 25th April 21:16

[Mr Whippy]

Hmmm… good point on AAVs.

I’m assuming the soil is as described wrt venting.

But it now makes me wonder if the tile vent has in some way become occluded, which would cause the same issues.

Ie, bird nest or similar.

[Vanden Saab]

Is the bath panel actually solid. We either silicone one tile in so the silicone can be cut and the tile removed or silicone around the whole panel so again it can be cut and removed. If the tiles are all grouted check there is nothing stopping the panel from being removed and try cutting the silicone around the panel then slide a scraper under the panel and then slide/lift the bottom out.
If you can then get to it put an anti-syphon valve in the bath waste pipe.
Of course they might have been utter fkwits and solid fixed the panel.

[gruffgriff]

Do you have a little aav on your basin waste? Quick and easy to swap one in which may help before looking at the stack aav..but if it's a new phenomenon that suggests something's changed and possibly a failure or gumming-up of the existing system of course. I just retch at the hair/soap sludge in waste bends!

Eta depends on the run between bath, basin and loo of course..but any aav in the room can only help.

[sherman]

Tip a bottle of Buster down the drain and see how it is.
Probably your cheapest and easiest fix to try.

https://www.google.co.uk/shopping/product/16527752...

[smokey mow]

gruffgriff said:

Do you have a little aav on your basin waste? Quick and easy to swap one in which may help before looking at the stack aav.

The OP needs to identify what the problem is before doing anything else.

An AAV needs to be fitted correctly with its cowl above the spill-over level of the basin. If he goes cutting one in to the waste below basin level and his issue turns out to be a blockage somewhere else in the bathroom there’s a likelihood of flooding the bathroom with the waste water through the incorrectly fitted AAV.

[Portofino]

Do you have baths often? Might sound silly but our bath had a bad smell coming from it & it turned out it’s stale water in the plughole. Run some water ( or have more baths!) through it regularly & the smell goes.

[Mr Whippy]

The kids use it a few times a week.

They’d had a bath the night before and it was smelly last night again.



Hmmm so a gammy clogged up I-bend could be causing some capillary draining and leaving it exposed?

Or pumping action… maybe when the loo goes its suction is draining the u-bend a bit because the air admittance (top of soil stack) is insufficient.


I’ll take a closer look at the bath panel to see how removable it is… but it looks rigid to me.
I could always remove and them silicone back for future access.



For all I know it could be some strange exotic u-bend or anti-smell system that’s gone awry, but the risk of water leaking (like a bath full at some point) and this smell being an early warning for months now has me thinking time to look more closely.

[BrokenSkunk]

Mr Whippy said:

Or pumping action… maybe when the loo goes its suction is draining the u-bend a bit because the air admittance (top of soil stack) is insufficient.

With the gurgling from the bath U-Bend when you flush, I think it's almost certainly this.

[Dunclane]

Portofino said:

Do you have baths often? Might sound silly but our bath had a bad smell coming from it & it turned out it’s stale water in the plughole. Run some water ( or have more baths!) through it regularly & the smell goes.

We had the same but it was the water had evaporated from the U bend sufficiently to let odour through. As you quite rightly say run the tap for a good while or have a bath!.

[Mark V GTD]

Possibly a blocked roof tile vent or the connection of the SVP/ducting to it - considering everything thats been said so far. Is the underside of the vent tile accessible in the roof void?

[Chrisgr31]

It could also be hair etc that has accumulated in the u bend

[timbobalob]

We had something similar to this in our 2017 build house from the kitchen sink - some days it would stink out the kitchen!

You couldn't smell it all of the time and we tried all products to try and clear it, but it was only when I decided to clear out the u-bend I noticed it was plumbed in such a way that the bend was below the waste pipe so negated the trap!

It might be worth double checking it's installed like it should be (at a glance ours looked fine)

[AW10]

It's worth seeing if there's a removable insert in the plug hole that acts as the trap - it might simply not be seated properly.

[soxboy]

We had similar in our 2003-build house.

It was traced to one of our sons and a combination of either massive turds or loads of toilet roll (most likely both) causing the siphoning effect as mentioned above and the flushing action sucking the water also out of the trap in the bath.

This was solved by firstly getting them to block the toilet less and by making sure that if there was a siphoning effect (you can tell by the gurgling noise) that the taps are run in the bath and the sink soon after for say 30 seconds each to make sure the traps are refilled.

No issues with smells since.

[Jeremy-75qq8]

The purpose of venting the stack is to make a vacuum in the stack impossible hence stopping the suction emptying the traps. So large turd or not should not make any odds !

[Mr Whippy]

The joys of modern new builds eh.

Just as many jobs as an old house, just chasing up shoddy workmanship or plain bad design, vs an old house chasing u shoddy DIY it just plain old worn out bits.



The roof vent tile is sort of accessible so can try get into that to inspect the vent tile flow.

Given the recent failure/symptoms I’m guessing it’s been damaged/occluded/blocked up with a bird nest or rubbish or something.

[DonkeyApple]

BrokenSkunk said:

Mr Whippy said:

Or pumping action… maybe when the loo goes its suction is draining the u-bend a bit because the air admittance (top of soil stack) is insufficient.

With the gurgling from the bath U-Bend when you flush, I think it's almost certainly this.

The gurgling from the bath trap when the lavatory is flushed just sounds like a vacuum in the system, I'd agree. And if it's a new thing then something has changed somewhere in the system to cause that vacuum. The first thing I'd check is the top of the soil stack and if there's a nest in it etc.